Simply invoked, date prints the date and
time to stdout. Where this command gets
interesting is in its formatting and parsing options.
Example 16.10. 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 "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$dateFri Mar 29 21:07:39 MST 2002bash$date -uSat Mar 30 04:07:42 UTC 2002
This option facilitates calculating the time between different dates.
Example 16.11. Date calculations
#!/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 Example 3.4, “Backup of all files changed in last day” and Example A.43, “A command-line stopwatch”.
Time zone dump: echoes the time in a specified time zone.
bash$zdump ESTEST Tue Sep 18 22:09:22 2001 EST
Outputs verbose timing statistics for executing a command.
time ls -l / gives something
like this:
real 0m0.067s
user 0m0.004s
sys 0m0.005s
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 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.listjob 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 32.6, “Cleaning up after Control-C”.
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.
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.
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.