How do I set a file's mtime in epoch format?

Question

I know how to get a file's mtime in epoch format:

stat --format=%Y <file>

But I have not been able to figure out how to set a file's mtime in epoch format. The touch(1) man page appears to only accept a "timestamp" value (more-or-less fixed format which uses months, days, hours, minues, etc) or a "mostly free format human readable date string".

Is there another utility I should be looking into?

Thanks.

Answer 1

At least in the GNU world:

touch --date=@1403970787 file

Like with date.

Answer 2

With the touch command from GNU coreutils (i.e. on non-embedded Linux and Cygwin), look at the full manual (usually available locally in info format) for the documentation of date input formats. Epoch dates are indicated with the prefix @:

touch -d @1234567890 foo

This also works with BusyBox (at least on some systems, it might depend on compilation options).

With *BSD, I don't think you can do this with touch alone, but you can call date to format the epoch time into a format that touch accepts.

touch -d "$(date -r 1234567890 +%Y%m%d%H%M.%S)" foo

POSIX notoriously lacks ways to manipulate epoch dates. You can use perl.

Answer 3

With perl:

$ perl -e 'utime (stat($_))[8], time(), $_ for @ARGV' file1 file2 ...

This will change mtime of all files in @ARGV. (stat($_))[8] is atime of file.

utime can receive list of files, if you don't care about changing atime, you can try:

 $ perl -e '$t = time(); utime $t, $t, @ARGV' file1 file2 ...

Note

utime depends on C runtime library and filesystem is using. see more in perldoc -f utime and perldoc perlport.