I have this shell script to update IP addresses in my configuration files (any that match $old_address_pattern must be changed to $new_address):
grep -rl "$old_address_pattern" /etc \
| xargs sed -i "s/$old_address_pattern/$new_address/g"
If the grep command finds no matching files, then sed will complain 'no input files'. How can I make this pipeline succeed when the list of files is empty?
If you want to avoid running sed when grep produces no output, then (since you've tagged this with Ubuntu), you can give the -r or --no-run-if-empty argument to xargs:
--no-run-if-empty
-r
If the standard input does not contain any nonblanks, do not run the command. Normally, the command is run once even if there is no input. This option is a GNU extension.
So your command should look like:
grep -rlZ "$old" /etc | xargs -0 -r sed -i "s/$old/$new/g"
(I added grep -Z and xargs -0 flags, since these are supported on your platform, and they make the commands more robust to malicious filenames)
For platforms without xargs -r, then the usual solution is to pass /dev/null as a first filename argument:
grep -rl "$old" /etc | xargs sed -i "s/$old/$new/g" /dev/null
In this case, when there are no matches, sed will operate harmlessly on the null device.