Command chaining (&&, ||, ;) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the && operator do in command chaining?

It runs the second command only if the first command succeeds (returns exit status 0). Think of it as "and then if successful."

beginner

What happens when you use the || operator between two commands?

The second command runs only if the first command fails (returns non-zero exit status). It's like saying "or else run this."

beginner

How does the ; operator work in command chaining?

It runs commands sequentially, one after another, regardless of whether the previous command succeeded or failed.

intermediate

Example: What will happen with mkdir test && cd test if the directory already exists?

The mkdir test will fail because the directory exists, so cd test will NOT run because of the && operator.

intermediate

Why might you use command1 || command2 in a script?

To try command1 and if it fails, automatically run command2 as a fallback or error handler.

What does cmd1 && cmd2 do?

A Runs cmd2 first, then cmd1

B Runs cmd2 only if cmd1 fails

C Runs cmd2 only if cmd1 succeeds

D Runs cmd1 and cmd2 regardless of success

&& runs the second command only if the first command succeeds.

What happens with cmd1 || cmd2?

A Runs cmd2 only if cmd1 succeeds

B Runs cmd2 only if cmd1 fails

C Runs both commands always

D Runs cmd1 twice

|| runs the second command only if the first command fails.

Which operator runs commands one after another no matter what?

A ;

B ||

C &&

D |

The semicolon ; runs commands sequentially regardless of success or failure.

If you want to run cmd2 only when cmd1 fails, which do you use?

A cmd1 | cmd2

B cmd1 && cmd2

C cmd1 ; cmd2

D cmd1 || cmd2

|| runs the second command only if the first fails.

What will mkdir newdir; cd newdir do?

A Run mkdir, then cd regardless of mkdir success

B Run cd only if mkdir succeeds

C Run cd only if mkdir fails

D Run neither command

The semicolon runs commands one after another no matter what.

Explain how the &&, ||, and ; operators differ in chaining commands.

Think about success and failure conditions for each operator.

Describe a real-life situation where you would use command1 || command2 in a script.

Imagine trying something and doing something else if it doesn't work.

Practice

1. What does the command chaining operator && do in Linux shell?

easy

A. Runs the next command only if the previous command succeeded

B. Runs the next command only if the previous command failed

C. Runs all commands regardless of previous command success

D. Runs the next command in the background

Solution

1. Step 1: Understand the meaning of &&

   The && operator runs the next command only if the previous command returns success (exit status 0).

2. Step 2: Compare with other operators

   || runs next command only if previous failed, and ; runs commands regardless of success.

3. Final Answer:

   Runs the next command only if the previous command succeeded -> Option A

4. Quick Check:

   && means run next if success [OK]

Hint: Remember: && means next runs only on success [OK]

Common Mistakes:

• Confusing && with ||
• Thinking && runs commands regardless
• Assuming && runs commands in background

2. Which of the following is the correct syntax to run cmd1 and then cmd2 regardless of cmd1's success or failure?

easy

A. cmd1 && cmd2

B. cmd1 || cmd2

C. cmd1 ; cmd2

D. cmd1 & cmd2

Solution

1. Step 1: Identify the operator that runs commands regardless

   The semicolon ; runs commands one after another no matter if the first succeeds or fails.

2. Step 2: Check other options

   && runs next only if first succeeds, || runs next only if first fails, & runs command in background.

3. Final Answer:

   cmd1 ; cmd2 -> Option C

4. Quick Check:

   ; runs commands regardless [OK]

Hint: Use ; to run commands one after another always [OK]

Common Mistakes:

• Using && or || when unconditional run is needed
• Confusing & (background) with ;
• Missing spaces around operators

3. What will be the output of the following commands if file.txt does not exist?
cat file.txt && echo "File exists" || echo "File missing"

medium

A. File exists

B. File missing

C. No output

D. cat: file.txt: No such file or directory

Solution

1. Step 1: Understand command chaining behavior

   cat file.txt fails because file does not exist, so && echo "File exists" is skipped, and || echo "File missing" runs.

2. Step 2: Predict output

   The error message from cat is printed to stderr (not shown here), but the command output from chaining is File missing.

3. Final Answer:

   File missing -> Option B

4. Quick Check:

   Failed cat triggers || echo [OK]

Hint: Remember: || runs next if previous fails [OK]

Common Mistakes:

• Expecting 'File exists' output
• Ignoring that cat prints error to stderr
• Thinking no output appears

4. Identify the error in this command chaining:
mkdir newdir || echo "Created" && cd newdir

medium

A. The echo message is incorrect

B. mkdir should be followed by ; not ||

C. The chaining logic runs cd only if echo runs

D. No error, command works as intended

Solution

1. Step 1: Analyze the chaining logic

   Operators || and && have equal precedence and associate left-to-right: parsed as (mkdir newdir || echo "Created") && cd newdir.

2. Step 2: Trace execution

   mkdir success: skip echo, success && cd runs. mkdir fail: run echo "Created" (success) && cd runs. Error: echo "Created" runs only on failure (dir exists), wrong message.

3. Final Answer:

   The echo message is incorrect -> Option A

4. Quick Check:

   || echo "Created" runs on mkdir fail [OK]

Hint: || triggers echo only on failure, check message [OK]

Common Mistakes:

• Misparsing as right-associative (mkdir || (echo && cd))
• Thinking cd skips on mkdir success
• Overlooking echo on mkdir failure

5. You want to run backup.sh only if prepare.sh succeeds, but if prepare.sh fails, you want to run cleanup.sh. Which command chaining achieves this?

hard

A. prepare.sh || cleanup.sh && backup.sh

B. prepare.sh && (backup.sh || cleanup.sh)

C. prepare.sh ; backup.sh ; cleanup.sh

D. prepare.sh && backup.sh || cleanup.sh

Solution

1. Step 1: Understand the requirement

   Run backup.sh only if prepare.sh succeeds; if prepare.sh fails, run cleanup.sh.

2. Step 2: Analyze prepare.sh && backup.sh || cleanup.sh

   prepare.sh && backup.sh || cleanup.sh means: if prepare.sh succeeds, run backup.sh. If either prepare.sh or backup.sh fails, run cleanup.sh. This matches the requirement.

3. Final Answer:

   prepare.sh && backup.sh || cleanup.sh -> Option D

4. Quick Check:

   Run backup if prepare succeeds, else cleanup [OK]

Hint: Use && then || to handle success and failure paths [OK]

Common Mistakes:

• Running cleanup even if prepare succeeds
• Running backup even if prepare fails
• Using ; which ignores success/failure