How to Append Output to a File in Linux Shell

In the Linux operating system, appending output to an existing file allows you to add new data to the end of a file without overwriting its current contents. This guide covers the most efficient methods to accomplish this using native shell redirection operators like >>, handling standard error with 2>>, and utilizing the tee command for simultaneous terminal viewing and file appending.

1. Using the >> Redirection Operator

The primary and most common method to append standard output (stdout) to an existing file is the double greater-than operator (>>). If the destination file already exists, the output is added to the end. If the file does not exist, the shell automatically creates it.

To append a simple text string:

echo "This is a new line" >> filename.txt

To append the output of a command:

date >> system_log.txt
ls -la >> directory_contents.txt

Note: Avoid using a single > operator, as it will overwrite (truncate) the existing contents of the file instead of appending to it.

2. Appending Error Messages (stderr)

By default, the >> operator only appends standard output (stream 1). If a command generates an error message (stream 2), it is displayed on the screen rather than written to the file.

To append only standard error to a file, use 2>>:

command_with_errors 2>> error_log.txt

To append both standard output and standard error into the same file, use &>>:

command_name &>> combined_output.txt

Alternatively, in older shells (such as legacy Bourne shells), you can redirect standard error to standard output:

command_name >> combined_output.txt 2>&1

3. Using the tee Command with the -a Flag

The tee utility reads from standard input and writes to both standard output (the terminal) and files simultaneously. By default, tee overwrites files, but adding the -a (or --append) flag forces it to append.

Basic syntax:

echo "New entry" | tee -a filename.txt

Appending with Root Privileges

A common problem occurs when attempting to append to a file owned by the root user using standard redirection:

# This will fail with a "Permission denied" error:
sudo echo "127.0.0.1 custom.domain" >> /etc/hosts

This fails because the redirection is handled by your user shell, not sudo. To resolve this, use tee -a with sudo:

echo "127.0.0.1 custom.domain" | sudo tee -a /etc/hosts