mjhjmtu

I have files in a directory where filenames are like

01 ABC DEF.m4a
02 DEF ABC.m4a
etc...

I want to convert these to

1.m4a
2.m4a
etc...

How can I do this using the command line?

If you have the perl version of rename (sometimes called prename) you can use this

rename -n 's/^0*(d+).*(.m4a)z/$1$2/s' [0-9]*.m4a

When you're happy it's going to do what you want, remove the -n flag.

This uses a Regular Expression match:

• 
  ^ - require start-of-subject


• 
  0* - match zero or more "0"


• 
  (d+) - match and remember one or more decimal digits


• 
  .* - match everything until...


• 
  (.m4a) - match and remember literal ".m4a"


• 
  z - require end-of-subject¹


• 
  s flag - make sure . matches any byte (including newline, also valid in file names)

and then uses $1 and $2 to reference the value of the bracketed expressions.

^{¹ Not strictly necessary here as the file names given by the shell as argument all end in .m4a and the previous .* is greedy. $ would also work here, but in rename which works on file names (which can be any sequence of non-0 bytes including newline), z is preferable as $ matches at the end of the subject like z but also before an eventual last newline character.}

How for would

for FN in *.m4a; do mv -i "$FN" "$FN%% *.m4a"; done

get you?