Alright, let's dig into why your cron job might not be running even though the script works perfectly when you run it manually. I've troubleshooted this exact scenario countless times, so let's break down the most common fixes step by step.

1. Fix Environment Variable Differences (The #1 Culprit) #

Cron runs in a stripped-down environment—its PATH is way shorter than your regular user shell, and it doesn't load your .bashrc or .profile by default. That means:

• Always use absolute paths for commands (like ping) and files in your script.
  • Find ping's absolute path with which ping (usually /bin/ping or /usr/bin/ping).
  • Use full paths for log files instead of relative ones (e.g., /home/youruser/ping_logs/google_ping.log instead of ping_logs/google_ping.log).

Example fixed script snippet:

#!/bin/bash
LOG_PATH="/home/youruser/ping_results.log"
/bin/ping -c 4 google.com >> $LOG_PATH 2>&1

2. Log Cron's Execution Output #

Cron doesn't show you errors by default—you have to capture them. Either:

• Check system cron logs (location varies by distro: /var/log/cron, /var/log/syslog, or /var/log/messages). Run tail -f /var/log/cron to watch real-time activity.
• Or, redirect all output from your cron job to a debug log:

  * * * * * /full/path/to/your/script.sh >> /home/youruser/cron_debug.log 2>&1
  

  This logs both standard output and errors, so you'll see exactly what's failing.

3. Verify Script Shebang & Permissions #

Double-check two key things:

• The script's shebang uses an absolute path for bash: #!/bin/bash (not #!/bash—cron might not find bash otherwise).
• Ensure the script is executable: chmod +x /full/path/to/your/script.sh
• Also confirm you have write permissions for the log directory (create it first if needed: mkdir -p /home/youruser/ping_logs).

4. Validate Cron Syntax & Task #

Run crontab -l to list your current cron jobs and make sure:

• The path to your script is absolute (no relative paths here either).
• The timing syntax is correct:
  • Test mode (every minute): * * * * * /full/path/to/script.sh
  • Production mode (every 10 minutes): */10 * * * * /full/path/to/script.sh
    (Remember: cron's order is minute, hour, day, month, week—don't mix these up!)

5. Simulate Cron's Environment Manually #

To replicate exactly how cron runs your script, use this command (replace paths with your own):

env -i PATH=/usr/bin:/bin HOME=/home/youruser /full/path/to/your/script.sh

If this fails, you'll know the issue is with missing environment variables, not the script itself.

6. Check if Cron Service is Running #

It sounds obvious, but sometimes cron can stop unexpectedly. Verify it's active:

# Systemd systems (Ubuntu 16.04+, Debian 9+, RHEL 7+)
systemctl status cron

# Older sysvinit systems
service cron status

If it's not running, start it with systemctl start cron or service cron start.

Example Working Setup #

Here's a complete example to reference:

Script (/home/jermayne/ping_google.sh)

#!/bin/bash
LOG_DIR="/home/jermayne/logs"
LOG_FILE="$LOG_DIR/google_ping.log"

# Create log directory if it doesn't exist
mkdir -p $LOG_DIR

# Log timestamp and ping results
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Starting ping test..." >> $LOG_FILE
/bin/ping -c 4 google.com >> $LOG_FILE 2>&1
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Ping test finished" >> $LOG_FILE
echo "----------------------------------------" >> $LOG_FILE

Set Permissions

chmod +x /home/jermayne/ping_google.sh

Cron Task (Add via crontab -e)

# Test: Run every minute
* * * * * /home/jermayne/ping_google.sh

# Production: Run every 10 minutes (uncomment later)
# */10 * * * * /home/jermayne/ping_google.sh

内容的提问来源于stack exchange，提问作者Jermayne Williams