|Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash|
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Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options.
Example 12-7. Using date
#!/bin/bash # Exercising the 'date' command echo "The number of days since the year's beginning is `date +%j`." # Needs a leading '+' to invoke formatting. # %j gives day of year. echo "The number of seconds elapsed since 01/01/1970 is `date +%s`." # %s yields number of seconds since "UNIX epoch" began, #+ but how is this useful? prefix=temp suffix=`eval date +%s` # The "+%s" option to 'date' is GNU-specific. filename=$prefix.$suffix echo $filename # It's great for creating "unique" temp filenames, #+ even better than using $$. # Read the 'date' man page for more formatting options. exit 0
Echoes the time in a specified time zone.
bash$ zdump EST EST Tue Sep 18 22:09:22 2001 EST
Outputs very verbose timing statistics for executing a command.
time ls -l / gives something like this:
0.00user 0.01system 0:00.05elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (149major+27minor)pagefaults 0swaps
See also the very similar times command in the previous section.
As of version 2.0 of Bash, time became a shell reserved word, with slightly altered behavior in a pipeline.
Utility for updating access/modification times of a file to current system time or other specified time, but also useful for creating a new file. The command touch zzz will create a new file of zero length, named zzz, assuming that zzz did not previously exist. Time-stamping empty files in this way is useful for storing date information, for example in keeping track of modification times on a project.
The touch command is equivalent to : >> newfile (for ordinary files).
The at job control command executes a given set of commands at a specified time. Superficially, it resembles crond, however, at is chiefly useful for one-time execution of a command set.
at 2pm January 15 prompts for a set of commands to execute at that time. These commands should be shell-script compatible, since, for all practical purposes, the user is typing in an executable shell script a line at a time. Input terminates with a Ctl-D.
Using either the -f option or input redirection (<), at reads a command list from a file. This file is an executable shell script, though it should, of course, be noninteractive. Particularly clever is including the run-parts command in the file to execute a different set of scripts.
bash$ at 2:30 am Friday < at-jobs.list job 2 at 2000-10-27 02:30
The batch job control command is similar to at, but it runs a command list when the system load drops below .8. Like at, it can read commands from a file with the -f option.
Prints a neatly formatted monthly calendar to stdout. Will do current year or a large range of past and future years.
This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing. This can be useful for timing or in processes running in the background, checking for a specific event every so often (see Example 30-5).
sleep 3 # Pauses 3 seconds.
The sleep command defaults to seconds, but minute, hours, or days may also be specified.
Microsleep (the "u" may be read as the Greek "mu", or micro prefix). This is the same as sleep, above, but "sleeps" in microsecond intervals. This can be used for fine-grain timing, or for polling an ongoing process at very frequent intervals.
usleep 30 # Pauses 30 microseconds.
The usleep command does not provide particularly accurate timing, and is therefore unsuitable for critical timing loops.
The hwclock command accesses or adjusts the machine's hardware clock. Some options require root privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from the hardware clock at bootup.
The clock command is a synonym for hwclock.