Found a script on my cron.monthly, i am wondering what's it doing?

Question

I am new to bash-programming, sorry if this might be a kind of stupid question. I found a script on my server and i am trying to understand every line:

#!/bin/sh
#
# man-db cron weekly

set -e

if ! [ -d /var/cache/man ]; then
    # Recover from deletion, per FHS.
    mkdir -p /var/cache/man
    chown man:root /var/cache/man || true
    chmod 2755 /var/cache/man
fi

# regenerate man database
if [ -x /usr/bin/mandb ]; then
# --pidfile /dev/null so it always starts; mandb isn't really a daemon,
# but we want to start it like one.
start-stop-daemon --start --pidfile /dev/null \
          --startas /usr/bin/mandb --oknodo --chuid man \
          -- --quiet
fi

exit 0

Some things i don't understand like

set -e

I read about it, that it "checks" your commands. So it gives warnings or an exception if something goes wrong? Or a more detailed error-report?

if ! [ -d /var/cache/man ]; then OR
if [ -x /usr/bin/mandb ]; then

What i don't understand are the "-d" or "-x" commands at the if clause, imho there is something missing there.

chown man:root /var/cache/man || true

I don't understand the last part, what's the reason for this "OR TRUE".

Maybe somebody can help me?

tia && regards noircc

Answer 1

set -e # Exit immediately if a command exits with a non-zero status.

The -d option in the if test is true if the path is a directory. ! negates the test.

    if ! [ -d /var/cache/man ]  # true if /var/cache/man is not a directory
    if [ -x /usr/bin/mandb ]    # true if /usr/bin/mandb is executable

Regarding chown man:root /var/cache/man || true - the code to the right of the || is actioned if the preceding chown command returns a non-zero exit status. If the chown command failed, true is evaluated. This is to prevent the script from failing - otherwise the chown non-zero error status would exit the script because of the set -e.

Answer 2

In bash, the -d switch is a "check if directory exists" and -x is for checking files. Set -e tells the script to exit if any commands fail (i.e, exit with a non-zero status).

The || true prevents a non-zero response from chown (if the command fails).