← All writing

Linux

The weird case of 'yes' command in Linux

What the yes command does, how to use it in pipelines, and why it behaves as it does.

2 min read
1 view

The yes command in Linux is a simple utility that outputs a string repeatedly until interrupted.

  • Basic Functionality:

    • Continuously prints "y" (or a specified string) to the terminal until the process is stopped (e.g., with Ctrl+C).
    • Default output is "y" followed by a newline.

  • Syntax:

    • yes [STRING] — If no string is provided, it outputs "y". If a string is given, it repeats that instead (e.g., yes hello prints "hello" repeatedly).

  • Purpose:

    • Primarily used to automate responses to interactive scripts or commands that prompt for confirmation (e.g., apt-get or rm -i).
    • Simulates user input by providing a constant stream of "y" or another string.

  • Common Use Cases:

    • Automating package installations: yes | sudo apt-get install package to auto-confirm prompts.
    • Stress testing scripts or systems by generating continuous output.
    • Creating large test files: yes "test" | head -n 1000 > file.txt to write "test" 1000 times to a file.

  • Weird Traits:

    • Extremely fast output can overwhelm terminals or consume CPU if not redirected or limited.
    • No built-in delay or rate-limiting, making it a "firehose" of text.
    • Minimalistic design: no options beyond specifying the output string (e.g., no flags for speed control).

  • Stopping the Command:

    • Must be interrupted manually (Ctrl+C) or piped to a command that limits output (e.g., head, tail).
    • If misused (e.g., yes > file), it can quickly fill disk space.

  • Examples:

    • yes | rm -i *.txt: Automatically confirms deletion of multiple files.
    • yes "okay" | head -n 5: Outputs "okay" five times and stops.
    • yes | while true; do echo "Looping"; sleep 1; done: Feeds "y" into a loop for automation.

  • Limitations:

    • Only outputs one string; no support for complex patterns or multiple inputs.
    • Not ideal for nuanced automation where varied responses are needed.

  • Fun Fact:

    • Its simplicity makes it a favorite in scripting for quick-and-dirty automation, but it’s also a classic example of a Unix tool that does one thing and does it well (or excessively).

Also see.

More articles →