Unpack and repack a library archive file, allowing mapping a script over each contained object

repack.zsh

#!/usr/bin/zsh
# Not portable, sorry...
set -e 
set -o pipefail
# set -x

# Change to your command. Below we assume that this:
#  - deletes (or replaces) the original file
#  - the new/replacement object file must be in the same directory as the source file (need not be same name though)
COMMAND="ls -l"
# Calls COMMAND on objects in all archives under this dir, and repacks
TARGET_DIR=$(realpath "$@")

# ...or just take the body of this loop and adapt it if you want to pack a single archive
for f in `find "$TARGET_DIR" -name '*.a' -type f`; do 
  grep -q "ar archive" <(file "$f") 2>/dev/null || continue
  echo -n "$f:  "

  TMP_DIR=$(mktemp -d)
  cd "$TMP_DIR"
  
  # Individual libraries can and do have identical names, which makes this a huge pain.
  # We unpack each member into a separate numbered directory, and repack in the same order
  O_NUM=1
  for cnt nm in `ar t "$f" | sort | uniq -c`; do
    for c in {1..$cnt}; do
        mkdir $O_NUM
        cd $O_NUM
        ar xN $c $f $nm
        cd ..
        ((O_NUM++))
    done
  done

  # optimize all objects we just unpacked (some may not be named *.o)
  for f in */* ; do "$COMMAND" "$f"; done

  # rearchive; use zsh numeric blob qualifier.
  ar cq repackaged.a */*(n)
  # Backup original
  cp --force --backup=numbered "$f" "$f"
  cp repackaged.a "$f"

  rm -r "$TMP_DIR"

done