A Bash script that you can run to check if your commit message conforms to a specific format.

check-commit-message.sh

#!/usr/bin/bash

# How to use this Bash script:
# You can simply copy and paste the contents of this Bash script
# to your repository's .git/hooks/commit-msg file.
# Alternatively, copy and paste this file in your repository's .git/hooks
# directory and in commit-msg, type the following to execute this script:
#       ./.git/hooks/check-commit-message.sh "$1"
# Note that the script exits with 0 when no issues are found, and 1
# when issues are found.

# debugging options (uncomment set command to enable)
# x: Show what the lines looked like after evalution
# u: Warn us if we access variables that aren't set with any value
# set -xu

# check if we have the valid number of arguments
if [[ $# -eq 0 ]]; then
    echo "Missing argument for filename" >> /dev/stderr
    echo "Usage: $0 <file>" >> /dev/stderr
    exit 1
fi

if [[ $# -gt 1 ]]; then
    echo "Too many arguments" >> /dev/stderr
    echo "Usage: $0 <file>" >> /dev/stderr
    exit 1
fi

echo "Running a check on your message..."

# commit subject line issues
commitSubjectNotProvided=0
commitSubjectExceedLimit=0
commitSubjectHasTrailingPeriod=0

# second line issues
secondLinePresent=0
secondLineBlank=0

# commit description issues
hasExceedingLines=0
linesExceedingLimit=()

linesRead=0
# in order to handle files with no newline at the end, we need to add the additional
# check for a non-empty line since read returns a false if a line doesn't end with a newline (?)
while read -r currentLine || [[ -n "$currentLine" ]]; do
    linesRead=$((linesRead + 1))

    # ignore lines starting with a pound symbol (#)
    if [[ "$currentLine" =~ ^\# ]]; then
        continue;
    fi

    # assume first line is commit subject
    if [[ $linesRead -eq 1 ]]; then

        # a commit subject must be provided
        if [[ "$currentLine" =~ ^[[:space:]]+$ ]] ||
           [[ -z "$currentLine" ]]; then
            commitSubjectNotProvided=1
        fi

        # commit subject shall not exceed 50 characters
        if [[ ${#currentLine} -gt 50 ]]; then
            commitSubjectExceedLimit=1
        fi

        # commit subject shall not have a trailing period
        if [[ "$currentLine" =~ \.[[:space:]]+$ ]]; then
            commitSubjectHasTrailingPeriod=1
        fi

    # second line of commit message
    elif [[ $linesRead -eq 2 ]]; then
        secondLinePresent=1

        # the second line shall contain just whitespace characters
        if [[ "$currentLine" =~ ^[[:space:]]+$ ]] ||
           [[ -z "$currentLine" ]]; then
            secondLineBlank=1
        fi

    # third line ownwards will be the commit description
    else

        # the current line in the commit description should
        # not exceed 72 characters
        if [[ ${#currentLine} -gt 72 ]]; then
            # record all the lines that exceed the 72-character limit
            hasExceedingLines=1
            linesExceedingLimit+=("$currentLine")
        fi

    fi

done < "$1"

commitMessageContainsIssues=0

# issues in the subject of the commit message
if [[ $commitSubjectExceedLimit -eq 1 ]] || 
   [[ $commitSubjectHasTrailingPeriod -eq 1 ]] ||
   [[ $commitSubjectNotProvided -eq 1 ]] ||
   [[ $hasExceedingLines -eq 1 ]]; then
    commitMessageContainsIssues=1
fi

# issues in second line of the commit message
if [[ $secondLinePresent -eq 1 ]] && [[ $secondLineBlank -ne 1 ]]; then
    commitMessageContainsIssues=1
fi

if [[ $commitMessageContainsIssues -eq 1 ]]; then
    echo "Commit aborted due to issues with your message."
fi

if [[ $commitSubjectNotProvided -eq 1 ]]; then
    echo "- Commit subject not provided"
fi

if [[ $commitSubjectExceedLimit -eq 1 ]]; then
    echo "- Commit subject longer than 50 characters"
fi

if [[ $commitSubjectHasTrailingPeriod -eq 1 ]]; then
    echo "- Commit subject has a trailing period"
fi

if [[ $secondLinePresent -eq 1 ]] && [[ $secondLineBlank -ne 1 ]]; then
    echo "- Second line not blank"
fi

if [[ $hasExceedingLines -eq 1 ]]; then
    echo "- Description line(s) longer than 72 chars"
    echo ""
    echo "The following line(s) are longer than 72 chars:"

    for descLine in "${linesExceedingLimit[@]}"; do
        echo " - $descLine"
    done
fi

if [[ $commitMessageContainsIssues -eq 1 ]]; then
    echo ""
    echo "Resolve the issues above to continue."
    exit 1
fi

echo "No issues found."
exit 0