ShellCheck for static analysis in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is ShellCheck?

ShellCheck is a tool that checks shell scripts for common mistakes and potential bugs before running them. It helps you write safer and cleaner scripts.

beginner

How does ShellCheck help beginners?

ShellCheck points out errors and warnings in your script with clear messages. It explains what might go wrong and suggests how to fix it, like a friendly script coach.

beginner

Which command runs ShellCheck on a script named myscript.sh?

You run: shellcheck myscript.sh. This checks the script and shows any issues found.

intermediate

What kind of problems can ShellCheck detect?

ShellCheck finds syntax errors, unsafe variable use, quoting mistakes, unused variables, and common pitfalls that can cause your script to fail or behave unexpectedly.

beginner

Why is static analysis useful before running scripts?

Static analysis checks your code without running it. This means you can catch bugs early, avoid crashes, and save time debugging later.

What does ShellCheck primarily do?

A Checks shell scripts for errors before running

B Runs shell scripts faster

C Converts shell scripts to Python

D Formats shell scripts with colors

ShellCheck analyzes shell scripts to find errors and warnings before execution.

How do you use ShellCheck on a script named script.sh?

A bashcheck script.sh

B run script.sh

C check script.sh

D shellcheck script.sh

The correct command is shellcheck script.sh to analyze the script.

Which problem is NOT typically detected by ShellCheck?

A Unsafe variable use

B Network connectivity issues

C Syntax errors

D Quoting mistakes

ShellCheck does not check runtime issues like network connectivity.

Why is quoting variables important in shell scripts?

A To make scripts run faster

B To change variable values

C To prevent word splitting and errors

D To comment out code

Quoting variables prevents unexpected splitting and errors in scripts.

What type of tool is ShellCheck?

A Static analysis tool

B Debugger

C Code formatter

D Package manager

ShellCheck is a static analysis tool that checks code without running it.

Explain what ShellCheck does and why it is helpful for shell scripting.

Think about how checking your work before running saves time and prevents mistakes.

Describe common mistakes in shell scripts that ShellCheck can detect.

Consider errors that cause scripts to fail or behave unexpectedly.

Practice

1. What is the main purpose of using ShellCheck in shell scripting?

easy

A. To find errors and suggest fixes before running the script

B. To execute the script faster

C. To convert shell scripts to Python

D. To add colors to the terminal output

Solution

1. Step 1: Understand what ShellCheck does

   ShellCheck is a tool that analyzes shell scripts without running them to find mistakes.

2. Step 2: Identify the main benefit

   It helps catch errors early, like missing quotes or bad syntax, making scripts safer.

3. Final Answer:

   To find errors and suggest fixes before running the script -> Option A

4. Quick Check:

   ShellCheck finds errors [OK]

Hint: ShellCheck checks scripts for errors before execution [OK]

Common Mistakes:

• Thinking ShellCheck runs the script
• Believing it speeds up script execution
• Confusing it with a code converter

2. Which of the following is the correct way to run ShellCheck on a script named myscript.sh?

easy

A. bash shellcheck myscript.sh

B. checkshell myscript.sh

C. run shellcheck myscript.sh

D. shellcheck myscript.sh

Solution

1. Step 1: Recall the ShellCheck command syntax

   The correct command to run ShellCheck is simply shellcheck followed by the script name.

2. Step 2: Verify the options

   Options B, C, and D use incorrect command names or extra words.

3. Final Answer:

   shellcheck myscript.sh -> Option D

4. Quick Check:

   Use 'shellcheck scriptname' [OK]

Hint: Use 'shellcheck' command followed by script name [OK]

Common Mistakes:

• Typing 'checkshell' instead of 'shellcheck'
• Adding extra words like 'run' before the command
• Using 'bash shellcheck' which is invalid

3. Given this script echo Hello $name and running shellcheck script.sh, what warning will ShellCheck most likely show?

medium

A. SC1000: Syntax error near unexpected token.

B. SC2001: Use 'sed' instead of 'echo'.

C. SC2086: Double quote to prevent globbing and word splitting.

D. No warnings, script is perfect.

Solution

1. Step 1: Understand ShellCheck's common warnings

   ShellCheck warns when variables are unquoted because it can cause word splitting or globbing.

2. Step 2: Match warning to the script

   The script uses $name without quotes, so ShellCheck will warn about missing quotes (SC2086).

3. Final Answer:

   SC2086: Double quote to prevent globbing and word splitting. -> Option C

4. Quick Check:

   Unquoted variables cause SC2086 warning [OK]

Hint: Look for unquoted variables to spot SC2086 warnings [OK]

Common Mistakes:

• Expecting syntax errors for simple echo
• Thinking no warning appears for unquoted variables
• Confusing warning codes with unrelated messages

4. You have this script snippet:

if [ $count -eq 10 ]; then
echo "Count is 10"
fi

ShellCheck reports an error. What is the best fix?

medium

A. Add quotes around $count: if [ "$count" -eq 10 ]; then

B. Remove the spaces inside [ ]

C. Change -eq to ==

D. Add a semicolon after echo statement

Solution

1. Step 1: Identify the ShellCheck warning cause

   Using $count without quotes can cause errors if $count is empty or contains spaces.

2. Step 2: Apply the fix

   Adding quotes around $count ensures the test works safely even if the variable is empty.

3. Final Answer:

   Add quotes around $count: if [ "$count" -eq 10 ]; then -> Option A

4. Quick Check:

   Quote variables in tests to avoid errors [OK]

Hint: Always quote variables inside [ ] tests [OK]

Common Mistakes:

• Removing spaces inside [ ] which breaks syntax
• Using == instead of -eq for numbers
• Adding unnecessary semicolons

5. You want to write a script that reads a filename from user input and safely prints its contents. Which ShellCheck advice should you follow?

hard

A. Use backticks instead of $( ) for command substitution

B. Always quote variables when using them, e.g., cat "$filename"

C. Avoid quoting variables to allow filename expansion

D. Use eval to run commands with variables

Solution

1. Step 1: Understand safe variable usage

   Quoting variables prevents word splitting and globbing, which can cause errors or security issues.

2. Step 2: Apply ShellCheck best practice

   ShellCheck recommends quoting variables like "$filename" when used in commands like cat.

3. Final Answer:

   Always quote variables when using them, e.g., cat "$filename" -> Option B

4. Quick Check:

   Quote variables to avoid unsafe expansions [OK]

Hint: Quote variables to prevent unsafe expansions [OK]

Common Mistakes:

• Using backticks which are outdated
• Not quoting variables causing errors
• Using eval which can be dangerous