Crontab Task Not Running? 90% of Ops Die in These 3 Environment Variable Traps

Mysterious "Disappearance" Scene

You wrote a perfect backup script backup.sh:

#!/bin/bash
# Backup database
mysqldump -u root -p123456 dbname > /backup/db.sql
echo "Backup done" >> /tmp/log.txt

Running ./backup.sh manually works perfectly.
So you confidently add it to Crontab:

0 3 * * * /home/user/backup.sh

The next morning, you check /backup/db.sql —— file does not exist.
Check /tmp/log.txt —— no logs either.
You start questioning life: Did the server restart at midnight? Is Crontab not running?

Stop guessing, 90% of the reasons are environment variable issues.

Trap 1: "Amnesia" of the PATH Variable

When you log into the terminal (Shell), the system automatically loads configuration files like /etc/profile and ~/.bashrc. Your PATH variable contains common paths like /usr/local/bin and /usr/bin.

However, the environment when Crontab executes is "barebones".

In the Crontab execution environment, PATH is usually only /usr/bin:/bin. If your mysqldump is installed at /usr/local/mysql/bin/mysqldump, the script will throw an error: command not found.

✅ Solution

Method 1: Use Absolute Paths (Recommended)

Write all commands in the script with absolute paths:

#!/bin/bash
/usr/local/mysql/bin/mysqldump -u root ...

Method 2: Manually Define PATH

Redefine PATH at the beginning of the script:

#!/bin/bash
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
# Your business logic

Trap 2: "Getting Lost" with Relative Paths

#!/bin/bash
# Wrong demonstration
cat ./config.ini

When executing in Crontab, the Current Working Directory (CWD) is usually the user's home directory (~), not the directory where the script is located. If you use ./config.ini in your script, it will look in the home directory, not in the same directory as the script.

✅ Solution

cd to the script's directory first:

#!/bin/bash
cd "$(dirname "$0")" || exit
# Now you can use relative paths
cat ./config.ini

Trap 3: Special Meaning of the % Symbol

You want to write a single line command in Crontab with a timestamp:

# Wrong demonstration
* * * * * echo "Run at $(date +%Y-%m-%d)" >> /tmp/cron.log

You'll find that the command doesn't execute at all, or throws an error.
The reason is: In Crontab files, % is treated as a newline character. Everything after the first % is sent to the command as standard input.

✅ Solution

You must escape %:

# Correct demonstration
* * * * * echo "Run at $(date +\%Y-\%m-\%d)" >> /tmp/cron.log

Ultimate Debugging Method

If you've checked all of the above and it still doesn't work, add this to your Crontab entry:

* * * * * /path/to/script.sh > /tmp/cron_debug.log 2>&1

This will redirect both standard output (stdout) and standard error (stderr) to a log file. Open the log, and the error message will usually be clear at a glance.

Ops rely not only on experience but also on tools. Having trouble with time calculations? Try our Timestamp Converter, which supports second/millisecond conversion and time zone handling.