File descriptors and redirection in Bash Scripting

Introduction

File descriptors let your script control where input comes from and where output goes. Redirection sends output to files or reads input from files instead of the screen or keyboard.

Save the output of a command to a file instead of showing it on the screen.

Read input for a command from a file instead of typing it manually.

Send error messages to a separate file to keep logs clean.

Combine output and error messages into one file for easier debugging.

Ignore output or errors when you don't need to see them.

Syntax

Bash Scripting

command > file       # Redirect standard output (stdout) to a file
command < file       # Redirect standard input (stdin) from a file
command 2> file      # Redirect standard error (stderr) to a file
command >> file      # Append standard output to a file
command &> file      # Redirect both stdout and stderr to a file

File descriptor 0 is standard input (stdin).

File descriptor 1 is standard output (stdout), and 2 is standard error (stderr).

Examples

Saves the list of files in the current folder to files.txt instead of showing it on the screen.

Bash Scripting

ls > files.txt

Reads the content of unsorted.txt as input to the sort command.

Bash Scripting

sort < unsorted.txt

Searches for 'error' in logfile.txt and sends any error messages to errors.txt.

Bash Scripting

grep 'error' logfile.txt 2> errors.txt

Adds the word 'Hello' to the end of greetings.txt without deleting existing content.

Bash Scripting

echo 'Hello' >> greetings.txt

Sample Program

This script runs ls commands and redirects outputs and errors to different files. Then it shows what was saved in those files.

Bash Scripting

#!/bin/bash

# Redirect output to a file
ls > output.txt

# Redirect errors to a file
ls non_existing_file 2> error.txt

# Redirect both output and errors to one file
ls existing_file non_existing_file &> combined.txt

# Show contents of the files
cat output.txt
cat error.txt
cat combined.txt

Important Notes

Use >> to add to a file without erasing it.

Use > to redirect only standard output, and 2> for standard error.

&> redirects both output and error together.

Summary

File descriptors 0, 1, and 2 represent input, output, and error streams.

Redirection changes where commands get input or send output.

Use redirection to save outputs, handle errors, or combine both.

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