The Mighty tee Command: More Than just a Pipe Dream!

We’ve all been there. You’re running a command, its output is flying by, and you desperately need to see it and save it to a file. Do you run it twice? Copy and paste from your terminal emulator? There’s a better way, and it’s called tee.

The tee command is a humble but incredibly powerful utility in the Unix-like operating system toolkit. Its name hints at its function: like a “T” junction in a pipe, it takes its standard input and simultaneously sends it to both standard output and one or more files.

Why is tee so Useful?

Let’s dive into some scenarios where tee truly shines:

• Simultaneous Viewing and Logging: This is tee‘s bread and butter. Imagine you’re compiling a large project or running a lengthy script. You want to monitor its progress in real-time on your screen, but you also need a complete log of the output for debugging or auditing later.

make | tee compilation_log.txt

In this example, the output of make is piped to tee. tee then displays the output on your terminal and writes it to compilation_log.txt.

• Debugging Pipelines: When you have a complex pipeline of commands, it can be tricky to see the output at intermediate stages. tee allows you to “tap into” the pipeline.

cat input.txt | grep "pattern" | tee intermediate_results.txt | sort

Here, you can see the results of the grep command on your screen and in intermediate_results.txt before they are passed to sort.

• Appending to Files: By default, tee will overwrite the specified file. However, with the -a (or --append) option, you can append the output to an existing file. This is great for continuous logging.

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

Each time my_script.sh runs, its output will be added to the end of daily_report.log without overwriting previous entries.

• Writing to Multiple Files: Need to send the same output to more than one file? tee handles that with ease.

ls -l | tee file_list.txt another_file.log

Now, both file_list.txt and another_file.log will contain the long listing of files in the current directory.

• Elevated Privileges (with sudo): A common frustration is trying to redirect output to a file that requires root privileges. A simple command > /path/to/file will often fail because the redirection happens before sudo takes effect on command. tee comes to the rescue:

echo "Some configuration" | sudo tee /etc/some_config_file.conf

Here, echo runs as your user, its output is piped to sudo tee, and then tee (running with root privileges) writes the content to the protected file.

Common tee Options:

• -a, --append: Append to the given files, do not overwrite.
• -i, --ignore-interrupts: Ignore interrupt signals (like Ctrl+C). This can be useful for ensuring logs are complete even if a process is interrupted.
• --help: Display help and exit.
• --version: Output version information and exit.

Beyond the Basics:

While tee is conceptually simple, its versatility makes it an indispensable tool for system administrators, developers, and anyone who frequently works in the command line. It promotes efficient workflow by eliminating the need for repetitive commands or manual copy-pasting.

The next time you find yourself wishing you could see and save the output of a command, remember the mighty tee. It’s a small command with a huge impact on your command-line productivity!