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Laas
Asked: 2017-01-13 11:22:37 +0800 CST

Convert modify time from stat into timestamp

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I just started my journey into bash scripting and I wrote a little script which copies certain files to my USB stick and external harddrive as a backup. So far it copies the entire directory with all files including those which I didn't change which naturally takes quite some time. In order to change that, I want it to compare the two files (on the pc and on the USB stick) with regards to the time when they were modified. So now I wonder how I can convert the output from

stat -c %y /test_folder/test.txt

into the format "YmDHMS"? Or maybe someone knows a better way to compare the files.

timestamp

2 Answers

  1. For comparison purposes you are probably better off using seconds since epoch (i.e. stat -a %Y) directly or even simpler use one of the bash built-in file tests [[ FILE1 -nt FILE2 ]] or [[ FILE1 -ot FILE2 ]] instead:

       file1 -nt file2
          True if file1 is newer (according  to  modification  date)  than
          file2, or if file1 exists and file2 does not.
   file1 -ot file2
          True  if file1 is older than file2, or if file2 exists and file1
          does not.

    However since the question as asked is about conversion, since %y is modification time, if you have a recent version of GNU date you could cheat and use that instead. From man date:

    -r, --reference=FILE
       display the last modification time of FILE

    So

    $ date +%Y%m%d%H%M%S -r file
20170111075620

    Alternatively, you could pass the seconds-since-epoch from stat to date:

    $ date +%Y%m%d%H%M%S -d "@$(stat -c '%Y' file)"
20170111075620
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  2. For comparing modification (or access or inode change) time of files, you should use seconds since Epoch, it is easier to handle in comparisons. stat's -c option has %Y format specifier for getting the modification time in seconds since Epoch.

    To compare two files based on modification time, you can write a small function:

    chk_mod () { (( $(stat -c '%Y' "$1") > $(stat -c '%Y' "$2") )) && echo "$1 is newer" \
            || echo "$2 is newer" ;}

    Input the two input files as first and second positional parameters, and the function will output which one is newer; when they have exactly the same modification times, no output will be shown.

    Example:

    $ chk_mod () { (( $(stat -c '%Y' "$1") > $(stat -c '%Y' "$2") )) && echo "$1 is newer" || echo "$2 is newer" ;}

$ chk_mod foobar spamegg 
spamegg is newer
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