Public bug reported:
I wrote a script to automatically edit a crontab across a fleet of machines.
The script works something like this:
ex $crontab << EOT
<ex commands to effect the changes I want>
wq!
EOT
I set EDITOR to point at that script, and then run "crontab -e", only to
find that the program says "No modification made". I carefully debugged
my script to ensure that it was actually making a change. In spite of
the change, crontab(1) claims "No modification made" and rejects the
changes.
After other attempts at troubleshooting failed, I delved into the source
code. The "No modification made" error is reported after "crontab"
checks the file modification time, which it does both before and after
calling $EDITOR.
The check is done like this:
if (statbuf.st_mtime == fsbuf->st_mtime) {
return 1; /* No change to file */
}
According to the stat(2) Linux man page, "st_mtime" is actually a macro
defined as:
#define st_mtime st_mtim.tv_sec
So it seems that the problem is that editing a file by script takes less
than a second, and this method of checking a file's modification time
only provides 1-second resolution.
It seems that attempting to move away from using the st_mtime field
would introduce some portability problems, since Linux and MacOS X
provide incompatible ways to get subsecond precision on a file's
modification time.
In the meantime, I will call sleep(1) from my script to ensure that
enough time passes that crontab can see that there was a change.
** Affects: cron (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1854891
Title:
Editing crontab by script doesn't work
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