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tee -a: Append Output to File

The `tee` command reads standard input and writes it to standard output and one or more files simultaneously. The `-a` (append) option, in particular, is used to add new content to the end of a file without overwriting its existing content. This is useful in various scenarios such as log file management and recording script execution results.

Overview

`tee` acts as an interceptor in a pipeline, displaying data on the screen while simultaneously saving it to a file. The `-a` option changes this saving behavior to 'append' mode, allowing new data to be added while preserving the existing file content. This is similar to `>>` redirection, unlike `>` which overwrites the file, but `tee` differs in that it continues to pass data to standard output.

Key Features

  • Processes standard input and standard output concurrently
  • Appends content to files (-a option)
  • Controls data flow in the middle of a pipeline
  • Can write to multiple files simultaneously

Key Options

The `tee` command offers various options to control how data is written to files. The `-a` option is particularly essential for appending data while preserving existing file content.

Writing Modes

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Usage Examples

Learn how to append data to files in various scenarios using the `tee -a` command.

Appending Content to an Existing File

echo "New log message" | tee -a my_log.txt

Appends the output of the `echo` command to `my_log.txt` while simultaneously displaying it on the screen.

Appending to System Log Files (Using sudo)

echo "[$(date)] System event occurred" | sudo tee -a /var/log/syslog

Use `sudo` with `tee -a` to append content to system log files that require elevated privileges. Direct redirection like `>>` might fail because the shell checks permissions before processing the redirection.

Appending to Multiple Files Simultaneously

ls -l | tee -a file1.txt file2.txt

You can append a single output to multiple files at once.

Logging Script Execution Results

./my_script.sh | tee -a script_output.log

Record all output from a script or command to a file while viewing it on the screen in real-time.

Tips & Precautions

Tips and points to consider for more effective use of the `tee -a` command.

Using with sudo

`tee` is very useful when writing to files that require elevated privileges using `sudo`. Direct redirection like `echo "content" >> /path/to/protected_file` can fail because the shell checks permissions before processing the redirection. `tee` executes with `sudo`'s privileges through the pipe, resolving this issue.

  • Correct Usage: echo "content" | sudo tee -a /path/to/protected_file
  • Incorrect Usage (Permission Issue): sudo echo "content" >> /path/to/protected_file

Error Handling

`tee` outputs error messages to standard error if a file writing error occurs, but it attempts to continue passing data to the next command in the pipeline. For critical data, consider using shell options like `set -o pipefail` to ensure the entire pipeline fails if any command within it fails.

File Creation

`tee -a` automatically creates the specified file if it does not exist. Therefore, there is no need to create the file beforehand.

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