Stop cron script from destroying my mirrorlist with invalid data

Question

I have to following cron script that runs daily. As you should be able to see from the code, it outputs the results from reflector to /etc/pacman.d/mirrorlist.

$ cat /etc/cron.daily/update-mirrorlist
#!/bin/bash

reflector -l 5 -r -o /etc/pacman.d/mirrorlist

Sometimes, reflector outputs a empty file and thus a invalid mirrorlist is created.

How can I modify the script above to only write to /etc/pacman.d/mirrorlist IF there is valid output from relfector?

Answer 1

It's a good idea to first accumulate the data, then move it into place. That way the target file will always be valid, even while the data accumulator program is running.

set -e
target=/etc/pacman.d/mirrorlist
reflector -l 5 -r -o "$target.tmp"
mv -f -- "$target.tmp" "$target"

If reflector does not properly report errors by returning a nonzero status, add your own validation test before the mv command, for example test -s "$target.tmp" to test that the file is not empty.

If you want to keep a backup of the old version, add ln -f -- "$target" "$target.old" || true before the mv command.