Bash Input/Output Redirection

Input/output redirection is a fundamental concept in Bash that allows you to control the flow of data between commands and files. It's an essential skill for efficient shell scripting and command-line operations.

Understanding I/O Streams

Bash uses three standard I/O streams:

• stdin (0): Standard input
• stdout (1): Standard output
• stderr (2): Standard error

Output Redirection

To redirect output to a file, use the > operator:

echo "Hello, World!" > output.txt

This command writes the output to output.txt, overwriting its contents. To append instead, use >>:

echo "Appended line" >> output.txt

Input Redirection

To use a file as input, employ the < operator:

sort < unsorted_list.txt

This command sorts the contents of unsorted_list.txt.

Error Redirection

Redirect error messages using 2>:

ls non_existent_directory 2> error.log

This captures error messages in error.log.

Combining Redirections

You can redirect both output and error streams:

command > output.txt 2> error.log

To send both streams to the same file:

command &> combined_output.log

Here Documents

For multi-line input, use here documents:

cat << EOF > multiline.txt
Line 1
Line 2
Line 3
EOF

This creates a file with three lines of text. Here documents are particularly useful in Bash script structures.

Practical Applications

• Logging: Redirect output to log files for later analysis
• Silent execution: Redirect output to /dev/null to suppress it
• Input automation: Use input redirection to feed commands with prepared data

Best Practices

• Always consider potential file overwriting when using >
• Use >> for logs to append rather than overwrite
• Redirect errors to separate files for easier debugging
• Combine redirection with Bash pipes for complex data processing

Mastering input/output redirection enhances your ability to manipulate data streams effectively in Bash. It's a crucial skill for both everyday command-line use and advanced Bash scripting.