Get yesterday s date in bash on Linux DST-safe

Question

I have a shell script that runs on Linux and uses this call to get yesterday s date in YYYY-MM-DD format   date -d  1 day ago     Y- m- d    It works most of the time  but when the script ran yesterday morning at 2013-03-11 0 35 CDT it returned  2013-03-09  instead of  2013-03-10    Presumably daylight saving time  which started yesterday  is to blame  I m guessing the way  1 day ago  is implemented it subtracted 24 hours  and 24 hours before 2013-03-11 0 35 CDT was 2013-03-09 23 35 CST  which led to the result of  2013-03-09    So what s a good DST-safe way to get yesterday s date in bash on Linux

User · Answer

You can use   date -d  yesterday 13 55     Y- m- d    Or whatever time you want to retrieve will retrieved by bash   For month   date -d  30 days ago     Y- m- d

User · Answer

date under Mac OSX is slightly different   For yesterday  date -v-1d   F   For Last week  date -v-1w   F

User · Answer

Just use date and trusty seconds   As you rightly point out  a lot of the details about the underlying computation are hidden if you rely on English time arithmetic  E g  -d yesterday  and -d 1 day ago will have different behaviour   Instead  you can reliably depend on the  precisely documented  seconds since the unix epoch UTC  and bash arithmetic to obtain the moment you want   date -d        date    s   - 24 3600      Y- m- d    This was pointed out in another answer  This form is more portable across platforms with different date command line flags  is language-independent  e g   yesterday  vs  hier  in French locale   and frankly  in the long-term  will be easier to remember  because well  you know it already  You might otherwise keep asking yourself   Was it -d 2 hours ago or -d 2 hour ago again   or  Is it -d yesterday or -d 1 day ago that I want     The only tricky bit here is the     Armed with bash and nothing else   Bash solely on bash  you can also get yesterday s time  via the printf builtin      datefmt T      causes printf to output the date-time string resulting from using      datefmt as a format string for strftime 3    The  corresponding  argu-      ment  is an integer representing the number of seconds since the      epoch   Two special argument values may be used  -1 represents the      current time  and -2 represents the time the shell was invoked       If no argument is specified  conversion behaves as if -1 had      been  given       This is an exception to the usual printf behavior    So     inner printf gets you the current unix time in seconds   outer printf spits it out according to the format printf     Y- m- d T n        printf     s T  -1  - 24 3600      or  equivalently with a temp variable  outer subshell optional  but keeps environment vars clean         now   printf     s T  -1     printf     Y- m- d T n     now - 24 3600        Note  despite the manpage stating that no argument to the    T formatter will assume a default -1  i seem to get a 0 instead  thank you  bash manual version 4 3 48

User · Answer

Here a solution that will work with Solaris and AIX as well   Manipulating the Timezone is possible for changing the clock some hours  Due to the daylight saving time  24 hours ago can be today or the day before yesterday   You are sure that yesterday is 20 or 30 hours ago  Which one  Well  the most recent one that is not today   echo -e    TZ GMT 30 date   Y- m- d  n  TZ GMT 20 date   Y- m- d     grep -v   date   Y- m- d    tail -1   The -e parameter used in the echo command is needed with bash  but will not work with ksh  In ksh you can use the same command without the -e flag   When your script will be used in different environments  you can start the script with    bin ksh or    bin bash  You could also replace the  n by a newline   echo    TZ GMT 30 date   Y- m- d    TZ GMT 20 date   Y- m- d     grep -v   date   Y- m- d    tail -1

User · Answer

As this question is tagged bash  DST safe   And using fork to date command implie delay  there is a simple and more efficient way using pure bash built-in   printf -v tznow     z  s T  -1 TZ   tznow     printf -v yesterday     Y- m- d T        tznow     - 86400    echo  yesterday   This is a lot quicker on more system friendly than having to fork to date   From bash V  5 0  there is a new variable  EPOCHSECONDS  printf -v tz     z T  -1 TZ  tz printf -v yesterday     Y- m- d T      EPOCHSECONDS - 86400    echo  yesterday

User · Answer

I think this should work  irrespective of how often and when you run it       date -d  yesterday 13 00     Y- m- d

User · Answer

This should also work  but perhaps it is too much   date -d        date    s   - 86400      Y- m- d

User · Answer

you can use  date -d  30 days ago     d  m  Y    to get the date from 30 days ago  similarly you can replace 30 with x amount of days

User · Answer

date -d  yesterday     Y- m- d    To use this later   date   date -d  yesterday     Y- m- d

User · Answer

If you are certain that the script runs in the first hours of the day  you can simply do    date -d  12 hours ago     Y- m- d    BTW  if the script runs daily at 00 35  via crontab   you should ask yourself what will happen if a DST change falls in that hour  the script could not run  or run twice in some cases  Modern implementations of cron are quite clever in this regard  though

[linux] Get yesterday's date in bash on Linux, DST-safe

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