|Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting|
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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 15-10. Using date
1 #!/bin/bash 2 # Exercising the 'date' command 3 4 echo "The number of days since the year's beginning is `date +%j`." 5 # Needs a leading '+' to invoke formatting. 6 # %j gives day of year. 7 8 echo "The number of seconds elapsed since 01/01/1970 is `date +%s`." 9 # %s yields number of seconds since "UNIX epoch" began, 10 #+ but how is this useful? 11 12 prefix=temp 13 suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific. 14 filename=$prefix.$suffix 15 echo $filename 16 # It's great for creating "unique" temp filenames, 17 #+ even better than using $$. 18 19 # Read the 'date' man page for more formatting options. 20 21 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.
1 date +%N | sed -e 's/000$//' -e 's/^0//' 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 3 # Strip off leading and trailing zeroes, if present.
There are many more options (try man date).
1 date +%j 2 # Echoes day of the year (days elapsed since January 1). 3 4 date +%k%M 5 # Echoes hour and minute in 24-hour format, as a single digit string. 6 7 8 9 # The 'TZ' parameter permits overriding the default time zone. 10 date # Mon Mar 28 21:42:16 MST 2005 11 TZ=EST date # Mon Mar 28 23:42:16 EST 2005 12 # Thanks, Frank Kannemann and Pete Sjoberg, for the tip. 13 14 15 SixDaysAgo=$(date --date='6 days ago') 16 OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month). 17 OneYearAgo=$(date --date='1 year ago')
See also Example 3-4.
Time zone dump: 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 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.
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
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. 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.
1 sleep 3 # Pauses 2 3 seconds.
The sleep command defaults to seconds, but minute, hours, or days may also be specified.
The watch command may be a better choice than sleep for running commands at timed intervals.
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.
1 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.
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.