textutils
This manual is incomplete: No attempt is made to explain basic concepts in a way suitable for novices. Thus, if you are interested, please get involved in improving this manual. The entire GNU community will benefit.
The GNU text utilities are mostly compatible with the POSIX.2 standard.
Please report bugs to bug-textutils@gnu.org. Remember to include the version number, machine architecture, input files, and any other information needed to reproduce the bug: your input, what you expected, what you got, and why it is wrong. Diffs are welcome, but please include a description of the problem as well, since this is sometimes difficult to infer. See section `Bugs' in GNU CC.
This manual was originally derived from the Unix man pages in the
distribution, which were written by David MacKenzie and updated by Jim
Meyering. What you are reading now is the authoritative documentation
for these utilities; the man pages are no longer being maintained.
The original fmt man page was written by Ross Paterson.
Fran@,{c}ois Pinard did the initial conversion to Texinfo format.
Karl Berry did the indexing, some reorganization, and editing of the results.
Richard Stallman contributed his usual invaluable insights to the
overall process.
Certain options are available in all these programs. Rather than writing identical descriptions for each of the programs, they are described here. (In fact, every GNU program accepts (or should accept) these options.)
A few of these programs take arbitrary strings as arguments. In those cases, `--help' and `--version' are taken as these options only if there is one and exactly one command line argument.
These commands read and write entire files, possibly transforming them in some way.
cat: Concatenate and write files
cat copies each file (`-' means standard input), or
standard input if none are given, to standard output. Synopsis:
cat [option] [file]...
The program accepts the following options. Also see section Common options.
cat on MS-DOS/MS-Windows uses
binary mode only when standard output is redirected to a file or a pipe;
this option overrides that. Binary file I/O is used so that the files
retain their format (Unix text as opposed to DOS text and binary),
because cat is frequently used as a file-copying program. Some
options (see below) cause cat read and write files in text mode
because then the original file contents aren't important (e.g., when
lines are numbered by cat, or when line endings should be
marked). This is so these options work as DOS/Windows users would
expect; for example, DOS-style text files have their lines end with
the CR-LF pair of characters which won't be processed as an empty line
by `-b' unless the file is read in text mode.
cat to read and write files in
text mode.
cat to read and write files in
text mode.
cat to read and write files in text mode.
cat to read and write
files in text mode.
cat to
read files and standard input in DOS binary mode, so the CR
characters at the end of each line are also visible.
tac: Concatenate and write files in reverse
tac copies each file (`-' means standard input), or
standard input if none are given, to standard output, reversing the
records (lines by default) in each separately. Synopsis:
tac [option]... [file]...
Records are separated by instances of a string (newline by default). By default, this separator string is attached to the end of the record that it follows in the file.
The program accepts the following options. Also see section Common options.
tac
on MS-DOS/MS-Windows should note that, since tac reads files in
binary mode, each line of a text file might end with a CR/LF pair
instead of the Unix-style LF.
nl: Number lines and write files
nl writes each file (`-' means standard input), or
standard input if none are given, to standard output, with line numbers
added to some or all of the lines. Synopsis:
nl [option]... [file]...
nl decomposes its input into (logical) pages; by default, the
line number is reset to 1 at the top of each logical page. nl
treats all of the input files as a single document; it does not reset
line numbers or logical pages between files.
A logical page consists of three sections: header, body, and footer. Any of the sections can be empty. Each can be numbered in a different style from the others.
The beginnings of the sections of logical pages are indicated in the input file by a line containing exactly one of these delimiter strings:
The two characters from which these strings are made can be changed from `\' and `:' via options (see below), but the pattern and length of each string cannot be changed.
A section delimiter is replaced by an empty line on output. Any text
that comes before the first section delimiter string in the input file
is considered to be part of a body section, so nl treats a
file that contains no section delimiters as a single body section.
The program accepts the following options. Also see section Common options.
rn):
od: Write files in octal or other formats
od writes an unambiguous representation of each file
(`-' means standard input), or standard input if none are given.
Synopsis:
od [option]... [file]... od -C [file] [[+]offset [[+]label]]
Each line of output consists of the offset in the input, followed by
groups of data from the file. By default, od prints the offset in
octal, and each group of file data is two bytes of input printed as a
single octal number.
The program accepts the following options. Also see section Common options.
bytes are interpreted as for the `-j' option.
od writes one copy
of each output line using each of the data types that you specified,
in the order that you specified.
Adding a trailing "z" to any type specification appends a display
of the ASCII character representation of the printable characters
to the output line generated by the type specification.
a outputs things like `sp' for space, `nl' for
newline, and `nul' for a null (zero) byte. Type c outputs
` ', `\n', and \0, respectively.
Except for types `a' and `c', you can specify the number
of bytes to use in interpreting each number in the given data type
by following the type indicator character with a decimal integer.
Alternately, you can specify the size of one of the C compiler's
built-in data types by following the type indicator character with
one of the following characters. For integers (`d', `o',
`u', `x'):
f):
od outputs only
the first line, and puts just an asterisk on the following line to
indicate the elision.
n input bytes per output line. This must be a multiple of
the least common multiple of the sizes associated with the specified
output types. If n is omitted, the default is 32. If this option
is not given at all, the default is 16.
The next several options map the old, pre-POSIX format specification
options to the corresponding POSIX format specs. GNU od accepts
any combination of old- and new-style options. Format specification
options accumulate.
od
accepted. The following syntax:
od --traditional [file] [[+]offset[.][b] [[+]label[.][b]]]can be used to specify at most one file and optional arguments specifying an offset and a pseudo-start address, label. By default, offset is interpreted as an octal number specifying how many input bytes to skip before formatting and writing. The optional trailing decimal point forces the interpretation of offset as a decimal number. If no decimal is specified and the offset begins with `0x' or `0X' it is interpreted as a hexadecimal number. If there is a trailing `b', the number of bytes skipped will be offset multiplied by 512. The label argument is interpreted just like offset, but it specifies an initial pseudo-address. The pseudo-addresses are displayed in parentheses following any normal address.
These commands reformat the contents of files.
fmt: Reformat paragraph text
fmt fills and joins lines to produce output lines of (at most)
a given number of characters (75 by default). Synopsis:
fmt [option]... [file]...
fmt reads from the specified file arguments (or standard
input if none are given), and writes to standard output.
By default, blank lines, spaces between words, and indentation are preserved in the output; successive input lines with different indentation are not joined; tabs are expanded on input and introduced on output.
fmt prefers breaking lines at the end of a sentence, and tries to
avoid line breaks after the first word of a sentence or before the last
word of a sentence. A sentence break is defined as either the end
of a paragraph or a word ending in any of `.?!', followed by two
spaces or end of line, ignoring any intervening parentheses or quotes.
Like TeX, fmt reads entire "paragraphs" before choosing line
breaks; the algorithm is a variant of that in "Breaking Paragraphs Into
Lines" (Donald E. Knuth and Michael F. Plass, Software--Practice
and Experience, 11 (1981), 1119--1184).
The program accepts the following options. Also see section Common options.
fmt
initially tries to make lines about 7% shorter than this, to give it
room to balance line lengths.
pr: Paginate or columnate files for printing
pr writes each file (`-' means standard input), or
standard input if none are given, to standard output, paginating and
optionally outputting in multicolumn format; optionally merges all
files, printing all in parallel, one per column. Synopsis:
pr [option]... [file]...
By default, a 5-line header is printed at each page: two blank lines; a line with the date, the filename, and the page count; and two more blank lines. A footer of five blank lines is also printed. With the `-F' option, a 3-line header is printed: the leading two blank lines are omitted; no footer is used. The default page_length in both cases is 66 lines. The default number of text lines changes from 56 (without `-F') to 63 (with `-F'). The text line of the header takes up the full page_width in the form `yyyy-mm-dd HH:MM string Page nnnn'. String is a centered header string.
Form feeds in the input cause page breaks in the output. Multiple form feeds produce empty pages.
Columns are of equal width, separated by an optional string (default is `space'). For multicolumn output, lines will always be truncated to page_width (default 72), unless you use the `-J' option. For single column output no line truncation occurs by default. Use `-W' option to truncate lines in that case.
Including version 1.22i:
Some small letter options (`-s', `-w') has been redefined with the object of a better posix compliance. The output of some further cases has been adapted to other unixes. A violation of downward compatibility has to be accepted.
Some new capital letter options (`-J', `-S', `-W') has been introduced to turn off unexpected interferences of small letter options. The `-N' option and the second argument last_page of `+FIRST_PAGE' offer more flexibility. The detailed handling of form feeds set in the input files requires `-T' option.
Capital letter options dominate small letter ones.
Some of the option-arguments (compare `-s', `-S', `-e', `-i', `-n') cannot be specified as separate arguments from the preceding option letter (already stated in the posix specification).
The program accepts the following options. Also see section Common options.
pr uses the default output
separator, TAB.
Without `-S' or `-J', pr uses a `space'
(same as `-S" "').
Using `-S' with no string is equivalent to `-S""'.
Note that for some of pr's options the single-letter option
character must be followed immediately by any corresponding argument;
there may not be any intervening white space.
`-S/-s' is one of them. Don't use `-S "STRING"'.
POSIX requires this.
fold: Wrap input lines to fit in specified width
fold writes each file (`-' means standard input), or
standard input if none are given, to standard output, breaking long
lines. Synopsis:
fold [option]... [file]...
By default, fold breaks lines wider than 80 columns. The output
is split into as many lines as necessary.
fold counts screen columns by default; thus, a tab may count more
than one column, backspace decreases the column count, and carriage
return sets the column to zero.
The program accepts the following options. Also see section Common options.
These commands output pieces of the input.
head: Output the first part of files
head prints the first part (10 lines by default) of each
file; it reads from standard input if no files are given or
when given a file of `-'. Synopses:
head [option]... [file]... head -number [option]... [file]...
If more than one file is specified, head prints a
one-line header consisting of
==> file name <==
before the output for each file.
head accepts two option formats: the new one, in which numbers
are arguments to the options (`-q -n 1'), and the old one, in which
the number precedes any option letters (`-1q').
The program accepts the following options. Also see section Common options.
-c, or `l' to mean count by lines,
or other option letters (`cqv').
tail: Output the last part of files
tail prints the last part (10 lines by default) of each
file; it reads from standard input if no files are given or
when given a file of `-'. Synopses:
tail [option]... [file]... tail -number [option]... [file]... tail +number [option]... [file]...
If more than one file is specified, tail prints a
one-line header consisting of
==> file name <==
before the output for each file.
GNU tail can output any amount of data (some other versions of
tail cannot). It also has no `-r' option (print in
reverse), since reversing a file is really a different job from printing
the end of a file; BSD tail (which is the one with -r) can
only reverse files that are at most as large as its buffer, which is
typically 32k. A more reliable and versatile way to reverse files is
the GNU tac command.
tail accepts two option formats: the new one, in which numbers
are arguments to the options (`-n 1'), and the old one, in which
the number precedes any option letters (`-1' or `+1').
If any option-argument is a number n starting with a `+',
tail begins printing with the nth item from the start of
each file, instead of from the end.
The program accepts the following options. Also see section Common options.
-c, or `l' to mean count by lines,
or other option letters (`cfqv').
tail prints a header whenever it
gets output from a different file, to indicate which file that output is
from.
There are two ways to specify how you'd like to track files with this option,
but that difference is noticeable only when a followed file is removed or
renamed.
If you'd like to continue to track the end of a growing file even after
it has been unlinked, use `--follow=descriptor'. This is the default
behavior, but it is not useful if you're tracking a log file that may be
rotated (removed or renamed, then reopened). In that case, use
`--follow=name' to track the named file by reopening it periodically
to see if it has been removed and recreated by some other program.
No matter which method you use, if the tracked file is determined to have
shrunk, tail prints a message saying the file has been truncated
and resumes tracking the end of the file from the newly-determined endpoint.
When a file is removed, tail's behavior depends on whether it is
following the name or the descriptor. When following by name, tail can
detect that a file has been removed and gives a message to that effect,
and if `--retry' has been specified it will continue checking
periodically to see if the file reappears.
When following a descriptor, tail does not detect that the file has
been unlinked or renamed and issues no message; even though the file
may no longer be accessible via its original name, it may still be
growing.
The option values `descriptor' and `name' may be specified only
with the long form of the option, not with `-f'.
make and tail
like this then the tail process will stop when your build completes.
Without this option, you would have had to kill the tail -f
process yourself.
$ make >& makerr & tail --pid=$! -f makerrIf you specify a pid that is not in use or that does not correspond to the process that is writing to the tailed files, then
tail
may terminate long before any files stop growing or it may not
terminate until long after the real writer has terminated.
tail follows the descriptor of a file
that continues growing at a rapid pace even after it is deleted or renamed.
After detecting n consecutive size changes for a file,
open/fstat the file to determine if that file name is
still associated with the same device/inode-number pair as before.
See the output of tail --help for the default value.
open/fstat the file to determine if that file name is
still associated with the same device/inode-number pair as before.
When following a log file that is rotated this is approximately the
number of seconds between when tail prints the last pre-rotation lines
and when it prints the lines that have accumulated in the new log file.
See the output of tail --help for the default value.
This option is meaningful only when following by name.
split: Split a file into fixed-size pieces
split creates output files containing consecutive sections of
input (standard input if none is given or input is
`-'). Synopsis:
split [option] [input [prefix]]
By default, split puts 1000 lines of input (or whatever is
left over for the last section), into each output file.
The output files' names consist of prefix (`x' by default)
followed by a group of letters `aa', `ab', and so on, such
that concatenating the output files in sorted order by file name produces
the original input file. (If more than 676 output files are required,
split uses `zaa', `zab', etc.)
The program accepts the following options. Also see section Common options.
csplit: Split a file into context-determined pieces
csplit creates zero or more output files containing sections of
input (standard input if input is `-'). Synopsis:
csplit [option]... input pattern...
The contents of the output files are determined by the pattern arguments, as detailed below. An error occurs if a pattern argument refers to a nonexistent line of the input file (e.g., if no remaining line matches a given regular expression). After every pattern has been matched, any remaining input is copied into one last output file.
By default, csplit prints the number of bytes written to each
output file after it has been created.
The types of pattern arguments are:
The output files' names consist of a prefix (`xx' by default) followed by a suffix. By default, the suffix is an ascending sequence of two-digit decimal numbers from `00' and up to `99'. In any case, concatenating the output files in sorted order by filename produces the original input file.
By default, if csplit encounters an error or receives a hangup,
interrupt, quit, or terminate signal, it removes any output files
that it has created so far before it exits.
The program accepts the following options. Also see section Common options.
printf(3)-style conversion specification, possibly including
format specification flags, a field width, a precision specifications,
or all of these kinds of modifiers. The format letter must convert a
binary integer argument to readable form; thus, only `d', `i',
`u', `o', `x', and `X' conversions are allowed. The
entire suffix is given (with the current output file number) to
sprintf(3) to form the file name suffixes for each of the
individual output files in turn. If this option is used, the
`--digits' option is ignored.
These commands generate just a few numbers representing entire contents of files.
wc: Print byte, word, and line counts
wc counts the number of bytes, whitespace-separated words, and
newlines in each given file, or standard input if none are given
or for a file of `-'. Synopsis:
wc [option]... [file]...
wc prints one line of counts for each file, and if the file was
given as an argument, it prints the file name following the counts. If
more than one file is given, wc prints a final line
containing the cumulative counts, with the file name `total'. The
counts are printed in this order: newlines, words, bytes.
By default, each count is output right-justified in a 7-byte field with
one space between fields so that the numbers and file names line up nicely
in columns. However, POSIX requires that there be exactly one space
separating columns. You can make wc use the POSIX-mandated
output format by setting the @env{POSIXLY_CORRECT} environment variable.
By default, wc prints all three counts. Options can specify
that only certain counts be printed. Options do not undo others
previously given, so
wc --bytes --words
prints both the byte counts and the word counts.
With the --max-line-length option, wc prints the length
of the longest line per file, and if there is more than one file it
prints the maximum (not the sum) of those lengths.
The program accepts the following options. Also see section Common options.
sum: Print checksum and block counts
sum computes a 16-bit checksum for each given file, or
standard input if none are given or for a file of `-'. Synopsis:
sum [option]... [file]...
sum prints the checksum for each file followed by the
number of blocks in the file (rounded up). If more than one file
is given, file names are also printed (by default). (With the
`--sysv' option, corresponding file name are printed when there is
at least one file argument.)
By default, GNU sum computes checksums using an algorithm
compatible with BSD sum and prints file sizes in units of
1024-byte blocks.
The program accepts the following options. Also see section Common options.
sum. Unless `-s' was also
given, it has no effect.
sum's default, and print file sizes in units of 512-byte blocks.
sum is provided for compatibility; the cksum program (see
next section) is preferable in new applications.
cksum: Print CRC checksum and byte counts
cksum computes a cyclic redundancy check (CRC) checksum for each
given file, or standard input if none are given or for a
file of `-'. Synopsis:
cksum [option]... [file]...
cksum prints the CRC checksum for each file along with the number
of bytes in the file, and the filename unless no arguments were given.
cksum is typically used to ensure that files
transferred by unreliable means (e.g., netnews) have not been corrupted,
by comparing the cksum output for the received files with the
cksum output for the original files (typically given in the
distribution).
The CRC algorithm is specified by the POSIX.2 standard. It is not
compatible with the BSD or System V sum algorithms (see the
previous section); it is more robust.
The only options are `--help' and `--version'. See section Common options.
md5sum: Print or check message-digests
md5sum computes a 128-bit checksum (or fingerprint or
message-digest) for each specified file.
If a file is specified as `-' or if no files are given
md5sum computes the checksum for the standard input.
md5sum can also determine whether a file and checksum are
consistent. Synopses:
md5sum [option]... [file]... md5sum [option]... --check [file]
For each file, `md5sum' outputs the MD5 checksum, a flag indicating a binary or text input file, and the filename. If file is omitted or specified as `-', standard input is read.
The program accepts the following options. Also see section Common options.
md5sum is usually the output of
a prior, checksum-generating run of `md5sum'.
Each valid line of input consists of an MD5 checksum, a binary/text
flag, and then a filename.
Binary files are marked with `*', text with ` '.
For each such line, md5sum reads the named file and computes its
MD5 checksum. Then, if the computed message digest does not match the
one on the line with the filename, the file is noted as having
failed the test. Otherwise, the file passes the test.
By default, for each valid line, one line is written to standard
output indicating whether the named file passed the test.
After all checks have been performed, if there were any failures,
a warning is issued to standard error.
Use the `--status' option to inhibit that output.
If any listed file cannot be opened or read, if any valid line has
an MD5 checksum inconsistent with the associated file, or if no valid
line is found, md5sum exits with nonzero status. Otherwise,
it exits successfully.
These commands work with (or produce) sorted files.
sort: Sort text files
sort sorts, merges, or compares all the lines from the given
files, or standard input if none are given or for a file of
`-'. By default, sort writes the results to standard
output. Synopsis:
sort [option]... [file]...
sort has three modes of operation: sort (the default), merge,
and check for sortedness. The following options change the operation
mode:
A pair of lines is compared as follows: if any key fields have been
specified, sort compares each pair of fields, in the order
specified on the command line, according to the associated ordering
options, until a difference is found or no fields are left.
Unless otherwise specified, all comparisons use the character
collating sequence specified by the @env{LC_COLLATE} locale.
If any of the global options `Mbdfinr' are given but no key fields
are specified, sort compares the entire lines according to the
global options.
Finally, as a last resort when all keys compare equal (or if no
ordering options were specified at all), sort compares the entire
lines. The last resort comparison
honors the `-r' global option. The `-s' (stable) option
disables this last-resort comparison so that lines in which all fields
compare equal are left in their original relative order. If no fields
or global options are specified, `-s' has no effect.
GNU sort (as specified for all GNU utilities) has no limits on
input line length or restrictions on bytes allowed within lines. In
addition, if the final byte of an input file is not a newline, GNU
sort silently supplies one. A line's trailing newline is part of
the line for comparison purposes; for example, with no options in an
ASCII locale, a line starting with a tab sorts before an empty line
because tab precedes newline in the ASCII collating sequence.
Upon any error, sort exits with a status of `2'.
If the environment variable @env{TMPDIR} is set, sort uses its
value as the directory for temporary files instead of `/tmp'. The
`-T tempdir' option in turn overrides the environment
variable.
The following options affect the ordering of output lines. They may be specified globally or as part of a specific key field. If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own. The `-b', `-d', `-f' and `-i' options classify characters according to the @env{LC_CTYPE} locale.
strtod to convert
a prefix of each line to a double-precision floating point number.
This allows floating point numbers to be specified in scientific notation,
like 1.0e-34 and 10e100.
Do not report overflow, underflow, or conversion errors.
Use the following collating sequence:
sort -n uses what might be considered an unconventional method
to compare strings representing floating point numbers. Rather than
first converting each string to the C double type and then
comparing those values, sort aligns the radix characters in the two
strings and compares the strings a character at a time. One benefit
of using this approach is its speed. In practice this is much more
efficient than performing the two corresponding string-to-double (or even
string-to-integer) conversions and then comparing doubles. In addition,
there is no corresponding loss of precision. Converting each string to
double before comparison would limit precision to about 16 digits
on most systems.
Neither a leading `+' nor exponential notation is recognized.
To compare such strings numerically, use the `-g' option.
Other options are:
sort copies
it to a temporary file before sorting and writing the output to
output-file.
sort breaks it
into fields ` foo' and ` bar'. The field separator is
not considered to be part of either the field preceding or the field
following.
In addition, when GNU sort is invoked with exactly one argument,
options `--help' and `--version' are recognized. See section Common options.
Historical (BSD and System V) implementations of sort have
differed in their interpretation of some options, particularly
`-b', `-f', and `-n'. GNU sort follows the POSIX
behavior, which is usually (but not always!) like the System V behavior.
According to POSIX, `-n' no longer implies `-b'. For
consistency, `-M' has been changed in the same way. This may
affect the meaning of character positions in field specifications in
obscure cases. The only fix is to add an explicit `-b'.
A position in a sort field specified with the `-k' or `+' option has the form `f.c', where f is the number of the field to use and c is the number of the first character from the beginning of the field (for `+pos') or from the end of the previous field (for `-pos'). If the `.c' is omitted, it is taken to be the first character in the field. If the `-b' option was specified, the `.c' part of a field specification is counted from the first nonblank character of the field (for `+pos') or from the first nonblank character following the previous field (for `-pos').
A sort key option may also have any of the option letters `Mbdfinr' appended to it, in which case the global ordering options are not used for that particular field. The `-b' option may be independently attached to either or both of the `+pos' and `-pos' parts of a field specification, and if it is inherited from the global options it will be attached to both. Keys may span multiple fields.
Here are some examples to illustrate various combinations of options. In them, the POSIX `-k' option is used to specify sort keys rather than the obsolete `+pos1-pos2' syntax.
sort -nrSort alphabetically, omitting the first and second fields. This uses a single key composed of the characters beginning at the start of field three and extending to the end of each line.
sort -k3
sort -t : -k 2,2n -k 5.3,5.4Note that if you had written `-k 2' instead of `-k 2,2' `sort' would have used all characters beginning in the second field and extending to the end of the line as the primary numeric key. For the large majority of applications, treating keys spanning more than one field as numeric will not do what you expect. Also note that the `n' modifier was applied to the field-end specifier for the first key. It would have been equivalent to specify `-k 2n,2' or `-k 2n,2n'. All modifiers except `b' apply to the associated field, regardless of whether the modifier character is attached to the field-start and/or the field-end part of the key specifier.
sort -t : -k 5b,5 -k 3,3n /etc/passwdAn alternative is to use the global numeric modifier `-n'.
sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
find src -type f -print0 | sort -t / -z -f | xargs -0 etags --appendThe use of `-print0', `-z', and `-0' in this case mean that pathnames that contain Line Feed characters will not get broken up by the sort operation. Finally, to ignore both leading and trailing white space, you could have applied the `b' modifier to the field-end specifier for the first key,
sort -t : -n -k 5b,5b -k 3,3 /etc/passwdor by using the global `-b' modifier instead of `-n' and an explicit `n' with the second key specifier.
sort -t : -b -k 5,5 -k 3,3n /etc/passwd
uniq: Uniquify files
uniq writes the unique lines in the given `input', or
standard input if nothing is given or for an input name of
`-'. Synopsis:
uniq [option]... [input [output]]
By default, uniq prints the unique lines in a sorted file, i.e.,
discards all but one of identical successive lines. Optionally, it can
instead show only lines that appear exactly once, or lines that appear
more than once.
The input must be sorted. If your input is not sorted, perhaps you want
to use sort -u.
If no output file is specified, uniq writes to standard
output.
The program accepts the following options. Also see section Common options.
comm: Compare two sorted files line by line
comm writes to standard output lines that are common, and lines
that are unique, to two input files; a file name of `-' means
standard input. Synopsis:
comm [option]... file1 file2
Before comm can be used, the input files must be sorted using the
collating sequence specified by the @env{LC_COLLATE} locale, with
trailing newlines significant. If an input file ends in a non-newline
character, a newline is silently appended. The sort command with
no options always outputs a file that is suitable input to comm.
With no options, comm produces three column output. Column one
contains lines unique to file1, column two contains lines unique
to file2, and column three contains lines common to both files.
Columns are separated by a single TAB character.
The options `-1', `-2', and `-3' suppress printing of the corresponding columns. Also see section Common options.
Unlike some other comparison utilities, comm has an exit
status that does not depend on the result of the comparison.
Upon normal completion comm produces an exit code of zero.
If there is an error it exits with nonzero status.
tsort: Topological sort
tsort performs a topological sort on the given file, or
standard input if no input file is given or for a file of
`-'. Synopsis:
tsort [option] [file]
tsort reads its input as pairs of strings, separated by blanks,
indicating a partial ordering. The output is a total ordering that
corresponds to the given partial ordering.
For example
tsort <<EOF a b c d e f b c d e EOF
will produce the output
a b c d e f
tsort will detect cycles in the input and writes the first cycle
encountered to standard error.
Note that for a given partial ordering, generally there is no unique total ordering.
The only options are `--help' and `--version'. See section Common options.
ptx: Produce permuted indexes
ptx reads a text file and essentially produces a permuted index, with
each keyword in its context. The calling sketch is either one of:
ptx [option ...] [file ...] ptx -G [option ...] [input [output]]
The `-G' (or its equivalent: `--traditional') option disables
all GNU extensions and revert to traditional mode, thus introducing some
limitations, and changes several of the program's default option values.
When `-G' is not specified, GNU extensions are always enabled. GNU
extensions to ptx are documented wherever appropriate in this
document. For the full list, see See section The GNU extensions to ptx.
Individual options are explained in incoming sections.
When GNU extensions are enabled, there may be zero, one or several file after the options. If there is no file, the program reads the standard input. If there is one or several file, they give the name of input files which are all read in turn, as if all the input files were concatenated. However, there is a full contextual break between each file and, when automatic referencing is requested, file names and line numbers refer to individual text input files. In all cases, the program produces the permuted index onto the standard output.
When GNU extensions are not enabled, that is, when the program
operates in traditional mode, there may be zero, one or two parameters
besides the options. If there is no parameters, the program reads the
standard input and produces the permuted index onto the standard output.
If there is only one parameter, it names the text input to be read
instead of the standard input. If two parameters are given, they give
respectively the name of the input file to read and the name of
the output file to produce. Be very careful to note that,
in this case, the contents of file given by the second parameter is
destroyed. This behaviour is dictated only by System V ptx
compatibility, because GNU Standards discourage output parameters not
introduced by an option.
Note that for any file named as the value of an option or as an input text file, a single dash - may be used, in which case standard input is assumed. However, it would not make sense to use this convention more than once per program invocation.
ptx and switch to traditional mode.
As it is setup now, the program assumes that the input file is coded
using 8-bit ISO 8859-1 code, also known as Latin-1 character set,
unless if it is compiled for MS-DOS, in which case it uses the
character set of the IBM-PC. (GNU ptx is not known to work on
smaller MS-DOS machines anymore.) Compared to 7-bit ASCII, the set of
characters which are letters is then different, this fact alters the
behaviour of regular expression matching. Thus, the default regular
expression for a keyword allows foreign or diacriticized letters.
Keyword sorting, however, is still crude; it obeys the underlying
character set ordering quite blindly.
ptx when this option is
not specified, usually found in `/usr/local/lib/eign' if this has
not been changed at installation time. If you want to deactivate the
default Ignore file, specify /dev/null instead.
[.?!][]\"')}]*\\($\\|\t\\| \\)[ \t\n]*Whenever GNU extensions are disabled or if `-r' option is used, end of lines are used; in this case, the default regexp is just:
\nUsing an empty regexp is equivalent to completely disabling end of line or end of sentence recognition. In this case, the whole file is considered to be a single big line or sentence. The user might want to disallow all truncation flag generation as well, through option `-F ""'. See section `Syntax of Regular Expressions' in The GNU Emacs Manual. When the keywords happen to be near the beginning of the input line or sentence, this often creates an unused area at the beginning of the output context line; when the keywords happen to be near the end of the input line or sentence, this often creates an unused area at the end of the output context line. The program tries to fill those unused areas by wrapping around context in them; the tail of the input line or sentence is used to fill the unused area on the left of the output line; the head of the input line or sentence is used to fill the unused area on the right of the output line. As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by
ptx itself.
ptx itself.
Output format is mainly controlled by `-O' and `-T' options,
described in the table below. When neither `-O' nor `-T' is
selected, and if GNU extensions are enabled, the program choose an
output format suited for a dumb terminal. Each keyword occurrence is
output to the center of one line, surrounded by its left and right
contexts. Each field is properly justified, so the concordance output
could readily be observed. As a special feature, if automatic
references are selected by option `-A' and are output before the
left context, that is, if option `-R' is not selected, then
a colon is added after the reference; this nicely interfaces with GNU
Emacs next-error processing. In this default output format, each
white space character, like newline and tab, is merely changed to
exactly one space, with no special attempt to compress consecutive
spaces. This might change in the future. Except for those white space
characters, every other character of the underlying set of 256
characters is transmitted verbatim.
Output format is further controlled by the following options.
ptx itself.
nroff, troff or TeX.
nroff or troff
processing. Each output line will look like:
.xx "tail" "before" "keyword_and_after" "head" "ref"so it will be possible to write an `.xx' roff macro to take care of the output typesetting. This is the default output format when GNU extensions are disabled. Option `-M' might be used to change `xx' to another macro name. In this output format, each non-graphical character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces. Each quote character: " is doubled so it will be correctly processed by
nroff or troff.
\xx {tail}{before}{keyword}{after}{head}{ref}
so it will be possible to write a \xx definition to take care of
the output typesetting. Note that when references are not being
produced, that is, neither option `-A' nor option `-r' is
selected, the last parameter of each \xx call is inhibited.
Option `-M' might be used to change `xx' to another macro
name.
In this output format, some special characters, like $, %,
&, # and _ are automatically protected with a
backslash. Curly brackets {, } are also protected with a
backslash, but also enclosed in a pair of dollar signs to force
mathematical mode. The backslash itself produces the sequence
\backslash{}. Circumflex and tilde diacritics produce the
sequence ^\{ } and ~\{ } respectively. Other
diacriticized characters of the underlying character set produce an
appropriate TeX sequence as far as possible. The other non-graphical
characters, like newline and tab, and all others characters which are
not part of ASCII, are merely changed to exactly one space, with no
special attempt to compress consecutive spaces. Let me know how to
improve this special character processing for TeX.
ptx
This version of ptx contains a few features which do not exist in
System V ptx. These extra features are suppressed by using the
`-G' command line option, unless overridden by other command line
options. Some GNU extensions cannot be recovered by overriding, so the
simple rule is to avoid `-G' if you care about GNU extensions.
Here are the differences between this program and System V ptx.
ptx reads only one file and produce the result on standard output
or, if a second file parameter is given on the command, to that
file.
Having output parameters not introduced by options is a quite dangerous
practice which GNU avoids as far as possible. So, for using ptx
portably between GNU and System V, you should pay attention to always
use it with a single input file, and always expect the result on
standard output. You might also want to automatically configure in a
`-G' option to ptx calls in products using ptx, if
the configurator finds that the installed ptx accepts `-G'.
ptx are options `-b',
`-f', `-g', `-i', `-o', `-r', `-t' and
`-w'. All other options are GNU extensions and are not repeated in
this enumeration. Moreover, some options have a slightly different
meaning when GNU extensions are enabled, as explained below.
troff or
nroff. It is rather formatted for a dumb terminal. troff
or nroff output may still be selected through option `-O'.
ptx does not accept 8-bit characters, a few
control characters are rejected, and the tilde ~ is condemned.
ptx processes only
the first 200 characters in each line.
ptx,
but still, there are some slight disposition glitches this program does
not completely reproduce.
ptx.
cut: Print selected parts of lines
cut writes to standard output selected parts of each line of each
input file, or standard input if no files are given or for a file name of
`-'. Synopsis:
cut [option]... [file]...
In the table which follows, the byte-list, character-list, and field-list are one or more numbers or ranges (two numbers separated by a dash) separated by commas. Bytes, characters, and fields are numbered from starting at 1. Incomplete ranges may be given: `-m' means `1-m'; `n-' means `n' through end of line or last field.
The program accepts the following options. Also see section Common options.
paste: Merge lines of files
paste writes to standard output lines consisting of sequentially
corresponding lines of each given file, separated by a TAB character.
Standard input is used for a file name of `-' or if no input files
are given.
Synopsis:
paste [option]... [file]...
The program accepts the following options. Also see section Common options.
join: Join lines on a common field
join writes to standard output a line for each pair of input
lines that have identical join fields. Synopsis:
join [option]... file1 file2
Either file1 or file2 (but not both) can be `-',
meaning standard input. file1 and file2 should be already
sorted in increasing textual order on the join fields, using the
collating sequence specified by the @env{LC_COLLATE} locale. Unless
the `-t' option is given, the input should be sorted ignoring blanks at
the start of the join field, as in sort -b. If the
`--ignore-case' option is given, lines should be sorted without
regard to the case of characters in the join field, as in sort -f.
The defaults are: the join field is the first field in each line; fields in the input are separated by one or more blanks, with leading blanks on the line ignored; fields in the output are separated by a space; each output line consists of the join field, the remaining fields from file1, then the remaining fields from file2.
The program accepts the following options. Also see section Common options.
join that functionality, POSIX invented the `0'
field specification notation.
The elements in field-list
are separated by commas or blanks. Multiple field-list
arguments can be given after a single `-o' option; the values
of all lists given with `-o' are concatenated together.
All output lines -- including those printed because of any -a or -v
option -- are subject to the specified field-list.
In addition, when GNU join is invoked with exactly one argument,
options `--help' and `--version' are recognized. See section Common options.
This commands operate on individual characters.
tr: Translate, squeeze, and/or delete charactersSynopsis:
tr [option]... set1 [set2]
tr copies standard input to standard output, performing
one of the following operations:
The set1 and (if given) set2 arguments define ordered
sets of characters, referred to below as set1 and set2. These
sets are the characters of the input that tr operates on.
The `--complement' (`-c') option replaces set1 with its
complement (all of the characters that are not in set1).
The format of the set1 and set2 arguments resembles the format of regular expressions; however, they are not regular expressions, only lists of characters. Most characters simply represent themselves in these strings, but the strings can contain the shorthands listed below, for convenience. Some of them can be used only in set1 or set2, as noted below.
tr
does not support the System V syntax that uses square brackets to
enclose ranges, translations specified in that format will still work as
long as the brackets in string1 correspond to identical brackets
in string2.
upper and lower classes,
which expand in ascending order. When the `--delete' (`-d')
and `--squeeze-repeats' (`-s') options are both given, any
character class can be used in set2. Otherwise, only the
character classes lower and upper are accepted in
set2, and then only if the corresponding character class
(upper and lower, respectively) is specified in the same
relative position in set1. Doing this specifies case conversion.
The class names are given below; an error results when an invalid class
name is given.
alnum
alpha
blank
cntrl
digit
graph
lower
print
punct
space
upper
xdigit
tr;
each character's equivalence class consists only of that character,
which is of no particular use.
tr performs translation when set1 and set2 are
both given and the `--delete' (`-d') option is not given.
tr translates each character of its input that is in set1
to the corresponding character in set2. Characters not in
set1 are passed through unchanged. When a character appears more
than once in set1 and the corresponding characters in set2
are not all the same, only the final one is used. For example, these
two commands are equivalent:
tr aaa xyz tr a z
A common use of tr is to convert lowercase characters to
uppercase. This can be done in many ways. Here are three of them:
tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ tr a-z A-Z tr '[:lower:]' '[:upper:]'
When tr is performing translation, set1 and set2
typically have the same length. If set1 is shorter than
set2, the extra characters at the end of set2 are ignored.
On the other hand, making set1 longer than set2 is not
portable; POSIX.2 says that the result is undefined. In this situation,
BSD tr pads set2 to the length of set1 by repeating
the last character of set2 as many times as necessary. System V
tr truncates set1 to the length of set2.
By default, GNU tr handles this case like BSD tr. When
the `--truncate-set1' (`-t') option is given, GNU tr
handles this case like the System V tr instead. This option is
ignored for operations other than translation.
Acting like System V tr in this case breaks the relatively common
BSD idiom:
tr -cs A-Za-z0-9 '\012'
because it converts only zero bytes (the first element in the complement of set1), rather than all non-alphanumerics, to newlines.
When given just the `--delete' (`-d') option, tr
removes any input characters that are in set1.
When given just the `--squeeze-repeats' (`-s') option,
tr replaces each input sequence of a repeated character that
is in set1 with a single occurrence of that character.
When given both `--delete' and `--squeeze-repeats', tr
first performs any deletions using set1, then squeezes repeats
from any remaining characters using set2.
The `--squeeze-repeats' option may also be used when translating,
in which case tr first performs translation, then squeezes
repeats from any remaining characters using set2.
Here are some examples to illustrate various combinations of options:
tr -d '\000'
tr -cs 'a-zA-Z0-9' '[\n*]'
tr -s '\n'
uniq with the `-d' option to print out only the words
that were adjacent duplicates.
#!/bin/sh cat "$@" \ | tr -s '[:punct:][:blank:]' '\n' \ | tr '[:upper:]' '[:lower:]' \ | uniq -d
Setting the environment variable @env{POSIXLY_CORRECT} turns off the following warning and error messages, for strict compliance with POSIX.2. Otherwise, the following diagnostics are issued:
tr by default prints
a usage message and exits, because set2 would not be used.
The POSIX specification says that set2 must be ignored in
this case. Silently ignoring arguments is a bad idea.
GNU tr does not provide complete BSD or System V compatibility.
For example, it is impossible to disable interpretation of the POSIX
constructs `[:alpha:]', `[=c=]', and `[c*10]'. Also, GNU
tr does not delete zero bytes automatically, unlike traditional
Unix versions, which provide no way to preserve zero bytes.
expand: Convert tabs to spaces
expand writes the contents of each given file, or standard
input if none are given or for a file of `-', to standard
output, with tab characters converted to the appropriate number of
spaces. Synopsis:
expand [option]... [file]...
By default, expand converts all tabs to spaces. It preserves
backspace characters in the output; they decrement the column count for
tab calculations. The default action is equivalent to `-8' (set
tabs every 8 columns).
The program accepts the following options. Also see section Common options.
unexpand: Convert spaces to tabs
unexpand writes the contents of each given file, or
standard input if none are given or for a file of `-', to
standard output, with strings of two or more space or tab characters
converted to as many tabs as possible followed by as many spaces as are
needed. Synopsis:
unexpand [option]... [file]...
By default, unexpand converts only initial spaces and tabs (those
that precede all non space or tab characters) on each line. It
preserves backspace characters in the output; they decrement the column
count for tab calculations. By default, tabs are set at every 8th
column.
The program accepts the following options. Also see section Common options.
This chapter originally appeared in Linux Journal, volume 1, number 2, in the What's GNU? column. It was written by Arnold Robbins.
This month's column is only peripherally related to the GNU Project, in that it describes a number of the GNU tools on your Linux system and how they might be used. What it's really about is the "Software Tools" philosophy of program development and usage.
The software tools philosophy was an important and integral concept in the initial design and development of Unix (of which Linux and GNU are essentially clones). Unfortunately, in the modern day press of Internetworking and flashy GUIs, it seems to have fallen by the wayside. This is a shame, since it provides a powerful mental model for solving many kinds of problems.
Many people carry a Swiss Army knife around in their pants pockets (or purse). A Swiss Army knife is a handy tool to have: it has several knife blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps a number of other things on it. For the everyday, small miscellaneous jobs where you need a simple, general purpose tool, it's just the thing.
On the other hand, an experienced carpenter doesn't build a house using a Swiss Army knife. Instead, he has a toolbox chock full of specialized tools--a saw, a hammer, a screwdriver, a plane, and so on. And he knows exactly when and where to use each tool; you won't catch him hammering nails with the handle of his screwdriver.
The Unix developers at Bell Labs were all professional programmers and trained computer scientists. They had found that while a one-size-fits-all program might appeal to a user because there's only one program to use, in practice such programs are
Instead, they felt that programs should be specialized tools. In short, each program "should do one thing well." No more and no less. Such programs are simpler to design, write, and get right--they only do one thing.
Furthermore, they found that with the right machinery for hooking programs together, that the whole was greater than the sum of the parts. By combining several special purpose programs, you could accomplish a specific task that none of the programs was designed for, and accomplish it much more quickly and easily than if you had to write a special purpose program. We will see some (classic) examples of this further on in the column. (An important additional point was that, if necessary, take a detour and build any software tools you may need first, if you don't already have something appropriate in the toolbox.)
Hopefully, you are familiar with the basics of I/O redirection in the shell, in particular the concepts of "standard input," "standard output," and "standard error". Briefly, "standard input" is a data source, where data comes from. A program should not need to either know or care if the data source is a disk file, a keyboard, a magnetic tape, or even a punched card reader. Similarly, "standard output" is a data sink, where data goes to. The program should neither know nor care where this might be. Programs that only read their standard input, do something to the data, and then send it on, are called "filters", by analogy to filters in a water pipeline.
With the Unix shell, it's very easy to set up data pipelines:
program_to_create_data | filter1 | .... | filterN > final.pretty.data
We start out by creating the raw data; each filter applies some successive transformation to the data, until by the time it comes out of the pipeline, it is in the desired form.
This is fine and good for standard input and standard output. Where does the
standard error come in to play? Well, think about filter1 in
the pipeline above. What happens if it encounters an error in the data it
sees? If it writes an error message to standard output, it will just
disappear down the pipeline into filter2's input, and the
user will probably never see it. So programs need a place where they can send
error messages so that the user will notice them. This is standard error,
and it is usually connected to your console or window, even if you have
redirected standard output of your program away from your screen.
For filter programs to work together, the format of the data has to be
agreed upon. The most straightforward and easiest format to use is simply
lines of text. Unix data files are generally just streams of bytes, with
lines delimited by the ASCII LF (Line Feed) character,
conventionally called a "newline" in the Unix literature. (This is
'\n' if you're a C programmer.) This is the format used by all
the traditional filtering programs. (Many earlier operating systems
had elaborate facilities and special purpose programs for managing
binary data. Unix has always shied away from such things, under the
philosophy that it's easiest to simply be able to view and edit your
data with a text editor.)
OK, enough introduction. Let's take a look at some of the tools, and then we'll see how to hook them together in interesting ways. In the following discussion, we will only present those command line options that interest us. As you should always do, double check your system documentation for the full story.
who command
The first program is the who command. By itself, it generates a
list of the users who are currently logged in. Although I'm writing
this on a single-user system, we'll pretend that several people are
logged in:
$ who arnold console Jan 22 19:57 miriam ttyp0 Jan 23 14:19(:0.0) bill ttyp1 Jan 21 09:32(:0.0) arnold ttyp2 Jan 23 20:48(:0.0)
Here, the `$' is the usual shell prompt, at which I typed who.
There are three people logged in, and I am logged in twice. On traditional
Unix systems, user names are never more than eight characters long. This
little bit of trivia will be useful later. The output of who is nice,
but the data is not all that exciting.
cut command
The next program we'll look at is the cut command. This program
cuts out columns or fields of input data. For example, we can tell it
to print just the login name and full name from the `/etc/passwd
file'. The `/etc/passwd' file has seven fields, separated by
colons:
arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/ksh
To get the first and fifth fields, we would use cut like this:
$ cut -d: -f1,5 /etc/passwd root:Operator ... arnold:Arnold D. Robbins miriam:Miriam A. Robbins ...
With the `-c' option, cut will cut out specific characters
(i.e., columns) in the input lines. This command looks like it might be
useful for data filtering.
sort command
Next we'll look at the sort command. This is one of the most
powerful commands on a Unix-style system; one that you will often find
yourself using when setting up fancy data plumbing. The sort
command reads and sorts each file named on the command line. It then
merges the sorted data and writes it to standard output. It will read
standard input if no files are given on the command line (thus
making it into a filter). The sort is based on the character collating
sequence or based on user-supplied ordering criteria.
uniq command
Finally (at least for now), we'll look at the uniq program. When
sorting data, you will often end up with duplicate lines, lines that
are identical. Usually, all you need is one instance of each line.
This is where uniq comes in. The uniq program reads its
standard input, which it expects to be sorted. It only prints out one
copy of each duplicated line. It does have several options. Later on,
we'll use the `-c' option, which prints each unique line, preceded
by a count of the number of times that line occurred in the input.
Now, let's suppose this is a large BBS system with dozens of users logged in. The management wants the SysOp to write a program that will generate a sorted list of logged in users. Furthermore, even if a user is logged in multiple times, his or her name should only show up in the output once.
The SysOp could sit down with the system documentation and write a C program that did this. It would take perhaps a couple of hundred lines of code and about two hours to write it, test it, and debug it. However, knowing the software toolbox, the SysOp can instead start out by generating just a list of logged on users:
$ who | cut -c1-8 arnold miriam bill arnold
Next, sort the list:
$ who | cut -c1-8 | sort arnold arnold bill miriam
Finally, run the sorted list through uniq, to weed out duplicates:
$ who | cut -c1-8 | sort | uniq arnold bill miriam
The sort command actually has a `-u' option that does what
uniq does. However, uniq has other uses for which one
cannot substitute `sort -u'.
The SysOp puts this pipeline into a shell script, and makes it available for all the users on the system:
# cat > /usr/local/bin/listusers who | cut -c1-8 | sort | uniq ^D # chmod +x /usr/local/bin/listusers
There are four major points to note here. First, with just four programs, on one command line, the SysOp was able to save about two hours worth of work. Furthermore, the shell pipeline is just about as efficient as the C program would be, and it is much more efficient in terms of programmer time. People time is much more expensive than computer time, and in our modern "there's never enough time to do everything" society, saving two hours of programmer time is no mean feat.
Second, it is also important to emphasize that with the combination of the tools, it is possible to do a special purpose job never imagined by the authors of the individual programs.
Third, it is also valuable to build up your pipeline in stages, as we did here. This allows you to view the data at each stage in the pipeline, which helps you acquire the confidence that you are indeed using these tools correctly.
Finally, by bundling the pipeline in a shell script, other users can use your command, without having to remember the fancy plumbing you set up for them. In terms of how you run them, shell scripts and compiled programs are indistinguishable.
After the previous warm-up exercise, we'll look at two additional, more complicated pipelines. For them, we need to introduce two more tools.
The first is the tr command, which stands for "transliterate."
The tr command works on a character-by-character basis, changing
characters. Normally it is used for things like mapping upper case to
lower case:
$ echo ThIs ExAmPlE HaS MIXED case! | tr '[A-Z]' '[a-z]' this example has mixed case!
There are several options of interest:
We will be using all three options in a moment.
The other command we'll look at is comm. The comm
command takes two sorted input files as input data, and prints out the
files' lines in three columns. The output columns are the data lines
unique to the first file, the data lines unique to the second file, and
the data lines that are common to both. The `-1', `-2', and
`-3' command line options omit the respective columns. (This is
non-intuitive and takes a little getting used to.) For example:
$ cat f1
11111
22222
33333
44444
$ cat f2
00000
22222
33333
55555
$ comm f1 f2
00000
11111
22222
33333
44444
55555
The single dash as a filename tells comm to read standard input
instead of a regular file.
Now we're ready to build a fancy pipeline. The first application is a word frequency counter. This helps an author determine if he or she is over-using certain words.
The first step is to change the case of all the letters in our input file to one case. "The" and "the" are the same word when doing counting.
$ tr '[A-Z]' '[a-z]' < whats.gnu | ...
The next step is to get rid of punctuation. Quoted words and unquoted words should be treated identically; it's easiest to just get the punctuation out of the way.
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | ...
The second tr command operates on the complement of the listed
characters, which are all the letters, the digits, the underscore, and
the blank. The `\012' represents the newline character; it has to
be left alone. (The ASCII tab character should also be included for
good measure in a production script.)
At this point, we have data consisting of words separated by blank space. The words only contain alphanumeric characters (and the underscore). The next step is break the data apart so that we have one word per line. This makes the counting operation much easier, as we will see shortly.
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | > tr -s '[ ]' '\012' | ...
This command turns blanks into newlines. The `-s' option squeezes multiple newline characters in the output into just one. This helps us avoid blank lines. (The `>' is the shell's "secondary prompt." This is what the shell prints when it notices you haven't finished typing in all of a command.)
We now have data consisting of one word per line, no punctuation, all one case. We're ready to count each word:
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | > tr -s '[ ]' '\012' | sort | uniq -c | ...
At this point, the data might look something like this:
60 a 2 able 6 about 1 above 2 accomplish 1 acquire 1 actually 2 additional
The output is sorted by word, not by count! What we want is the most
frequently used words first. Fortunately, this is easy to accomplish,
with the help of two more sort options:
The final pipeline looks like this:
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | > tr -s '[ ]' '\012' | sort | uniq -c | sort -nr 156 the 60 a 58 to 51 of 51 and ...
Whew! That's a lot to digest. Yet, the same principles apply. With six commands, on two lines (really one long one split for convenience), we've created a program that does something interesting and useful, in much less time than we could have written a C program to do the same thing.
A minor modification to the above pipeline can give us a simple spelling checker! To determine if you've spelled a word correctly, all you have to do is look it up in a dictionary. If it is not there, then chances are that your spelling is incorrect. So, we need a dictionary. If you have the Slackware Linux distribution, you have the file `/usr/lib/ispell/ispell.words', which is a sorted, 38,400 word dictionary.
Now, how to compare our file with the dictionary? As before, we generate a sorted list of words, one per line:
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | > tr -s '[ ]' '\012' | sort -u | ...
Now, all we need is a list of words that are not in the
dictionary. Here is where the comm command comes in.
$ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | > tr -s '[ ]' '\012' | sort -u | > comm -23 - /usr/lib/ispell/ispell.words
The `-2' and `-3' options eliminate lines that are only in the dictionary (the second file), and lines that are in both files. Lines only in the first file (standard input, our stream of words), are words that are not in the dictionary. These are likely candidates for spelling errors. This pipeline was the first cut at a production spelling checker on Unix.
There are some other tools that deserve brief mention.
grep
egrep
grep, but with more powerful regular expressions
wc
tee
sed
awk
The software tools philosophy also espoused the following bit of advice: "Let someone else do the hard part." This means, take something that gives you most of what you need, and then massage it the rest of the way until it's in the form that you want.
To summarize:
As of this writing, all the programs we've discussed are available via
anonymous ftp from prep.ai.mit.edu as
`/pub/gnu/textutils-1.9.tar.gz'.(1)
None of what I have presented in this column is new. The Software Tools
philosophy was first introduced in the book Software Tools,
by Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN
0-201-03669-X). This book showed how to write and use software
tools. It was written in 1976, using a preprocessor for FORTRAN named
ratfor (RATional FORtran). At the time, C was not as ubiquitous
as it is now; FORTRAN was. The last chapter presented a ratfor
to FORTRAN processor, written in ratfor. ratfor looks an
awful lot like C; if you know C, you won't have any problem following
the code.
In 1981, the book was updated and made available as Software Tools in Pascal (Addison-Wesley, ISBN 0-201-10342-7). Both books remain in print, and are well worth reading if you're a programmer. They certainly made a major change in how I view programming.
Initially, the programs in both books were available (on 9-track tape)
from Addison-Wesley. Unfortunately, this is no longer the case,
although you might be able to find copies floating around the Internet.
For a number of years, there was an active Software Tools Users Group,
whose members had ported the original ratfor programs to essentially
every computer system with a FORTRAN compiler. The popularity of the
group waned in the middle '80s as Unix began to spread beyond universities.
With the current proliferation of GNU code and other clones of Unix programs, these programs now receive little attention; modern C versions are much more efficient and do more than these programs do. Nevertheless, as exposition of good programming style, and evangelism for a still-valuable philosophy, these books are unparalleled, and I recommend them highly.
Acknowledgment: I would like to express my gratitude to Brian Kernighan of Bell Labs, the original Software Toolsmith, for reviewing this column.
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