How to handle a missing environment variable set in `crontab`

Question

I have a bash script that I need to be able to start from either:

1. cron
2. an interactive (logon) shell

This script needs to know whether it was started from cron, or started from an interactive shell. I thought I had resolved this issue when I declared an environment variable in the root crontab:

RUN_BY_CRON="TRUE"

In the script, I test RUN_BY_CRON and use the result to set another variable:

if [ "$RUN_BY_CRON" = "TRUE" ]; then            
    ((wait_time=DELAY_HALT*60))
fi

This worked until I added set -u to my script (as a "common defensive programming strategy"). Since then, when I run the script from the command line, set -u flags RUN_BY_CRON as an "unbound variable" error:

$ sudo ./skedrtc.sh
./skedrtc.sh: line 24: RUN_BY_CRON: unbound variable

FWIW, I ran shellcheck on this script, and got no warning or error.

I tried adding a test for RUN_BY_CRON, but got the same error. I tried testing for an interactive shell, but testing from within the script itself isn't helpful:

...
if [ -z "$RUN_BY_CRON" ]; then        # test for null string
    RUN_BY_CRON="FALSE"
fi
...

if [[ $- == *i* ]]; then              # test for interactive shell
    RUN_BY_CRON="FALSE"
fi

This feels like a "catch 22" situation. I've looked for ways to create a try-catch block, but AIUI there's nothing like that in bash.

Q: How can I avoid this "unbound variable" error without removing the set -u ?

Answer 1

Potentially you could use the standard ${var-string} syntax¹ which means that if $var is unset then it expands to string otherwise it expands to the value of the variable and (like the other related variants: ${var+string}, ${var=string}, ${var?string}, ${var:-string}, ${var:+string}, ${var:?string}¹) bypasses the effect of the nounset option.

e.g

#!/bin/bash

set -u # same as, shorter, though less legible than set -o nounset

printf '%s\n' "${NOSUCHVARIABLE-FALSE}"

Now if I run this normally:

% ./x
FALSE

So there's no error, and the value is expanded to a "default" value.

But if the value it set:

% NOSUCHVARIABLE=hello ./x
hello

This even works if the variable is set but is empty

% NOSUCHVARIABLE= ./x

If you also want it to replace the empty string then use :- instead;

#!/bin/bash

set -u

printf '%s\n' "${NOSUCHVARIABLE:-FALSE}"

% NOSUCHVARIABLE="" ./x
FALSE

A not uncommon pattern is to have code do something like:

NOSUCHVARIABLE="${NOSUCHVARIABLE-default value}"

Or:

: "${NOSUCHVARIABLE=default value}"

at the beginning to ensure there's a default value set.

In ksh, bash or zsh, you can also the the -v predicate of their builtin [ utility or of the [[ ... ]] construct to test whether a variable is set³ which would also allow bypassing nounset:

if [[ ! -v RUN_BY_CRON || -z "$RUN_BY_CRON" || $- = *i* ]]; then
  RUN_BY_CRON=FALSE
fi

The standard sh equivalent of ksh's [ -v var ] being [ -n "${var+set}" ].

---

^{¹ Originally from the Bourne shell, like the -u option, aliases to -o nounset in ksh. All of that specified by POSIX for sh}

^{² Also in zsh: ${var::=string} for unconditional assignment and ${+var} which expands to 1 or 0 depending on whether $var is set or not.}

^{³ With some variations between shells for variables that are declared but not set, or set to an empty list for arrays or associative arrays.}

Answer 2

The genesis of the Question here concerns how to make a reliable determination as to whether a script was started from a cron job - or from the command line (or some other method; e.g. systemd). In fact, it was my attempt to implement a suggestion in another answer here on SE that led to the question above.

In thinking about this problem, I developed another approach that also seems to work. It's shown below as a bash function; it utilizes the ability of pstree to provide the entire process tree for the PID of the current shell ($$). Also note that usage of the regex [c]ron, prevents grep from including its own process in the output. I'll share it here as it may be of interest to some:

cron_ancestry() {
    if [[ -n $(pstree -aps $(echo $$) | grep -m 1 "[c]ron,") ]]; then
        echo "cron is your daddy"
    else
        echo "spawned by unknown"
    fi
}