- date
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 |
The -u option gives the UTC (Universal
Coordinated Time).
bash$ date
Fri Mar 29 21:07:39 MST 2002
bash$ date -u
Sat Mar 30 04:07:42 UTC 2002
|
- zdump
Echoes the time in a specified time zone.
bash$ zdump EST
EST Tue Sep 18 22:09:22 2001 EST
|
- time
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. |
- touch
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
or >> newfile (for ordinary
files). |
- at
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
|
- batch
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.
- cal
Prints a neatly formatted monthly calendar to
stdout. Will do current year or a large
range of past and future years.
- sleep
This is the shell equivalent of a wait loop. It pauses for a
specified number of seconds, doing nothing. It can be useful
for timing or in processes running in the background,
checking for a specific event every so often (polling),
as in Example 30-6.
sleep 3
# Pauses 3 seconds. |
| The sleep command defaults to
seconds, but minute, hours, or days may also be specified.
sleep 3 h
# Pauses 3 hours! |
|
| The watch command may
be a better choice than sleep for running
commands at timed intervals. |
- usleep
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. It can be used for fine-grain timing, or for
polling an ongoing process at very frequent intervals.
usleep 30
# Pauses 30 microseconds. |
This command is part of the Red Hat initscripts /
rc-scripts package.
 | The usleep command does not
provide particularly accurate timing, and is therefore
unsuitable for critical timing loops. |
- hwclock, clock
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.
