After installing Subversion a folder /volume1/svn/repositories/hooks was created. This folder contains several template files (ending with .tmpl). If you want you can use one of these pre-defined templates. We are going to adjust the pre-commit.tmpl in such way that it won’t allow users to commit files without a comment.

1. Go to cd /volume1/svn/repositories/hooks
2. Type cp pre-commit.tmpl pre-commit to copy the template file to a new file without the .tmpl extension.
3. Open the file with the build-in VI editor. To do this type vi pre-commit
4. Search for a string which looks like $SVNLOOK log -t "$TXN" "$REPOS" | \ grep "[a-zA-Z0-9]" > /dev/null || exit 1 . You can use the arrows. This piece of code already checks whether a comment has been filled in but won’t give you the reason why it couldn’t commit.
5. Now press (SHIFT + A). This will jump to the end of the line. VI editor is now in “edit” mode.
6. Change that line to

$SVNLOOK log -t "$TXN" "$REPOS" | \
grep "[a-zA-Z0-9]" > /dev/null || { echo "<<Empty Log Message>>" >&2; exit 1;} 

It will check if a comment is filled in, otherwise it will exit with exit code 1. If all checks passed then the script will exit with exit code 0.
7. Press ESC to get out the “edit” mode.
8. Type :wq to save the file and exit the VI editor.
9. Type chmod u+x pre-commit to set the right permissions, otherwise you will get an exit code 255, instead of the desired exit code 1.
10. Now try to commit without a comment you will get something like the screenshot below.

You can also download more examples or pre-written scripts, here and
here.

Complete script (Without comments)

#!/bin/sh

REPOS="$1"
TXN="$2"

# Make sure that the log message contains some text.
SVNLOOK=/opt/bin/svnlook
$SVNLOOK log -t "$TXN" "$REPOS" | \
   grep "[a-zA-Z0-9]" > /dev/null || { echo "<<Empty log message>>" >& 2; exit 1; }

# All checks passed, so allow the commit.
exit 0

To understand a bit more of how Subversion exactly works, visit this website, where you can download a free e-book in a lot of different formats (like pdf / HTML).