commit-msg-check.sh

#!/bin/sh
#
# A (client-side) git hook script which warns if there are too long lines in
# the commit message, not fitting with 50/72 formatting.
#
# To use this script, copy it as .git/hooks/commit-msg and give it an executable
# file permission.
#
FILE=$1

msg_lines() {
  cat "$FILE" \
  | sed 's/^#.*//' \
  | sed '/./=' \
  | sed '/^$/{p;d}; N; s/^/     /; s/ *\(.\{6,\}\)\n/\1  /' \
  | sed -n '/./,$p'
}

first_line() {
  msg_lines | head -1
}

second_line() {
  N=$(msg_lines | wc -l | cut --characters=-7)
  if [ $N -gt 1 ] ; then
    msg_lines | head -2 | tail -1
  else
    echo ""
  fi
}

rest_lines() {
  N=$(msg_lines | wc -l | cut --characters=-7)
  if [ $N -gt 2 ] ; then
    msg_lines | sed '1,2d'
  else
    echo ""
  fi
}

my_warned=false

if first_line | sed -n '/\(.\)\{81\}/p' | grep '.' >/dev/null; then
  echo '' >&2
  echo 'WARNING: The first line should be 72 characters or less.' >&2
  first_line >&2
  my_warned=true
elif first_line | sed -n '/\(.\)\{59\}/p' | grep '.' >/dev/null; then
  echo '' >&2
  echo 'WARNING: The first line is recommended to be 50 characters or less.' >&2
  first_line >&2
  my_warned=true
fi

if second_line | grep '.' >/dev/null; then
  echo '' >&2
  echo 'WARNING: The second line should be blank.' >&2
  second_line >&2
  my_warned=true
fi

if rest_lines | sed -n '/\(.\)\{81\}/p' | grep '.' >/dev/null; then
  echo '' >&2
  echo 'WARNING: Text should be wrapped at 72 characters.' >&2
  rest_lines | sed -n '/\(.\)\{81\}/p' | grep '.' >&2
  my_warned=true
fi

if $my_warned; then
  echo '' >&2
  echo '    use "git commit --amend" etc. to reedit the commit message.' >&2
  echo '' >&2
fi