15.3. Time / Date Commands
- date
Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options.
Example 15-10. 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=$(date +%s) # The "+%s" option to 'date' is GNU-specific. filename=$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
The date command has quite a number of output options. For example %N gives the nanosecond portion of the current time. One interesting use for this is to generate six-digit random integers.
date +%N | sed -e 's/000$//' -e 's/^0//' ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ # Strip off leading and trailing zeroes, if present.There are many more options (try man date).
date +%j # Echoes day of the year (days elapsed since January 1). date +%k%M # Echoes hour and minute in 24-hour format, as a single digit string. # The 'TZ' parameter permits overriding the default time zone. date # Mon Mar 28 21:42:16 MST 2005 TZ=EST date # Mon Mar 28 23:42:16 EST 2005 # Thanks, Frank Kannemann and Pete Sjoberg, for the tip. SixDaysAgo=$(date --date='6 days ago') OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month). OneYearAgo=$(date --date='1 year ago')
See also Example 3-4.
- zdump
Time zone dump: 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).
Before doing a cp -u (copy/update), use touch to update the time stamp of files you don't wish overwritten.
As an example, if the directory /home/bozo/tax_audit contains the files spreadsheet-051606.data, spreadsheet-051706.data, and spreadsheet-051806.data, then doing a touch spreadsheet*.data will protect these files from being overwritten by files with the same names during a cp -u /home/bozo/financial_info/spreadsheet*data /home/bozo/tax_audit.
- at
The at job control command executes a given set of commands at a specified time. Superficially, it resembles cron, 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 29-6.
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-grained 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.