mjhjmtu

I need to take the following filename structure and rename to the appropriate day of the week:

GMT20161003-randomtext.mp4

would end up as monday.mp4

I have a lot of these files in various subdirectories so it would be better if it could be run recursively...

With zsh:

zmodload zsh/datetime
autoload zmv
zmv -n '(**/)GMT(<->)*(.mp4)(#qD.)' '$1$(L)$(
 strftime %A "$(strftime -r %Y%m%d $2)")$3'

Remove the -n to actually do the renaming.

• 
  <-> matches any decimal number.


• That second (...) is captured in $2, (.mp4) in $3 and the directory ((**/), recursive) in $1.


• 
  (#qD.) is a glob qualifier that only selects regular files (.: not directories, nor symlinks nor fifos/devices...) and also traverse hidden directories (D for dotfile/dotdir).


• 
  $(L)...: converts the expansion to lower-case.


• 
  strftime -r %Y%m%d: reverse-strftime (strptime) to convert the date to an epoch time.


• 
  strftime %A ...: format time for that epoch time with %A being for the full week day. Beware it's locale-dependany. (in a French locale, you'll get the French week day).

On a GNU system, and with the GNU shell (bash), you could do:

find . -name 'GMT*-*.mp4' -type f -exec bash -c '
 for file do
 base=$file##*/
 date=$base#GMT
 date=$date%%-*
 wday=$(date -d "$date" +%A)
 echo mv -i "$file" "$file%/*/$wday,,.mp4"
 done' bash +

(remove echo to perform the operation).

$var,, being bash's operator to convert to lower case. date -d being the GNU date way to parse a date (like strftime -r above).

While zmv would check for conflicts before starting renaming any file, this one wouldn't. So we add a -i above to at least give you a chance of avoiding clobbering files. GNU mv has a -v option to tell it to show what it's going to do which may be useful to revert the command later if anything went wrong.