Check if script is started by cron, rather than invoked manually

I'm not aware that cron does anything to its environment by default that can be of use here, but there are a couple of things you could do to get the desired effect.

1) Make a hard or soft link to the script file, so that, for example, myscript and myscript_via_cron point to the same file. You can then test the value of $0 inside the script when you want to conditionally run or omit certain parts of the code. Put the appropriate name in your crontab, and you're set.

2) Add an option to the script, and set that option in the crontab invocation. For example, add an option -c, which tells the script to run or omit the appropriate parts of the code, and add -c to the command name in your crontab.

And of course, cron can set arbitrary environment variables, so you could just put a line like RUN_BY_CRON="TRUE" in your crontab, and check its value in your script.

Scripts run from cron are not run in interactive shells. Neither are startup scripts. The differentiation is that interactive shells have STDIN and STDOUT attached to a tty.

Method 1: check if $- includes the i flag. i is set for interactive shells.

case "$-" in
    *i*)
        interactive=1
        ;;
    *)
        not_interactive=1
        ;;
esac

Method 2: check is $PS1 is empty.

if [ -z "$PS1" ]; then
    not_interactive=1 
else
    interactive=1
fi

reference: http://techdoc.kvindesland.no/linux/gnubooks/bash/bashref_54.html

Method 3: test your tty. it's not as reliable, but for simple cron jobs you should be ok, as cron does not by default allocate a tty to a script.

if [ -t 0 ]; then
    interactive=1
else
    non_interactive=1
fi

Keep in mind that you can however force an interactive shell using -i, but you'd probably be aware if you were doing this...

First, get cron's PID, then get the current process's parent PID (PPID), and compare them:

CRONPID=$(ps ho %p -C cron)
PPID=$(ps ho %P -p $$)
if [ $CRONPID -eq $PPID ] ; then echo Cron is our parent. ; fi

If your script is started by another process that might have been started by cron, then you can walk your way back up the parent PIDs until you get to either $CRONPID or 1 (init's PID).

something like this, maybe (Untested-But-It-Might-Work<TM>):

PPID=$$   # start from current PID
CRON_IS_PARENT=0
CRONPID=$(ps ho %p -C cron)
while [ $CRON_IS_PARENT -ne 1 ] && [ $PPID -ne 1 ] ; do
  PPID=$(ps ho %P -p $PPID)
  [ $CRONPID -eq $PPID ] && CRON_IS_PARENT=1
done

From Deian: This is a version tested on RedHat Linux

# start from current PID
MYPID=$$
CRON_IS_PARENT=0
# this might return a list of multiple PIDs
CRONPIDS=$(ps ho %p -C crond)

CPID=$MYPID
while [ $CRON_IS_PARENT -ne 1 ] && [ $CPID -ne 1 ] ; do
        CPID_STR=$(ps ho %P -p $CPID)
        # the ParentPID came up as a string with leading spaces
        # this will convert it to int
        CPID=$(($CPID_STR))
        # now loop the CRON PIDs and compare them with the CPID
        for CRONPID in $CRONPIDS ; do
                [ $CRONPID -eq $CPID ] && CRON_IS_PARENT=1
                # we could leave earlier but it's okay like that too
        done
done

# now do whatever you want with the information
if [ "$CRON_IS_PARENT" == "1" ]; then
        CRON_CALL="Y"
else
        CRON_CALL="N"
fi

echo "CRON Call: ${CRON_CALL}"