How-to check if Linux shell script is executed by a cronjob?

Question

Is it possible to identify, if a Linux shell script is executed by a user or a cronjob?

If yes, how can i identify/check, if the shell script is executed by a cronjob?

I want to implement a feature in my script, that returns some other messages as if it is executed by a user. Like this for example:

    if [[ "$type" == "cron" ]]; then
        echo "This was executed by a cronjob. It's an automated task.";
    else
        USERNAME="$(whoami)"
        echo "This was executed by a user. Hi ${USERNAME}, how are you?";
    fi

Answer 1

One option is to test whether the script is attached to a tty.

#!/bin/sh

if [ -t 0 ]; then
   echo "I'm on a TTY, this is interactive."
else
   logger "My output may get emailed, or may not. Let's log things instead."
fi

Note that jobs fired by at(1) are also run without a tty, though not specifically by cron.

Note also that this is POSIX, not Linux- (or bash-) specific.