How to use the command 'tee' (with examples)

The tee command in Unix-like systems is an exceptionally versatile tool that allows users to read from standard input (stdin) and write to both standard output (stdout) and one or more files simultaneously. This command is particularly useful in pipelines, enabling the user to display outputs on the terminal while simultaneously saving it to a file or further processing it through additional commands. The functionality of tee makes it an indispensable tool for anyone looking to manipulate data streams efficiently.

Use case 1: Copy input to file and terminal

Code:

echo "example" | tee path/to/file

Motivation:

This use case is useful when you want to monitor the output on the terminal while also saving it directly to a file for future reference. By using tee, you can avoid running the command twice and save time and processing power.

Explanation:

• echo "example": This command generates the string “example” and sends it to stdout.
• |: This pipe operator directs the output of the preceding command into the input of the tee command.
• tee path/to/file: The tee command receives the input from the echo command and writes it to “path/to/file”. Simultaneously, it also displays the input on the terminal (stdout).

Example output:

example

Where the word “example” is both printed to the terminal and saved in the specified file.

Use case 2: Append input to an existing file

Code:

echo "example" | tee -a path/to/file

Motivation:

In many situations, it’s crucial to add content to the end of a file without overwriting the existing data. For instance, when logging data, appending provides a seamless record of activities without losing historical information.

Explanation:

• echo "example": Produces the input “example”.
• |: Sends the output to tee.
• tee -a path/to/file: The -a flag tells tee to append the input to the file, rather than overwriting it. As before, it also prints to stdout.

Example output:

example

The word “example” is shown on the terminal and appended to the file.

Use case 3: Pipe output to another program

Code:

echo "example" | tee /dev/tty | xargs printf "[%s]"

Motivation:

Sometimes it’s necessary to process data further while also visualizing it on the terminal. This pipeline enables such flexibility by displaying output, then formatting it through another command like xargs.

Explanation:

• echo "example": Generates “example”.
• |: Directs the data flow into the tee command.
• tee /dev/tty: Writes the input to the terminal (/dev/tty) and also passes it down the line.
• | xargs printf "[%s]": Takes the input from tee, uses xargs to format it, and wraps it in square brackets with printf.

Example output:

example
[example]

Where “example” initially appears on the terminal, followed by its formatted version.

Use case 4: Create a directory and character count

Code:

echo "example" | tee >(xargs mkdir) >(wc -c)

Motivation:

This command showcases the power of process substitution, where input can be handled by multiple subshells simultaneously. Here, it creates a directory named after the input string and counts the characters in that string.

Explanation:

• echo "example": Creates the input “example”.
• |: Directs the echo output into tee.
• tee >(xargs mkdir) >(wc -c): Uses process substitution >(...) to send the input to commands in separate shells. xargs mkdir uses the input to make a directory, wc -c counts characters in the input and displays the count.

Example output:

7

The number “7” represents the character count, while a directory named “example” is created.

Conclusion:

The tee command is a versatile and powerful utility for handling input and output simultaneously across Unix-like systems. Its ability to duplicate its input, display it, and send it for further processing or storage makes it an indispensable tool for efficient command-line operations. With the examples above, users can understand and leverage the capabilities of tee to enhance their workflow and data management skills.