Safe Bash Defaults — set -euo pipefail¶
Overview¶
In Bash scripting, the combination set -euo pipefail enforces strict error handling and predictable execution.
It ensures that your script fails fast, loudly, and for the right reasons — essential when automating cron jobs, backups, or server tasks.
What Each Option Does¶
set -e → Exit on Error¶
Stops the script immediately if any command fails (non-zero exit code).
set -e
echo "Starting..."
cp /backup/source.txt /backup/dest/ # If file missing, script stops here
echo "Done!" # Never reached if cp fails
````
**Use Case:**
Avoids cascading errors when an early step (like a copy or query) fails silently.
---
### **`set -u` → Treat Undefined Variables as Errors**
If the script tries to use a variable that hasn’t been defined, Bash will throw an error and exit.
```bash
set -u
echo "Home: $HOME"
echo "Tmp: $TMP_DIR" # If TMP_DIR isn’t set → error
Use Case:
Catches typos like $HMOE instead of $HOME.
set -o pipefail → Catch Failures in Pipelines¶
Without pipefail, Bash ignores failures from all but the last command in a pipeline.
With it, Bash treats any failure within a pipeline as an overall failure.
set -o pipefail
grep "pattern" file.txt | sort | uniq > output.txt
If grep fails (file missing), the pipeline now reports a failure correctly.
Use Case:
Accurate error handling in multi-stage operations (log parsing, monitoring, data processing).
Safe Script Template¶
#!/bin/bash
set -euo pipefail
IFS=$'\n\t' # Optional: safer word splitting
LOGFILE="/var/log/safe-script.log"
{
echo "=== Script started at $(date) ==="
cp /important/data.txt /backup/
echo "Backup complete!"
} >> "$LOGFILE" 2>&1
Behavior:
-
Script stops on any error.
-
Undefined variables trigger failure.
-
Pipeline errors are caught.
-
Output is logged with timestamps.
Why It Matters¶
| Problem | Without set -euo pipefail |
With set -euo pipefail |
|---|---|---|
| Missing file | Script continues silently | Script stops immediately |
| Typo in variable | Expands to empty string | Script aborts |
| Pipeline failure | Ignored | Properly detected |
This makes your scripts reliable and reproducible, especially under automation frameworks like cron, systemd, or Nagios plugins.
Pro Tips¶
-
Always include this at the start of production scripts:
```bash
!/bin/bash¶
set -euo pipefail IFS=$'\n\t' ```
-
When a command may fail intentionally, handle it explicitly:
bash grep "pattern" file.txt || true -
Log important steps with timestamps:
bash echo "$(date): Task completed" >> /var/log/your-script.log -
Test interactively before scheduling via cron.