File descriptors and redirection in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a file descriptor in bash scripting?

A file descriptor is a number that represents an open file or input/output resource in bash. Common ones are 0 for standard input, 1 for standard output, and 2 for standard error.

beginner

How do you redirect standard output to a file in bash?

Use the > symbol followed by the filename. For example, echo "Hello" > file.txt sends the output to file.txt instead of the screen.

intermediate

What does the command 2>&1 do in bash?

It redirects standard error (file descriptor 2) to the same place as standard output (file descriptor 1). This way, both outputs go to the same destination.

beginner

Explain the difference between > and >> in bash redirection.

> overwrites the file with new output, while >> appends the output to the end of the file without deleting existing content.

intermediate

How can you redirect only standard error to a file, leaving standard output unchanged?

Use 2> filename. For example, command 2> error.log sends error messages to error.log but shows normal output on the screen.

Which file descriptor number represents standard input in bash?

A 0

B 1

C 2

D 3

Standard input is always file descriptor 0.

What does the command ls > files.txt do?

A Appends the list of files to files.txt

B Sends the list of files to the screen

C Sends the list of files to the file files.txt

D Deletes files.txt

The > symbol redirects output to the file, overwriting it.

How do you append output to a file instead of overwriting it?

A >

B <

C 2>

D >>

The >> operator appends output to the file.

What does command 2>&1 do?

A Redirects standard output to standard error

B Redirects standard error to standard output

C Redirects both to a file

D Runs command twice

It sends standard error (2) to the same place as standard output (1).

How to redirect only error messages to a file named error.log?

A command 2> error.log

B command >> error.log

C command > error.log

D command 1> error.log

2> redirects only standard error to the file.

Describe what file descriptors 0, 1, and 2 represent in bash and how you can redirect each.

Think about where data comes from and goes to in commands.

Explain how to combine standard output and standard error into one file using redirection.

Remember the order matters in redirection.

Practice

1. Which file descriptor number represents the standard error stream in bash?

easy

A. 3

B. 1

C. 2

D. 0

Solution

1. Step 1: Recall standard file descriptors

   Standard input is 0, standard output is 1, and standard error is 2.

2. Step 2: Identify the error stream descriptor

   The error stream is always file descriptor 2 in bash.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Standard error = 2 [OK]

Hint: Remember: 0=input, 1=output, 2=error [OK]

Common Mistakes:

• Confusing standard output (1) with error (2)
• Thinking input (0) is error
• Using non-standard descriptor numbers

2. Which of the following is the correct syntax to redirect standard output to a file named output.txt in bash?

easy

A. command >> output.txt 2>&1

B. command < output.txt

C. command 2> output.txt

D. command > output.txt

Solution

1. Step 1: Understand output redirection syntax

   Using > redirects standard output (file descriptor 1) to a file, overwriting it.

2. Step 2: Match syntax to redirect output only

   command > output.txt uses > correctly to redirect output to output.txt.

3. Final Answer:

   command > output.txt -> Option D

4. Quick Check:

   Output redirection uses > [OK]

Hint: Use > for output, < for input redirection [OK]

Common Mistakes:

• Using < instead of > for output
• Redirecting error (2>) instead of output
• Appending (>>) when overwrite is needed

3. What will be the output of the following command if file.txt does not exist?

cat file.txt > output.txt 2>&1

medium

A. Redirects both output and error messages to output.txt

B. Creates an empty output.txt file with no error message

C. Outputs the error message to the terminal and creates output.txt

D. Throws a syntax error and does nothing

Solution

1. Step 1: Understand redirection 2>&1

   This redirects standard error (2) to the same place as standard output (1).

2. Step 2: Analyze command behavior when file missing

   cat tries to read file.txt, fails, and sends error to stderr, which is redirected to output.txt along with stdout.

3. Final Answer:

   Redirects both output and error messages to output.txt -> Option A

4. Quick Check:

   2>&1 merges error into output [OK]

Hint: 2>&1 sends errors to the same place as output [OK]

Common Mistakes:

• Thinking error shows on terminal
• Assuming output.txt stays empty
• Confusing 2>&1 with separate redirections

4. Identify the error in this command that attempts to redirect only standard error to error.log:

ls -l > error.log 2>

medium

A. Output redirection > should be 1>

B. Missing filename after 2> for error redirection

C. Should use 2>&1 instead of 2>

D. Command syntax is correct

Solution

1. Step 1: Check error redirection syntax

   Redirection 2> must be followed by a filename to redirect standard error.

2. Step 2: Identify missing filename

   The command ends with 2> but no file is specified, causing a syntax error.

3. Final Answer:

   Missing filename after 2> for error redirection -> Option B

4. Quick Check:

   2> needs a target file [OK]

Hint: Always specify a file after 2> for error redirection [OK]

Common Mistakes:

• Leaving out filename after 2>
• Confusing 2> with 2>&1
• Thinking > needs to be 1>

5. You want to run a script that outputs normal messages to output.log and error messages to error.log. Which command correctly achieves this?

hard

A. ./script.sh > output.log 2> error.log

B. ./script.sh 2>&1 > output.log error.log

C. ./script.sh > output.log 2>&1 error.log

D. ./script.sh > output.log &> error.log

Solution

1. Step 1: Understand separate redirection for output and error

   To send stdout and stderr to different files, use > for stdout and 2> for stderr separately.

2. Step 2: Analyze each option

   ./script.sh > output.log 2> error.log correctly redirects stdout to output.log and stderr to error.log. Others mix or misuse redirection syntax.

3. Final Answer:

   ./script.sh > output.log 2> error.log -> Option A

4. Quick Check:

   Separate > and 2> for output and error [OK]

Hint: Use > for output, 2> for errors to separate files [OK]

Common Mistakes:

• Using &> redirects both to one file
• Mixing order of redirections incorrectly
• Omitting file names after redirection operators