How to Declare and Use Boolean Variables in Bash Scripts

Bash does not have a native boolean type, but you can represent boolean values using numeric 0 (false) and 1 (true) or the strings "true" and "false". The article shows how to declare such variables, test them, and use them in real‑world scripts.

Declaring Boolean Variables

You can assign numeric values:

failed=0   # false
jobdone=1  # true

Or use more readable string literals:

failed=false
jobdone=true

Testing Numeric Booleans

When the variable holds 0 or 1, use a numeric comparison:

if [ $failed -eq 1 ]
then
    echo "Job failed"
else
    echo "Job done"
fi

Testing String Booleans

When using "true"/"false" strings, compare the values as strings:

failed="false"
if [ "$failed" == "true" ]
then
    echo "Job failed"
else
    echo "Job done"
fi

You can also use the double‑bracket syntax for string comparison:

bool=false
if [[ "$bool" = true ]]
then
    echo "Done."
else
    echo "Failed."
fi

Practical Example: Backup Script with Failure Flag

A sample script shows how to initialize failed=0, run rsync for multiple servers, set failed=1 if any rsync command returns a non‑zero status, and then send an email alert if failed equals 1.

#!/bin/bash
source "/apps/.keychain/$HOSTNAME-sh"
source "/apps/scripts/cli_app.sh"
failed=0
D="/nfsefs/ec2mum/prodwwwroot"
log="/tmp/server.log.$$.txt"
exec 3>&1 4>&2
trap 'exec 2>&4 1>&3' 0 1 2 3
exec 1>"${log}" 2>&1
for s in www-0{1..8}
do
    /usr/bin/rsync -az --delete backupt@${s}:/data/apps/ ${D}/${s}/
    [ $? -ne 0 ] && failed=1
done
if [ $failed -eq 1 ]
then
    echo "$0 script error: rsync backup failed. See attached log file." |
        mail -A ${log} -s "$HOSTNAME - LXD backup failed" -r sysuser@my-corp-tld alert@somewhere-tld
    push_to_mobile "$0" "Backup failed at $HOSTNAME"
fi
[ -f "${log}" ] && rm -f "${log}"

Additional Boolean Checks

The article also demonstrates capturing command exit status, scanning logs for errors, and sending alerts based on combined conditions:

status=$?
alogs="$(egrep -w '^ERROR:|ERROR' $log)"
if [ $status -ne 0 ] || [ "$alogs" != "" ]; then
    # send failure notification
else
    # success handling
fi

Summary

By treating 0/1 or "true"/"false" as boolean equivalents, Bash scripts can implement clear success/failure logic, especially for cron jobs, backups, and monitoring tasks. Refer to man bash, help test, and help if for further details.