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[Jaap Fieret]

11

Given that you're using the stat (similar functionality, but different output 
format on BSDs and GNU), you could also use the test utility, which does this 
comparison directly:

FILE1 -nt FILE2 FILE1 is newer (modification date) than FILE2 FILE1 -ot FILE2 
FILE1 is older than FILE2
In your example,

if [ "$source_file" -nt "$target_file" ] then printf '%s\n' "$source_file is 
newer than $target_file" fi
The feature is not available in POSIX (see its documentation for test), which 
provides as a rationale:

Some additional primaries newly invented or from the KornShell appeared in an 
early proposal as part of the conditional command ([[]]): s1 > s2, s1 < s2, str 
= pattern, str != pattern, f1 -nt f2, f1 -ot f2, and f1 -ef f2. They were not 
carried forward into the test utility when the conditional command was removed 
from the shell because they have not been included in the test utility built 
into historical implementations of the sh utility.
That might change in the future though as the feature is widely supported.

Note that when operands are symlinks, it's the modification time of the target 
of the symlink that is considered (which is generally what you want, use find 
-newer instead if not). When symlinks cannot be resolved, the behaviour between 
implementations (some consider an existing file as always newer than one that 
can't resolve, some will always report false if any of the operands can't be 
resolved).

Also note that not all implementations support sub-second granularity (bash's 
test/[ builtin as of version 4.4 still doesn't for instance, GNU test and the 
test builtin of zshor ksh93 do, at least on GNU/Linux).

For reference:

For the GNU test utility implementation (though note that your shell, if fish 
or Bourne-like, will also have a test/[builtin that typically shadows it, use 
env test instead of test to bypass it), get_mtime in test.c reads struct 
timespec, and
option -nt uses that data
shareimprove this answer
edited Jun 26 '17 at 9:48

Stéphane Chazelas
317k57601961
answered Jun 23 '17 at 0:50

Thomas Dickey
54.5k5106182
thanks, will this give me a modified time that is more granular than seconds? 
about to find out... – Alexander Mills Jun 23 '17 at 0:50
my guess is that the modification time of files is no more granular than 
seconds, even with this tool? – Alexander Mills Jun 23 '17 at 0:52
1
According to the source-code, it compares struct timespec, which is potentially 
finer-grained (not all filesystems support subsecond timestamps, but you're 
likely to get what you wanted...)– Thomas Dickey Jun 23 '17 at 1:06
nice this is mucho bueno...accept accept accept – Alexander Mills Jun 23 '17 at 
1:14
seems to work on OSX sierra, but then again I installed some gnu tools on here 
I am sure at some point so...dunno for sure – Alexander Mills Jun 23 '17 at 1:14
show 2 more comments

1

In testing on this linux system. The usual way to test file times is the shell:

[ file1 -nt file2 ] && echo "yes"
Seems to work with seconds. This, which will touch the files with a time 
difference less than a second, doesn't detect that difference:

$ touch file2; sleep 0.1; touch file1; [ file1 -nt file2 ] && echo "yes"
To confirm the issue (time after the dot is nanoseconds):

$ ls --time-style=full-iso -l file? -rw-r--r-- 1 user user 0 2017-06-23 
01:37:01.707387495 -0400 file1 -rw-r--r-- 1 user user 0 2017-06-23 
01:37:01.599392538 -0400 file2
The file1 is (a bit) newer than file2.

The problem now will be to correctly process the time value.

One solution is to use a formatted output of ls:

$ ls --time-style=+%s.%N -l file? -rw-r--r-- 1 user user 0 1498196221.707387495 
file1 -rw-r--r-- 1 user user 0 1498196221.599392538 file2
Extracting the time to two variables (without the dot):

$ file1time=$(ls --time-style=+%s%N -l file1 | awk "{print(\$6)}") $ 
file2time=$(ls --time-style=+%s%N -l file2 | awk "{print(\$6)}")
And compare the times (times with nanoseconds just barely fit in a 64 bit 
value. If your system does not use 64 bit, this comparison will fail):

$ [ $file1time -gt $file2time ] && echo "yes" yes
That shows that file1 is newer than file2

If ls fails to have the format needed, then you may try stat. 

$ stat file1 File: file1 Size: 0 Blocks: 0 IO Block: 4096 regular file Device: 
805h/2053d Inode: 9180838 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 1000/ user) 
Gid: ( 1000/ user) Access: 2017-06-23 01:37:01.707387495 -0400 Modify: 
2017-06-23 01:37:01.707387495 -0400 Change: 2017-06-23 01:37:01.707387495 -0400 
 Birth: -
If the output shows nanoseconds, the we will need date to parse (and format) 
the time.

$ stat --printf='%y\n' file1 2017-06-23 01:37:01.707387495 -0400 $ date +'%s%N' 
-d "$(stat --printf='%y\n' file1)" 1498196221707387495
The rest is the same, assign the results of file1 and file2 to two variables 
and numerically compare them.

shareimprove this answer
answered Jun 23 '17 at 7:45

Arrow
2,518218
so you're saying that -ot and -nt only use seconds for comparison?– Alexander 
Mills Jun 26 '17 at 2:18
looks like it, that's pretty lame, this is a good answer – Alexander Mills Jun 
26 '17 at 2:21
add a comment

1

If you're willing to assume non-embedded Linux, then you can use the test 
external command, which is part of GNU coreutils. (test is another name for [ 
and is a builtin in most shells). It has nanosecond granularity (up to the 
precision reported by the filesystem).

/usr/bin/test "$target" -nt "$source"
The -nt operator isn't defined by POSIX, but it's present in many 
implementations including dash, bash, pdksh, mksh, ATT ksh, zsh, GNU coreutils 
test, and BusyBox. However many implementations (dash, bash, pdksh, mksh, 
BusyBox — tested on Debian jessie) only support 1-second granularity.

But it would be a better idea to use tools dedicated to this job, such as make. 
Running a command only if a certain file is newer than some other file is the 
whole point of make. With the following content in a file called Makefile (note 
that you need a tab character before eacho command line).

target: source echo This command is only executed if target is newer than 
source do_stuff <source >$@
Run make target to execute the commands that generate it. If target exists and 
is newer than source, the commands are not executed. Read some documentation of 
make for more information.

shareimprove this answer
edited Jun 26 '17 at 6:19
answered Jun 25 '17 at 23:14

Gilles
551k13211231637
nice, yeah something more granular than seconds would be very handy. Can you 
add an example using make? – Alexander MillsJun 26 '17 at 1:03
you are saying that /usr/bin/test is different than /usr/bin/bash or 
/usr/bin/sh? – Alexander Mills Jun 26 '17 at 1:04
@AlexanderMills I added an example but I do recommend that you read some 
documentation or tutotials. /usr/bin/test and shell builtins test implement 
mostly the same functionality but there can be minor diferences, and support 
for sub-second resolution of timestamps is one of these minor differences. – 
GillesJun 26 '17 at 6:21
add a comment

0

POSIXly, you'd use find:

if find "$source_file" -prune -newer "$target_file" | grep -q '^'; then printf 
'%s\n' "$source_file is newer than $target_file" else echo "It's not newer or 
one of the files is not accessible" fi
For symlinks, that compares the mtime of the symlinks themselves. To compare 
the targets of the symlinks, add the -Hor -L option.

That assumes $source_file doesn't start with - and doesn't otherwise correspond 
to one of the find predicates. If you need to deal with arbitrary file names, 
you'll need to do things like this first:

case $source_file in (["-+()!"]*) source_file=./$source_file;; esac
GNU and FreeBSD find implementations at least do support sub-second 
granularity. AFAICT, macos doesn't seem to even store sub-second time 
information in the file attributes on the HFS+ file system at least.

shareimprove this answer
answered Jun 26 '17 at 10:40

Stéphane Chazelas
317k57601961
add a comment

0

GNU date can format the mtime of a file to be the nanoseconds since epoch:

date +%s%N --reference file
Those numbers are comparable with test or '['. It even works on bash v3.00 on 
an i386 (32-bit) system (circa 2008).

xxx=$(date +%s%N --reference file1) yyy=$(date +%s%N --reference file2) [ $xxx 
-lt $yyy ] && echo file1 is older than file2
shareimprove this answer
edited Jul 20 '18 at 23:40
answered Jul 20 '18 at 23:29

jmccanta
11
add a comment
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