I want to check whether link lnkfile is older than a regular reference file reffile.
The bash test builtin dereferences links, so test lnkfile -ot reffile compares the target of lnkfile, not the link itself.
Is there a way to make the test builtin not follow symlinks? Otherwise, how can I compare the time of a symlink?
I don't think there is a way with test, but you could use find.
test "$(find reffile -prune -newer lnkfile)" && command Here, find returns output if lnkfile is older than reffile. test without an option is equivalent to test -n. This is true if the length of the string is nonzero. Hence, if there is output from find, command is executed.
In the comments, I was asked to make a comparison of this solution vs. stat. Firstly, I find the stat approach perfectly fine.
I did a benchmark to compare. I repeated the test a few times, alternating, and got similar results.
$ time (for i in {1..1000}; do test "$(stat --format=%Z a)" -lt "$(stat --format=%Z b)" && echo foo > /dev/null ; done) ================ CPU 101% CPU 104% user 1.264 system 0.942 total 2.108 $ time (for i in {1..1000}; do test "$(find b -newer a)" && echo foo > /dev/null ; done) ================ CPU 104% user 0.693 system 0.526 total 1.164 I looks like find is almost twice as fast, perhaps because it's a single process rather than two stats? I'm not sure how else to compare them; please comment if there are other relevant aspects you can think of.
Here are some other differences, as per Stéphane Chazelas's comments below:
Other differences are: the
findone is standard. while thestatone needs the GNU implementation ofstat. Thestatone won't work for files modified within the same second (whilefindshould work on systems where sub-second granularity for timestamps is supported). Neitherfindnorstatsupport arbitrary file names.You'll find other differences if either of the files cannot be
stat()ed.
test implementation do you have that does not have -n? It's a standard option... Also quote the command substitution. -n, so it's easier to grep the man. Good point with the quote. Thanks. -n and read it as "there is no -n option" instead of "test without an option is the same as test -n". As far as I know, bash doesn't offer any versions of -ot (and the like) which avoid dereferencing symlinks.
What you can do instead is use GNU stat (which doesn't dereference symbolic links without -L) and compare their mtime epochs numerically:
if (( "$(stat --format=%Z lnkfile)" < "$(stat --format=%Z reffile)" )); then # lnkfile is older fi With zsh 5.6 or above (for the nanosecond precision), you could do it with builtins only with:
zmodload zsh/stat if stat -LA lnktime -F '%021s%N' +mtime -- $lnkfile && stat -A reftime -F '%021s%N' +mtime -- $reffile && [[ $lnktime < $reftime ]] then print -r -- $lnkfile is older than $reffile fi Which would work regardless of what characters or non-characters the file names contain and compare timestamps down to the nanosecond.
We're comparing the timestamps as strings (number of nanoseconds as a decimal string representation zero-padded to 30 digits) instead of floating points because the typical double precision floating points of a x86_64 PC running GNU/Linux at least don't have enough precision to store numbers like 1548195897.775033155 so you would not be able to tell apart two files modified today within the same 100 nanoseconds.
$ ((1548195897.775033155 < 1548195897.775033255)) && echo yes $ [[ 1548195897775033155 < 1548195897775033255 ]] && echo yes yes 
stat shows no problem. find will replace the newline with ? if output goes to the terminal, otherwise it is preserved (which is the case in Sparhawk's answer, but anyway we're just testing if there's any output, so ? would be fine). All substitutions are double-quoted to avoid word splitting.