Bash script to add constant to file names

Question

I need a bash script to rename the files in a directory. The names have the format <name>_<number> (eg bob_12 , alice_233 ). The script must change them to <name>_<number+k> (eg, if k=20, bob_32 , alice_253 ). Can someone help?

Answer 1

$ ls
alice_253  bob_32
$ k=20
$ for old in *; do new=${old%_*}_$((${old#*_}+k)); mv "$old" "$new"; done
$ ls
alice_273  bob_52

This is just an example though. It won't work properly if there are files whose names have:

• dash as the first character,
• any number of underscores but one,
• non-digit characters at the righthand side of the underscore,
• a zero immediately following the underscore.

If we wanted to cover those cases as well, assuming filenames may not overlap (eg foo_100 and foo_120 when k=20 , in which case we'd need to sort them in reverse order first), we'd do:

# handle -foo_123
# skip foo, foo_bar, foo_012,
# and foo_bar_123
k=20
for old in ./*_*; do
  case $old in
    (*_*_*) ;&
    (*_*[!0-9]*) ;&
    (*_0*) continue
  esac
  new=${old%_*}_$((${old#*_}+k))
  mv "$old" "$new"
done