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>Time / Date Commands</TITLE
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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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>Chapter 16. External Filters, Programs and Commands</TD
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>16.3. Time / Date Commands</H1
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>Time/date and timing</B
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> prints the date and
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>. Where this command gets
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interesting is in its formatting and parsing options.</P
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>Example 16-10. Using <I
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CLASS="PROGRAMLISTING"
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2 # Exercising the 'date' command
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4 echo "The number of days since the year's beginning is `date +%j`."
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5 # Needs a leading '+' to invoke formatting.
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6 # %j gives day of year.
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8 echo "The number of seconds elapsed since 01/01/1970 is `date +%s`."
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9 # %s yields number of seconds since "UNIX epoch" began,
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10 #+ but how is this useful?
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13 suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific.
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14 filename=$prefix.$suffix
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15 echo "Temporary filename = $filename"
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16 # It's great for creating "unique and random" temp filenames,
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17 #+ even better than using $$.
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19 # Read the 'date' man page for more formatting options.
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> option gives the UTC (Universal
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Coordinated Time).</P
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CLASS="COMPUTEROUTPUT"
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>Fri Mar 29 21:07:39 MST 2002</TT
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CLASS="COMPUTEROUTPUT"
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>Sat Mar 30 04:07:42 UTC 2002</TT
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>This option facilitates calculating the time between
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CLASS="PROGRAMLISTING"
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2 # date-calc.sh
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3 # Author: Nathan Coulter
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4 # Used in ABS Guide with permission (thanks!).
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6 MPHR=60 # Minutes per hour.
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7 HPD=24 # Hours per day.
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10 printf '%s' $(( $(date -u -d"$TARGET" +%s) -
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11 $(date -u -d"$CURRENT" +%s)))
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12 # %d = day of month.
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16 CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z')
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17 TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z')
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18 # %F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.
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20 printf '\nIn 2007, %s ' \
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21 "$(date -d"$CURRENT +
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22 $(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"
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23 # %B = name of month ^ halfway
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24 printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')"
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25 printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"
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27 printf '\nOn %s at %s, there were\n' \
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28 $(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T)
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29 DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD ))
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30 CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z')
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31 HOURS=$(( $(diff) / $MPHR / $MPHR ))
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32 CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z')
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33 MINUTES=$(( $(diff) / $MPHR ))
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34 CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z')
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35 printf '%s days, %s hours, ' "$DAYS" "$HOURS"
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36 printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)"
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37 printf 'until Christmas Dinner!\n\n'
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41 # Rewrite the diff () function to accept passed parameters,
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42 #+ rather than using global variables.</PRE
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> command has quite a
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> gives the nanosecond portion
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of the current time. One interesting use for this is to
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generate random integers.
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CLASS="PROGRAMLISTING"
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> 1 date +%N | sed -e 's/000$//' -e 's/^0//'
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2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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3 # Strip off leading and trailing zeroes, if present.
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4 # Length of generated integer depends on
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5 #+ how many zeroes stripped off.
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9 # 394504284</PRE
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>There are many more options (try <B
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CLASS="PROGRAMLISTING"
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2 # Echoes day of the year (days elapsed since January 1).
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5 # Echoes hour and minute in 24-hour format, as a single digit string.
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9 # The 'TZ' parameter permits overriding the default time zone.
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10 date # Mon Mar 28 21:42:16 MST 2005
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11 TZ=EST date # Mon Mar 28 23:42:16 EST 2005
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12 # Thanks, Frank Kannemann and Pete Sjoberg, for the tip.
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15 SixDaysAgo=$(date --date='6 days ago')
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16 OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month!)
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17 OneYearAgo=$(date --date='1 year ago')</PRE
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HREF="special-chars.html#EX58"
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>Time zone dump: echoes the time in a specified time zone.</P
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>EST Tue Sep 18 22:09:22 2001 EST</TT
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>Outputs verbose timing statistics for executing a command.</P
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>See also the very similar <A
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HREF="internal.html#TIMESREF"
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> command in the previous
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SRC="common/note.png"
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> became a shell reserved word,
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with slightly altered behavior in a pipeline.</P
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>Utility for updating access/modification times of a
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file to current system time or other specified time,
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but also useful for creating a new file. The command
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> will create a new file
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of zero length, named <TT
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> did not previously exist.
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Time-stamping empty files in this way is useful for
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storing date information, for example in keeping track of
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modification times on a project.
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SRC="common/note.png"
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>: >> newfile</B
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>>> newfile</B
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HREF="external.html#CPREF"
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> to update the time stamp of files
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you don't wish overwritten.</P
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>As an example, if the directory <TT
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>/home/bozo/tax_audit</TT
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>spreadsheet-051606.data</TT
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>spreadsheet-051706.data</TT
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>spreadsheet-051806.data</TT
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>touch spreadsheet*.data</B
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will protect these files from being overwritten
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by files with the same names during a
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>cp -u /home/bozo/financial_info/spreadsheet*data
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/home/bozo/tax_audit</B
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> job control command executes
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a given set of commands at a specified time. Superficially,
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HREF="system.html#CRONREF"
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> is chiefly useful for one-time execution
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>at 2pm January 15</B
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> prompts for a set of
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commands to execute at that time. These commands should be
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shell-script compatible, since, for all practical
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purposes, the user is typing in an executable shell
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script a line at a time. Input terminates with a <A
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HREF="special-chars.html#CTLDREF"
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>Using either the <TT
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reads a command list from a file. This file is an
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executable shell script, though it should, of course,
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be non-interactive. Particularly clever is including the
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HREF="extmisc.html#RUNPARTSREF"
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the file to execute a different set of scripts.</P
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>at 2:30 am Friday < at-jobs.list</B
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>job 2 at 2000-10-27 02:30</TT
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> job control command is similar to
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>, but it runs a command list when the system
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>, it can read commands from a file with the
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dates back to the era of mainframe computers. It means
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running a set of commands without user intervention.</P
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>Prints a neatly formatted monthly calendar to
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>. Will do current year or a large
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range of past and future years.</P
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>This is the shell equivalent of a <I
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>. It pauses for a specified number of
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seconds, doing nothing. It can be useful for timing or
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in processes running in the background, checking for
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a specific event every so often (polling), as in <A
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HREF="debugging.html#ONLINE"
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> 1 sleep 3 # Pauses 3 seconds.</PRE
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> command defaults to
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seconds, but minute, hours, or days may also be specified.
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> 1 sleep 3 h # Pauses 3 hours!</PRE
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be a better choice than <B
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commands at timed intervals.</P
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> may be read as the Greek
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prefix). This is the same as <B
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intervals. It can be used for fine-grained timing,
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or for polling an ongoing process at very frequent
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> 1 usleep 30 # Pauses 30 microseconds.</PRE
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>This command is part of the Red Hat
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>initscripts / rc-scripts</I
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provide particularly accurate timing, and is therefore
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unsuitable for critical timing loops.</P
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> command accesses or
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adjusts the machine's hardware clock. Some options
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>/etc/rc.d/rc.sysinit</TT
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> to set the system time
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from the hardware clock at bootup.</P
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> command is a synonym for
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>Complex Commands</TD
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>Text Processing Commands</TD
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