Command chaining (&&, ||, ;) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this chained command?

Consider the following command run in a Linux shell:

echo Hello && echo World || echo Fail

What will be printed?

Linux CLI

echo Hello && echo World || echo Fail

A Hello\nWorld

B Hello\nFail

C World

D Fail

💡 Hint

Remember that && runs the next command only if the previous succeeds, and || runs the next command only if the previous fails.

The first echo prints 'Hello' and succeeds, so the second echo runs and prints 'World'. Since the second command succeeds, the || part does not run.

💻 Command Output

intermediate

What happens with semicolon chaining?

What is the output of this command?

echo Start; false; echo End

Note: 'false' is a command that always fails.

Linux CLI

echo Start; false; echo End

A End

B Start

C Start\nEnd

D Start\nfalse\nEnd

💡 Hint

Semicolon runs all commands regardless of success or failure.

Semicolon separates commands to run one after another no matter what. So 'Start' prints, then 'false' runs (fails silently), then 'End' prints.

💻 Command Output

advanced

What is the output of this mixed chaining?

What will this command print?

false && echo No || echo Yes; echo Done

Linux CLI

false && echo No || echo Yes; echo Done

A Yes\nDone

B No\nDone

C No

D Done

💡 Hint

&& runs next only if previous succeeds; || runs next only if previous fails; semicolon always runs next.

false fails, so 'echo No' after && does not run. The || runs 'echo Yes'. Then semicolon runs 'echo Done'.

💻 Command Output

advanced

What error or output occurs here?

What happens when running this command?

mkdir test_dir && cd test_dir || echo Failed

Linux CLI

mkdir test_dir && cd test_dir || echo Failed

A cd: no such file or directory: test_dir

B No output, current directory changes to test_dir

C mkdir: cannot create directory ‘test_dir’: File exists

D Failed

💡 Hint

If mkdir succeeds and cd succeeds, || echo Failed does not run.

If 'test_dir' does not exist, mkdir creates it and succeeds, cd changes directory successfully, so echo Failed does not run and no output appears.

💻 Command Output

expert

What is the output of this complex chain?

Analyze this command:

echo Start && false || echo Middle && echo End

What will be printed?

Linux CLI

echo Start && false || echo Middle && echo End

A Start\nMiddle

B Middle\nEnd

C Start\nEnd

D Start\nMiddle\nEnd

💡 Hint

Remember operator precedence: && has higher precedence than ||, so group accordingly.

The command groups as: (echo Start && false) || (echo Middle && echo End). 'echo Start' succeeds, then 'false' fails, so left side fails. Right side runs: 'echo Middle' prints, succeeds, so 'echo End' runs and prints.

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