sh-utils
First of all, this manual is incomplete. The stty section, in
particular, needs substantial reorganization and additional explanatory
text before it will be up to the standard of other GNU manuals.
Explanatory text in general is lacking; the manual presently assumes you
pretty much know what to do, and just need to be reminded of how. Thus,
if you are interested, please get involved in improving this manual.
The entire GNU community will benefit.
Some of these programs are useful only when writing shell scripts; utilities like these are, in fact, the "language" of shell scripts (to a great extent). Others are occasionally useful interactively.
The GNU shell utilities are mostly compatible with the POSIX.2 standard.
Please report bugs to `bug-sh-utils@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. 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.)
Many 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.
First, a quote:
Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time.
... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ...
--- Robert Grudin, Time and the Art of Living.
This section describes the textual date representations that GNU
programs accept. These are the strings you, as a user, can supply as
arguments to the various programs. The C interface (via the
getdate function) is not described here.
Although the date syntax here can represent any possible time since zero A.D., computer integers are not big enough for such a (comparatively) long time. The earliest date semantically allowed on Unix systems is midnight, 1 January 1970 UCT.
A date is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items:
We describe each of these item types in turn, below.
A few numbers may be written out in words in most contexts. This is most useful for specifying day of the week items or relative items (see below). Here is the list: `first' for 1, `next' for 2, `third' for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6, `seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10, `eleventh' for 11 and `twelfth' for 12. Also, `last' means exactly -1.
When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings.
Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored.
A calendar date item specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date:
1970-09-17 # ISO 8601. 70-9-17 # This century assumed by default. 70-09-17 # Leading zeros are ignored. 9/17/72 # Common U.S. writing. 24 September 1972 24 Sept 72 # September has a special abbreviation. 24 Sep 72 # Three-letter abbreviations always allowed. Sep 24, 1972 24-sep-72 24sep72
The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example:
9/17 sep 17
Here are the rules.
For numeric months, the ISO 8601 format `year-month-day' is allowed, where year is any positive number, month is a number between 01 and 12, and day is a number between 01 and 31. A leading zero must be present if a number is less than ten. If year is less than 100, then 1900 is added to it to force a date in this century. The construct `month/day/year', popular in the United States, is accepted. Also `month/day', omitting the year.
Literal months may be spelled out in full: `January', `February', `March', `April', `May', `June', `July', `August', `September', `October', `November' or `December'. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write `Sept' instead of `September'.
When months are written literally, the calendar date may be given as any of the following:
day month year day month month day year day-month-year
Or, omitting the year:
month day
A time of day item in date strings specifies the time on a given day. Here are some examples, all of which represent the same time:
20:02:0 20:02 8:02pm 20:02-0500 # In EST (Eastern U.S. Standard Time).
More generally, the time of the day may be given as `hour:minute:second', where hour is a number between 0 and 23, minute is a number between 0 and 59, and second is a number between 0 and 59. Alternatively, `:second' can be omitted, in which case it is taken to be zero.
If the time is followed by `am' or `pm' (or `a.m.' or `p.m.'), hour is restricted to run from 1 to 12, and `:minute' may be omitted (taken to be zero). `am' indicates the first half of the day, `pm' indicates the second half of the day. In this notation, 12 is the predecessor of 1: midnight is `12am' while noon is `12pm'.
The time may alternatively be followed by a time zone correction, expressed as `shhmm', where s is `+' or `-', hh is a number of zone hours and mm is a number of zone minutes. When a time zone correction is given this way, it forces interpretation of the time relative to Coordinated Universal Time (UTC), overriding any previous specification for the time zone or the local time zone. The minute part of the time of the day may not be elided when a time zone correction is used. This is the only way to specify a time zone correction by fractional parts of an hour.
Either `am'/`pm' or a time zone correction may be specified, but not both.
A time zone item specifies an international time zone, indicated by a small set of letters. They are supported for backward compatibility reasons, but they are not recommended because they are ambiguous in practice: for example, the abbreviation `EST' has different meanings in Australia and the United States. Any included period is ignored. Military time zone designations use a single letter. Currently, only integral zone hours may be represented in a time zone item. See the previous section for a finer control over the time zone correction.
Here are many non-daylight-saving-time time zones, indexed by the zone hour value.
Here are many daylight-saving time (DST) time zones, indexed by the zone hour value. Also, by following a non-DST time zone by the string `DST' in a separate word (that is, separated by some whitespace), the corresponding DST time zone may be specified.
The explicit mention of a day of the week will forward the date (only if necessary) to reach that day of the week in the future.
Days of the week may be spelled out in full: `Sunday', `Monday', `Tuesday', `Wednesday', `Thursday', `Friday' or `Saturday'. Days may be abbreviated to their first three letters, optionally followed by a period. The special abbreviations `Tues' for `Tuesday', `Wednes' for `Wednesday' and `Thur' or `Thurs' for `Thursday' are also allowed.
A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like `third monday'. In this context, `last day' or `next day' is also acceptable; they move one week before or after the day that day by itself would represent.
A comma following a day of the week item is ignored.
Relative items adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples:
1 year 1 year ago 3 years 2 days
The unit of time displacement may be selected by the string `year' or `month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are `fortnight' which is worth 14 days, `week' worth 7 days, `day' worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60 seconds, and `second' or `sec' worth one second. An `s' suffix on these units is accepted and ignored.
The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string `ago' is equivalent to preceding the unit by a multiplicator with value -1.
The string `tomorrow' is worth one day in the future (equivalent to `day'), the string `yesterday' is worth one day in the past (equivalent to `day ago').
The strings `now' or `today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise change by previous items. They may be used to stress other items, like in `12:00 today'. The string `this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like `this thursday'.
When a relative item makes the resulting date to cross the boundary between DST and non-DST (or vice-versa), the hour is adjusted according to the local time.
The precise intepretation of a pure decimal number is dependent of the context in the date string.
If the decimal number is of the form yyyymmdd and no other calendar date item (see section Calendar date item) appears before it in the date string, then yyyy is read as the year, mm as the month number and dd as the day of the month, for the specified calendar date.
If the decimal number is of the form hhmm and no other time of day item appears before it in the date string, then hh is read as the hour of the day and mm as the minute of the hour, for the specified time of the day. mm can also be omitted.
If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year.
getdate
getdate was originally implemented by Steven M. Bellovin
(smb@research.att.com) while at the University of North Carolina
at Chapel Hill. The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (rsalz@bbn.com)
and Jim Berets (jberets@bbn.com) in August, 1990. Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
and others.
This chapter was originally produced by Fran@,{c}ois Pinard (pinard@iro.umontreal.ca) from the `getdate.y' source code, and then edited by K. Berry (kb@cs.umb.edu).
This section describes commands that display text strings.
echo: Print a line of text
echo writes each given string to standard output, with a
space between each and a newline after the last one. Synopsis:
echo [option]... [string]...
The program accepts the following options. Also see section Common options.
printf: Format and print data
printf does formatted printing of text. Synopsis:
printf format [argument]...
printf prints the format string, interpreting `%'
directives and `\' escapes in the same way as the C printf
function. The format argument is re-used as necessary to convert
all of the given arguments.
printf has one additional directive, `%b', which prints its
argument string with `\' escapes interpreted in the same way as in
the format string.
printf interprets `\0ooo' in format as an octal number
(if ooo is 0 to 3 octal digits) specifying a character to print,
and `\xhhh' as a hexadecimal number (if hhh is 1 to 3 hex
digits) specifying a character to print.
An additional escape, `\c', causes printf to produce no
further output.
The only options are a lone `--help' or `--version'. See section Common options.
yes: Print a string until interrupted
yes prints the command line arguments, separated by spaces and
followed by a newline, forever until it is killed. If no arguments are
given, it prints `y' followed by a newline forever until killed.
The only options are a lone `--help' or `--version'. See section Common options.
This section describes commands that are primarily useful for their exit
status, rather than their output. Thus, they are often used as the
condition of shell if statements, or as the last command in a
pipeline.
false: Do nothing, unsuccessfully
false does nothing except return an exit status of 1, meaning
failure. It can be used as a place holder in shell scripts
where an unsuccessful command is needed.
false ignores all command line arguments, even `--help'
and `--version', since to do otherwise would change expected
behavior that some programmers may be relying on.
This version of false is implemented as a C program, and is thus
more secure and faster than a shell script implementation, and may safely
be used as a dummy shell for the purpose of disabling accounts.
true: Do nothing, successfully
true does nothing except return an exit status of 0, meaning
success. It can be used as a place holder in shell scripts
where a successful command is needed, although the shell built-in
command : (colon) does the same thing faster.
true ignores all command line arguments, even `--help'
and `--version', since to do otherwise would change expected
behavior that some programmers may be relying on.
This version of true is implemented as a C program, and is thus
more secure and faster than a shell script implementation, and may safely
be used as a dummy shell for the purpose of disabling accounts.
test: Check file types and compare values
test returns a status of 0 (true) or 1 (false) depending on the
evaluation of the conditional expression expr. Each part of the
expression must be a separate argument.
test has file status checks, string operators, and numeric
comparison operators.
Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.
Besides the options below, test accepts a lone `--help' or
`--version'. See section Common options. A single non-option argument
is also allowed: test returns true if the argument is not null.
These options test for particular types of files. (Everything's a file, but not all files are the same!)
These options test for particular access permissions.
These options test other file characteristics.
These options test string characteristics. Strings are not quoted for
test, though you may need to quote them to protect characters
with special meaning to the shell, e.g., spaces.
Numeric relationals. The arguments must be entirely numeric (possibly
negative), or the special expression -l string, which
evaluates to the length of string.
For example:
test -1 -gt -2 && echo yes => yes test -l abc -gt 1 && echo yes => yes test 0x100 -eq 1 error--> test: integer expression expected before -eq
testThe usual logical connectives.
expr: Evaluate expressions
expr evaluates an expression and writes the result on standard
output. Each token of the expression must be a separate argument.
Operands are either numbers or strings. expr coerces
anything appearing in an operand position to an integer or a string
depending on the operation being applied to it.
Strings are not quoted for expr itself, though you may need to
quote them to protect characters with special meaning to the shell,
e.g., spaces.
Operators may given as infix symbols or prefix keywords. Parentheses may be used for grouping in the usual manner (you must quote parentheses to avoid the shell evaluating them, however).
0 if the expression is neither null nor 0, 1 if the expression is null or 0, 2 for invalid expressions.
expr supports pattern matching and other string operators. These
have lower precedence than both the numeric and relational operators (in
the next sections).
grep) regular
expression, with a ^ implicitly prepended. The first argument is
then matched against this regular expression.
If the match succeeds and regex uses `\(' and `\)', the
: expression returns the part of string that matched the
subexpression; otherwise, it returns the number of characters matched.
If the match fails, the : operator returns the null string if
`\(' and `\)' are used in regex, otherwise 0.
Only the first `\( ... \)' pair is relevant to the return
value; additional pairs are meaningful only for grouping the regular
expression operators.
In the regular expression, \+, \?, and \| are
operators which respectively match one or more, zero or one, or separate
alternatives. SunOS and other expr's treat these as regular
characters. (POSIX allows either behavior.)
See section `Regular Expression Library' in Regex, for details of
regular expression syntax. Some examples are in section Examples of using expr.
/.
This makes it possible to test expr length quote "$x" or
expr quote "$x" : '.*/\(.\)' and have it do the right thing even if
the value of $x happens to be (for example) / or index.
This operator is a GNU extension. It is disabled when
the environment variable @env{POSIXLY_CORRECT} is set.
To make expr interpret keywords as strings, you must use the
quote operator.
expr supports the usual numeric operators, in order of increasing
precedence. The string operators (previous section) have lower precedence,
the connectives (next section) have higher.
expr
expr supports the usual logical connectives and relations. These
are higher precedence than either the string or numeric operators
(previous sections). Here is the list, lowest-precedence operator first.
== is a synonym for =. expr first tries to coerce
both arguments to numbers and do a numeric comparison; if either
coercion fails, it does a lexicographic comparison.
exprHere are a few examples, including quoting for shell metacharacters.
To add 1 to the shell variable foo, in Bourne-compatible shells:
foo=`expr $foo + 1`
To print the non-directory part of the file name stored in
$fname, which need not contain a /.
expr $fname : '.*/\(^.*\)' '^|' $fname
An example showing that \+ is an operator:
expr aaa : 'a\+' => 3
expr abc : 'a\(.\)c' => b expr index abcdef cz => 3 expr index index a error--> expr: syntax error expr index quote index a => 0
Unix shells commonly provide several forms of redirection---ways to change the input source or output destination of a command. But one useful redirection is performed by a separate command, not by the shell; it's described here.
tee: Redirect output to multiple files
The tee command copies standard input to standard output and also
to any files given as arguments. This is useful when you want not only
to send some data down a pipe, but also to save a copy. Synopsis:
tee [option]... [file]...
If a file being written to does not already exist, it is created. If a
file being written to already exists, the data it previously contained
is overwritten unless the -a option is used.
The program accepts the following options. Also see section Common options.
This section describes commands that manipulate file names.
basename: Strip directory and suffix from a file name
basename removes any leading directory components from
name. Synopsis:
basename name [suffix]
If suffix is specified and is identical to the end of name,
it is removed from name as well. basename prints the
result on standard output.
The only options are `--help' and `--version'. See section Common options.
dirname: Strip non-directory suffix from a file name
dirname prints all but the final slash-delimited component of
a string (presumably a filename). Synopsis:
dirname name
If name is a single component, dirname prints `.'
(meaning the current directory).
The only options are `--help' and `--version'. See section Common options.
pathchk: Check file name portability
pathchk checks portability of filenames. Synopsis:
pathchk [option]... name...
For each name, pathchk prints a message if any of
these conditions is true:
The program accepts the following option. Also see section Common options.
0 if all specified file names passed all of the tests, 1 otherwise.
This section describes commands that display or alter the context in which you are working: the current directory, the terminal settings, and so forth. See also the user-related commands in the next section.
pwd: Print working directory
pwd prints the fully resolved name of the current directory.
That is, all components of the printed name will be actual directory
names--none will be symbolic links.
Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.
The only options are a lone `--help' or `--version'. See section Common options.
stty: Print or change terminal characteristics
stty prints or changes terminal characteristics, such as baud rate.
Synopses:
stty [option] [setting]... stty [option]
If given no line settings, stty prints the baud rate, line
discipline number (on systems that support it), and line settings
that have been changed from the values set by `stty sane'.
By default, mode reading and setting are performed on the tty line
connected to standard input, although this can be modified by the
`--file' option.
stty accepts many non-option arguments that change aspects of
the terminal line operation, as described below.
The program accepts the following options. Also see section Common options.
O_NONDELAY flag to
prevent a POSIX tty from blocking until the carrier detect line is high if
the clocal flag is not set. Hence, it is not always possible
to allow the shell to open the device in the traditional manner.
stty command to restore the current settings. This option
may not be used in combination with any line settings.
Many settings can be turned off by preceding them with a `-'. Such arguments are marked below with "May be negated" in their description. The descriptions themselves refer to the positive case, that is, when not negated (unless stated otherwise, of course).
Some settings are not available on all POSIX systems, since they use extensions. Such arguments are marked below with "Non-POSIX" in their description. On non-POSIX systems, those or other settings also may not be available, but it's not feasible to document all the variations: just try it and see.
stop character when the system input buffer
is almost full, and start character when it becomes almost
empty again. May be negated.
These arguments specify output-related operations.
interrupt, quit, and suspend special
characters. May be negated.
erase, kill, werase, and rprnt
special characters. May be negated.
erase characters as backspace-space-backspace. May be
negated.
kill character. May be negated.
interrupt and quit special
characters. May be negated.
icanon is set.
Non-POSIX. May be negated.
kill special character by erasing each character on
the line as indicated by the echoprt and echoe settings,
instead of by the echoctl and echok settings. Non-POSIX.
May be negated.
parenb -parodd cs7. May be negated. If negated, same
as -parenb cs8.
parenb parodd cs7. May be negated. If negated, same
as -parenb cs8.
-icrnl -onlcr. May be negated. If negated, same as
icrnl -inlcr -igncr onlcr -ocrnl -onlret.
erase and kill special characters to their default
values.
cread -ignbrk brkint -inlcr -igncr icrnl -ixoff -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0 isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echokeand also sets all special characters to their default values.
brkint ignpar istrip icrnl ixon opost isig icanon, plus
sets the eof and eol characters to their default values
if they are the same as the min and time characters.
May be negated. If negated, same as raw.
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -opost -isig -icanon -xcase min 1 time 0May be negated. If negated, same as
cooked.
-icanon. May be negated. If negated, same as
icanon.
-parenb -istrip cs8. May be negated. If negated,
same as parenb istrip cs7.
-parenb -istrip -opost cs8. May be negated.
If negated, same as parenb istrip opost cs7.
-ixany. Non-POSIX. May be negated.
tab0. Non-POSIX. May be negated. If negated, same
as tab3.
xcase iuclc olcuc. Non-POSIX. May be negated.
echoe echoctl echoke.
echoe echoctl echoke -ixany intr ^C erase ^? kill C-u.
The special characters' default values vary from system to system. They are set with the syntax `name value', where the names are listed below and the value can be given either literally, in hat notation (`^c'), or as an integer which may start with `0x' to indicate hexadecimal, `0' to indicate octal, or any other digit to indicate decimal.
For GNU stty, giving a value of ^- or undef disables that
special character. (This is incompatible with Ultrix stty,
which uses a value of `u' to disable a special character. GNU
stty treats a value `u' like any other, namely to set that
special character to U.)
-icanon is set.
-icanon is set.
stty does not know anything about them.)
Non-POSIX.
exta extb. exta is the same as
19200; extb is the same as 38400. 0 hangs up the line if
-clocal is set.
printenv: Print all or some environment variables
printenv prints environment variable values. Synopsis:
printenv [option] [variable]...
If no variables are specified, printenv prints the value of
every environment variable. Otherwise, it prints the value of each
variable that is set, and nothing for those that are not set.
The only options are a lone `--help' or `--version'. See section Common options.
0 if all variables specified were found 1 if at least one specified variable was not found 2 if a write error occurred
tty: Print file name of terminal on standard input
tty prints the file name of the terminal connected to its standard
input. It prints `not a tty' if standard input is not a terminal.
Synopsis:
tty [option]...
The program accepts the following option. Also see section Common options.
0 if standard input is a terminal 1 if standard input is not a terminal 2 if given incorrect arguments 3 if a write error occurs
This section describes commands that print user-related information: logins, groups, and so forth.
id: Print real and effective uid and gid
id prints information about the given user, or the process
running it if no user is specified. Synopsis:
id [option]... [username]
By default, it prints the real user id, real group id, effective user id if different from the real user id, effective group id if different from the real group id, and supplemental group ids.
Each of these numeric values is preceded by an identifying string and followed by the corresponding user or group name in parentheses.
The options cause id to print only part of the above information.
Also see section Common options.
-u, -g, or -G.
-u, -g, or -G.
logname: Print current login name
logname prints the calling user's name, as found in the file
`/etc/utmp', and exits with a status of 0. If there is no
`/etc/utmp' entry for the calling process, logname prints
an error message and exits with a status of 1.
The only options are `--help' and `--version'. See section Common options.
whoami: Print effective user id
whoami prints the user name associated with the current
effective user id. It is equivalent to the command `id -un'.
The only options are `--help' and `--version'. See section Common options.
groups: Print group names a user is in
groups prints the names of the primary and any supplementary
groups for each given username, or the current process if no names
are given. If names are given, the name of each user is printed before
the list of that user's groups. Synopsis:
groups [username]...
The group lists are equivalent to the output of the command `id -Gn'.
The only options are `--help' and `--version'. See section Common options.
users: Print login names of users currently logged in
users prints on a single line a blank-separated list of user
names of users currently logged in to the current host. Each user name
corresponds to a login session, so if a user has more than one login
session, that user's name will appear the same number of times in the
output. Synopsis:
users [file]
With no file argument, users extracts its information from
the file `/etc/utmp'. If a file argument is given, users
uses that file instead. A common choice is `/etc/wtmp'.
The only options are `--help' and `--version'. See section Common options.
who: Print who is currently logged in
who prints information about users who are currently logged on.
Synopsis:
who [option] [file] [am i]
If given no non-option arguments, who prints the following
information for each user currently logged on: login name, terminal
line, login time, and remote hostname or X display.
If given one non-option argument, who uses that instead of
`/etc/utmp' as the name of the file containing the record of
users logged on. `/etc/wtmp' is commonly given as an argument
to who to look at who has previously logged on.
If given two non-option arguments, who prints only the entry
for the user running it (determined from its standard input), preceded
by the hostname. Traditionally, the two arguments given are `am
i', as in `who am i'.
The program accepts the following options. Also see section Common options.
who.
`+' allowingwritemessages `-' disallowingwritemessages `?' cannot find terminal device
This section describes commands that print or change system-wide information.
date: Print or set system date and time
date with no arguments prints the current time and date, in the
format of the `%c' directive (described below). Synopses:
date [option]... [+format] date [-u|--utc|--universal] [ MMDDhhmm[[CC]YY][.ss] ]
If given an argument that starts with a `+', date prints the
current time and date (or the time and date specified by the
--date option, see below) in the format defined by that argument,
which is the same as in the strftime function. Except for
directives, which start with `%', characters in the format string
are printed unchanged. The directives are described below.
date directives related to times.
date directives related to dates.
date directives that produce literal strings.
By default, date pads numeric fields with zeroes, so that, for
example, numeric months are always output as two digits. GNU date
recognizes the following numeric modifiers between the `%' and the
directive.
These are GNU extensions.
Here is an example illustrating the differences:
date +%d/%m -d "Feb 1" => 01/02 date +%-d/%-m -d "Feb 1" => 1/2 date +%_d/%_m -d "Feb 1" => 1/ 2
If given an argument that does not start with `+', date sets
the system clock to the time and date specified by that argument (as
described below). You must have appropriate privileges to set the
system clock. The `--date' and `--set' options may not be
used with such an argument. The `--universal' option may be used
with such an argument to indicate that the specified time and date are
relative to Coordinated Universal Time rather than to the local time
zone.
The argument must consist entirely of digits, which have the following meaning:
The `--set' option also sets the system clock; see the next section.
dateThe program accepts the following options. Also see section Common options.
date executable many times can
be considerable.
dateHere are a few examples. Also see the documentation for the `-d' option in the previous section.
date --date='2 days ago'
date --date='3 months 1 day'
date --date='25 Dec' +%j
date '+%B %d'But this may not be what you want because for the first nine days of the month, the `%d' expands to a zero-padded two-digit field, for example `date -d 1may '+%B %d'' will print `May 01'.
- modifier to suppress
the padding altogether.
date -d=1may '+%B %-d'
date when setting the system clock:
date +%m%d%H%M%Y.%S
date --set='+2 minutes'
Mon, 25 Mar 1996 23:34:17 -0600
date --date='1970-01-01 00:00:01 UTC +5 hours' +%s 18001Suppose you had not specified time zone information in the example above. Then, date would have used your computer's idea of the time zone when interpreting the string. Here's what you would get if you were in Greenwich, England:
# local time zone used date --date='1970-01-01 00:00:01' +%s 1
date --date='2000-01-01 UTC' +%s 946684800To convert such an unwieldy number of seconds back to a more readable form, use a command like this:
date -d '1970-01-01 946684800 sec' +"%Y-%m-%d %T %z" 2000-01-01 00:00:00 +0000
uname: Print system information
uname prints information about the machine and operating system
it is run on. If no options are given, uname acts as if the
-s option were given. Synopsis:
uname [option]...
If multiple options or -a are given, the selected information is
printed in this order:
sysname nodename release osversion machine
The osversion, at least, may well be multiple words. For example:
uname -a => Linux hayley 1.0.4 #3 Thu May 12 18:06:34 1994 i486
The program accepts the following options. Also see section Common options.
hostname: Print or set system name
With no arguments, hostname prints the name of the current host
system. With one argument, it sets the current host name to the
specified string. You must have appropriate privileges to set the host
name. Synopsis:
hostname [name]
The only options are `--help' and `--version'. See section Common options.
This section describes commands that run other commands in some context different than the current one: a modified environment, as a different user, etc.
chroot: Run a command with a different root directory
chroot runs a command with a specified root directory.
On many systems, only the super-user can do this.
Synopses:
chroot newroot [command [args]...] chroot option
Ordinarily, filenames are looked up starting at the root of the
directory structure, i.e., `/'. chroot changes the root to
the directory newroot (which must exist) and then runs
command with optional args. If command is not
specified, the default is the value of the @env{SHELL} environment
variable or /bin/sh if not set, invoked with the `-i' option.
The only options are `--help' and `--version'. See section Common options.
env: Run a command in a modified environment
env runs a command with a modified environment. Synopses:
env [option]... [name=value]... @c [command [args]...] env
Arguments of the form `variable=value' set the environment variable variable to value value. value may be empty (`variable='). Setting a variable to an empty value is different from unsetting it.
The first remaining argument specifies the program name to invoke; it is searched for according to the @env{PATH} environment variable. Any remaining arguments are passed as arguments to that program.
If no command name is specified following the environment
specifications, the resulting environment is printed. This is like
specifying a command name of printenv.
The program accepts the following options. Also see section Common options.
nice: Run a command with modified scheduling priority
nice prints or modifies the scheduling priority of a job.
Synopsis:
nice [option]... [command [arg]...]
If no arguments are given, nice prints the current scheduling
priority, which it inherited. Otherwise, nice runs the given
command with its scheduling priority adjusted. If no
adjustment is given, the priority of the command is incremented by
10. You must have appropriate privileges to specify a negative
adjustment. The priority can be adjusted by nice over the range
of -20 (the highest priority) to 19 (the lowest).
Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.
The program accepts the following option. Also see section Common options.
nohup: Run a command immune to hangups
nohup runs the given command with hangup signals ignored,
so that the command can continue running in the background after you log
out. Synopsis:
nohup command [arg]...
nohup increases the scheduling priority of command by 5, so
it has a slightly smaller change to run. If standard output is a terminal,
it and standard error are redirected so that they are appended to the
file `nohup.out'; if that cannot be written to, they are appended
to the file `$HOME/nohup.out'. If that cannot be written to, the
command is not run.
If nohup creates either `nohup.out' or
`$HOME/nohup.out', it creates it with no "group" or "other"
access permissions. It does not change the permissions if the output
file already existed.
nohup does not automatically put the command it runs in the
background; you must do that explicitly, by ending the command line
with an `&'.
The only options are `--help' and `--version'. See section Common options.
su: Run a command with substitute user and group id
su allows one user to temporarily become another user. It runs a
command (often an interactive shell) with the real and effective user
id, group id, and supplemental groups of a given user. Synopsis:
su [option]... [user [arg]...]
If no user is given, the default is root, the super-user.
The shell to use is taken from user's passwd entry, or
`/bin/sh' if none is specified there. If user has a
password, su prompts for the password unless run by a user with
effective user id of zero (the super-user).
By default, su does not change the current directory.
It sets the environment variables @env{HOME} and @env{SHELL}
from the password entry for user, and if user is not
the super-user, sets @env{USER} and @env{LOGNAME} to user.
By default, the shell is not a login shell.
Any additional args are passed as additional arguments to the shell.
GNU su does not treat `/bin/sh' or any other shells specially
(e.g., by setting argv[0] to `-su', passing -c only
to certain shells, etc.).
su can optionally be compiled to use syslog to report
failed, and optionally successful, su attempts. (If the system
supports syslog.) However, GNU su does not check if the
user is a member of the wheel group; see below.
The program accepts the following options. Also see section Common options.
-c option instead of starting an interactive shell.
-f option to the shell. This probably only makes sense
if the shell run is csh or tcsh, for which the -f
option prevents reading the startup file (`.cshrc'). With
Bourne-like shells, the -f option disables file name pattern
expansion (globbing), which is not likely to be useful.
su is not the superuser and
user's shell is restricted. A restricted shell is one that
is not listed in the file `/etc/shells', or in a compiled-in list
if that file does not exist. Parts of what this option does can be
overridden by --login and --shell.
su is not the superuser and user's
shell is restricted (see `-m' just above).
Why GNU su does not support the `wheel' group
(This section is by Richard Stallman.)
Sometimes a few of the users try to hold total power over all the rest. For example, in 1984, a few users at the MIT AI lab decided to seize power by changing the operator password on the Twenex system and keeping it secret from everyone else. (I was able to thwart this coup and give power back to the users by patching the kernel, but I wouldn't know how to do that in Unix.)
However, occasionally the rulers do tell someone. Under the usual
su mechanism, once someone learns the root password who
sympathizes with the ordinary users, he or she can tell the rest. The
"wheel group" feature would make this impossible, and thus cement the
power of the rulers.
I'm on the side of the masses, not that of the rulers. If you are used to supporting the bosses and sysadmins in whatever they do, you might find this idea strange at first.
sleep: Delay for a specified time
sleep pauses for an amount of time specified by the sum of
the values of the command line arguments.
Synopsis:
sleep [number[smhd]]...
Each argument is a number followed by an optional unit; the default is seconds. The units are:
The only options are `--help' and `--version'. See section Common options.
These programs do numerically-related operations.
factor: Print prime factors
factor prints prime factors. Synopses:
factor [number]... factor option
If no number is specified on the command line, factor reads
numbers from standard input, delimited by newlines, tabs, or spaces.
The only options are `--help' and `--version'. See section Common options.
seq: Print numeric sequences
seq prints a sequence of numbers to standard output. Synopses:
seq [option]... [first [step]] last...
seq prints the numbers from first to last by
step. By default, first and step are both 1, and each
number is printed on its own line. All numbers can be reals, not just
integers.
The program accepts the following options. Also see section Common options.
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