Bash Scripting

Also Known As

TL;DR

Explanation

Bash scripts combine shell commands with variables, conditionals, loops, and functions. Key practices: always start with #!/bin/bash, use set -euo pipefail to fail on errors, quote all variable expansions to handle spaces, use [[ ]] over [ ] for safer conditionals, and prefer $() over backticks for command substitution. ShellCheck is a static analyser that catches common Bash mistakes automatically.

Diagram

flowchart TD
    subgraph Script_Structure
        SHEBANG[shebang bin bash]
        SET[set -euo pipefail<br/>fail on error undefined var pipe fail]
        VARS[Variables and functions]
        MAIN[Main logic]
        SHEBANG --> SET --> VARS --> MAIN
    end
    subgraph Control_Flow
        IF[if condition<br/>then ... fi]
        FOR[for item in list<br/>do ... done]
        WHILE[while condition<br/>do ... done]
        CASE[case var in<br/>pattern action esac]
    end
    subgraph Best_Practices
        QUOTE[Always quote variables<br/>file with spaces]
        LOCAL[Use local in functions]
        TRAP[trap cleanup EXIT<br/>cleanup on exit]
    end
style SET fill:#238636,color:#fff
style TRAP fill:#1f6feb,color:#fff
style QUOTE fill:#d29922,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Not using set -euo pipefail — scripts continue silently after errors.
• Unquoted variable expansions — $filename breaks if the filename contains spaces; use "$filename".
• Using [ ] instead of [[ ]] — [[ ]] handles empty variables and complex expressions safely.
• Not validating input arguments — scripts crash with cryptic errors when arguments are missing.

Code Examples

#!/bin/bash
# Missing set -e — continues on error
cd /var/www/app
git pull
php artisan migrate  # If this fails, next line still runs
php artisan cache:clear
# Partial deployment with failed migration

#!/bin/bash
set -euo pipefail  # Exit on error, undefined vars, pipe failures

APP_DIR="/var/www/app"
cd "$APP_DIR"

git pull origin main
echo 'Running migrations...'
/usr/bin/php artisan migrate --force
echo 'Clearing caches...'
/usr/bin/php artisan cache:clear
echo 'Deploy complete'

References

• ↗ https://www.gnu.org/software/bash/manual/bash.html
• ↗ https://www.shellcheck.net/

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