Time / Date Commands

Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options.

!/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 "Temporary filename = $filename"
#  It's great for creating "unique and random" 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

This option facilitates calculating the time between different dates.

#!/bin/bash
# date-calc.sh
# Author: Nathan Coulter
# Used in ABS Guide with permission (thanks!).
MPHR=60    # Minutes per hour.
HPD=24     # Hours per day.
diff () {
        printf '%s' $(( $(date -u -d"$TARGET" +%s) -
                        $(date -u -d"$CURRENT" +%s)))
#                       %d = day of month.
}
CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z')
TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z')
# %F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.
printf '\nIn 2007, %s ' \
       "$(date -d"$CURRENT +
        $(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"
#       %B = name of month                ^ halfway
printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')"
printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"
printf '\nOn %s at %s, there were\n' \
        $(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T)
DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD ))
CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z')
HOURS=$(( $(diff) / $MPHR / $MPHR ))
CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z')
MINUTES=$(( $(diff) / $MPHR ))
CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z')
printf '%s days, %s hours, ' "$DAYS" "$HOURS"
printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)"
printf 'until Christmas Dinner!\n\n'
#  Exercise:
#  --------
#  Rewrite the diff () function to accept passed parameters,
#+ rather than using global variables.

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 random integers.

date +%N | sed -e 's/000$//' -e 's/^0//'
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#  Strip off leading and trailing zeroes, if present.
#  Length of generated integer depends on
#+ how many zeroes stripped off.
# 115281032
# 63408725
# 394504284

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 TODO Example 3-4 and Example A-43.

Time zone dump: echoes the time in a specified time zone.

bash$ zdump EST
EST  Tue Sep 18 22:09:22 2001 EST

Outputs verbose timing statistics for executing a command.

$ time ls -l /
<output of of ls here>
real    0m0.067s
user    0m0.004s
sys     0m0.005s

See also the very similar times command in the previous section.

Note: 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.

Note: The touch command is equivalent to : >> newfile or >> newfile (for ordinary files).

Tip: 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 non-interactive. 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.

History: The concept of batch processing dates back to the era of mainframe computers. It means running a set of commands without user intervention.

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 TODO Example 32-6.

sleep 3     # Pauses 3 seconds.

Note: The sleep command defaults to seconds, but minute, hours, or days may also be specified.

sleep 3 h   # Pauses 3 hours!

Note: The watch command may be a better choice than sleep for running commands at timed intervals.