Linux CLI HowTo 13 🐧 How to Input Output and Redirection

How to Redirect Standard Output to a File

Steps:

1. Use the > operator followed by the filename
2. The command output will be written to the specified file
3. If the file exists, it will be overwritten completely

Example: To save command output to a file:

echo "Hello World" > hello.txt

This creates a file named hello.txt containing “Hello World”

How to Append Output to a File

Steps:

1. Use the >> operator followed by the filename
2. The command output will be added at the end of the file
3. If the file doesn’t exist, it will be created

Example: To add content to an existing file:

echo "New line" >> log.txt

This adds “New line” to the end of log.txt without removing existing content

How to Redirect Input from a File

Steps:

1. Use the < operator followed by the filename
2. The command will read input from the specified file instead of keyboard
3. Useful for processing files with commands that normally expect stdin

Example: To sort contents of a file:

sort < data.txt

This sorts the content of data.txt and displays it on screen

How to Redirect Error Messages to a File

Steps:

1. Use the 2> operator followed by error filename
2. All error messages will be captured in the specified file
3. Normal output continues to display on terminal

Example: To capture errors from a command:

ls nonexistent.txt 2> errors.log

This shows normal output but saves any error messages to errors.log

How to Redirect Both Output and Error to Different Files

Steps:

1. Use > for stdout and 2> for stderr with different filenames
2. Standard output goes to first file, errors go to second file
3. Both streams can be captured separately

Example: To separate normal output and errors:

command > output.log 2> error.log

This saves regular output to output.log and errors to error.log

How to Redirect Both Output and Error to Same File

Steps:

1. Use &> operator followed by filename
2. Both standard output and standard error go to the same file
3. This captures everything in one location

Example: To capture all output including errors:

command &> complete.log

This saves both normal output and error messages to complete.log

How to Suppress All Output

Steps:

1. Use > or 2> with /dev/null as destination
2. Redirect either stdout (>), stderr (2>), or both (&>)
3. The /dev/null device discards all data sent to it

Example: To hide both output and errors:

command > /dev/null 2>&1

This discards everything from the command execution

How to Suppress Only Standard Output

Steps:

1. Redirect stdout to /dev/null using >
2. Keep stderr going to terminal or another destination
3. Useful when you want to see errors but hide normal output

Example: To hide regular output but show errors:

command > /dev/null

This suppresses normal output while showing any error messages

How to Suppress Only Standard Error

Steps:

1. Redirect stderr to /dev/null using 2>
2. Keep stdout going to terminal or another destination
3. Useful when you want to see normal output but hide errors

Example: To show normal output but hide errors:

command 2> /dev/null

This shows regular output while discarding error messages

How to Save Command Results to Multiple Files

Steps:

1. Use multiple redirection operators in one command
2. Redirect stdout to one file and stderr to another
3. Combine both operations using separate redirections

Example: To save both output and errors separately:

ls /nonexistent 1> success.txt 2> error.txt

This saves successful output to success.txt and error messages to error.txt

How to Create a Comprehensive Log File

Steps:

1. Redirect both stdout and stderr to the same file using &>
2. This captures all command output and errors in one location
3. Useful for logging complete command execution

Example: To create a full execution log:

process_data.sh &> execution.log

This saves everything from process_data.sh to execution.log, including both normal output and errors

How to Process File Contents with Command

Steps:

1. Use < redirection to feed file content as input
2. The command reads from the file instead of standard input
3. Useful for piping files through text processing commands

Example: To sort file contents:

sort < sorted_data.txt

This sorts the lines in sorted_data.txt and displays the result

How to Capture Command Errors Only

Steps:

1. Use 2> redirection to capture only error stream
2. Redirect stdout normally or suppress it if needed
3. This isolates error information for analysis

Example: To collect only errors from a command:

find /etc -name "*.conf" 2> error_report.txt

This finds configuration files and saves only error messages to error_report.txt

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