Use input output redirection

This guide shows how to manage standard input, output, and error streams in Linux. You’ll learn how to redirect data between commands, files, and devices to build powerful command-line workflows.

Default Input and Output

Most Linux utilities read from files or stdin and write to stdout by default. For example, the sort command orders lines of text:

$ cat file.txt
6
5
1
3
4
2

$ sort file.txt
1
2
3
4
5
6

By default, sort reads from file.txt, sorts the lines, and sends the result to your terminal.

Redirecting Standard Output (>)

Use the > operator to save a command’s output to a file. The target file is created if it doesn’t exist or overwritten if it does:

$ sort file.txt > sortedfile.txt
$ cat sortedfile.txt
1
2
3
4
5
6

Each time you run:

$ date > file.txt

the contents of file.txt are replaced by the new output.

Appending Standard Output (>>)

To append data instead of overwriting, use >>:

$ date >> file.txt
$ date >> file.txt
$ date >> file.txt
$ cat file.txt
Mon Nov  8 18:50:30 CST 2021
Mon Nov  8 18:50:31 CST 2021
Mon Nov  8 18:50:32 CST 2021

Standard Streams Overview

Every Linux process has three standard streams:

Redirecting Standard Error (2>)

Send error messages to a file (or discard them) by prefixing the redirection operator with 2:

$ grep -r '^The' /etc/
grep: /etc/cups/ssl: Permission denied
...
$ grep -r '^The' /etc/ 2>/dev/null
/etc/brltty/Input/tn/all.txt: The two keys at the left rear (2 columns, 1 row):
...

Here, 2>/dev/null discards all errors by sending them to the null device.

Capturing Both stdout and stderr

To redirect both streams:

# Separate files
$ grep -r '^The' /etc/ 1>output.txt 2>errors.txt

# Append instead of overwrite
$ grep -r '^The' /etc/ 1>>output.txt 2>>errors.txt

# Merge stderr into stdout
$ grep -r '^The' /etc/ > all_output.txt 2>&1

Redirecting > first sends stdout to all_output.txt, then 2>&1 points stderr at the same destination.

Redirecting Standard Input (<)

Feed a file as input using <:

$ sort < file.txt
1
2
3
4
5
6

For commands that expect interactive input (e.g., sendmail), supply a file:

$ sendmail [email protected] < emailcontent.txt

Here Documents

Embed multiline input directly in a script with a here document:

$ sort <<EOF
6
3
2
5
1
4
EOF
1
2
3
4
5
6

The shell reads stdin until it finds the terminating EOF marker.

Here Strings

Use a here string (<<<) for single-line input:

$ bc <<< "1+2+3+4"
10

Piping Between Commands

Chain commands with the pipe operator |, which sends stdout of one command to stdin of the next:

$ grep -v '^#' /etc/login.defs \
  | sort \
  | column -t

Example output:

CREATE_HOME      yes
ENCRYPT_METHOD   SHA512
GID_MAX          60000
GID_MIN          1000
... (and so on)

Pipes let you build complex data transformations by combining simple utilities.

Redirection Operators Summary

Links and References

• Bash Reference Manual
• Linux I/O Redirection (TLDP)
• Advanced Shell Scripting Guide