tee

The Basics of tee

At its core, tee takes the standard input (stdin) and writes it to both standard output (stdout) – your terminal – and one or more files. This is incredibly useful when you need to log the output of a command while also seeing it in real-time.

The basic syntax is straightforward:

command | tee output_file.txt

This pipes the output of command to tee, which then writes it to output_file.txt and displays it on your terminal.

Example 1: Logging the output of ls

Let’s say you want to log a directory listing to a file while simultaneously viewing it on your terminal. You can achieve this with:

ls -l /tmp | tee /tmp/directory_listing.txt

This command lists the contents of /tmp with detailed information (ls -l), pipes the output to tee, and writes it to /tmp/directory_listing.txt. The output also appears on your terminal.

Advanced tee Usage: Appending and Multiple Files

tee offers more advanced options to improve its functionality.

Appending to a file: By default, tee overwrites the output file if it exists. To append to an existing file, use the -a (or --append) option:

ls -l /etc | tee -a /tmp/system_files.txt

This appends the output of ls -l /etc to /tmp/system_files.txt. If /tmp/system_files.txt doesn’t exist, it will be created.

Writing to multiple files: You can specify multiple output files by separating them with spaces:

date | tee file1.txt file2.txt

This command writes the current date and time to both file1.txt and file2.txt, and also displays it on the terminal.

Combining tee with Other Commands for Powerful Workflows

The real power of tee shines when combined with other CLI commands.

Example 2: Filtering and Logging System Logs

Suppose you want to filter system logs for specific error messages and save them to a file for later analysis:

dmesg | grep "error" | tee error_log.txt

This command filters the system log (dmesg) for lines containing “error,” pipes the output to tee, and saves the filtered results to error_log.txt while also showing them on the terminal.

Example 3: Logging the output of a long-running process:

Monitoring a lengthy process and saving its output is easily done with tee. Let’s assume you have a script my_long_script.sh:

./my_long_script.sh | tee my_script_output.log

This will run your script, log its complete output to my_script_output.log, and show it on your console simultaneously, allowing you to monitor the progress.

Ignoring Interrupts

The -i (or --ignore-interrupts) option is useful when running long commands. It prevents the command from being interrupted by signals such as Ctrl+C, ensuring the output is fully written to the file even if you stop the process manually.

long_running_command | tee -i output.log

This ensures that even if you interrupt long_running_command, the output already processed will be saved to output.log.

By understanding and utilizing the versatile tee command, you can improve your command-line efficiency and streamline your workflow. Its ability to simultaneously display output and log it to files makes it a useful tool for any Linux user.