output

All posts tagged output by Linux Bash
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    The tee command in Unix-like operating systems is a powerful utility for capturing and duplicating command output. It allows you to both display the output of a command on the terminal and simultaneously write it to a file. Here's a detailed guide to understanding and using tee.

    Basic Syntax

    command | tee [options] [file...]
    • command: The command whose output you want to capture.
    • |: A pipe that passes the output of command to tee.
    • tee: The command that reads from standard input and writes to standard output and file(s).
    • file...: One or more files where the output will be saved.

    How tee Works

    1. Standard Output Display: tee sends the output to the terminal (standard output).
    2. File Saving: Simultaneously, it writes the same output to one or more specified files.

    Common Use Cases

    1. Log Output While Viewing

    To log output to a file while still seeing it in real time on the terminal:

    ls -l | tee output.txt

    2. Append to a File

    By default, tee overwrites the file. Use the -a option to append instead:

    ls -l | tee -a output.txt

    3. Capture and Process Output

    You can use tee in pipelines to split the output for multiple processing streams:

    cat file.txt | tee intermediate.txt | grep "pattern"

    Here: - The content of file.txt is saved in intermediate.txt. - The grep command processes the same output.

    4. Save Multiple Outputs

    Save output to multiple files simultaneously:

    ls -l | tee file1.txt file2.txt

    5. Debugging Scripts

    Log the output of a script while running it interactively:

    ./script.sh | tee debug.log

    Common Options

    • -a: Append the output to the file(s) instead of overwriting.
    • -i: Ignore interrupt signals, ensuring tee continues writing even if interrupted.

    Practical Examples

    Redirect Standard Error (stderr)

    To capture both standard output (stdout) and standard error (stderr):

    command 2>&1 | tee output.txt

    Save Output for Multiple Commands

    Using tee in a multi-stage pipeline:

    command | tee temp.txt | another_command

    Interactive Use with Sudo

    When saving files requiring elevated permissions:

    command | sudo tee /path/to/protected_file

    Tips

    • Use tee for monitoring logs in real-time during processes like backups or deployments.
    • Be cautious when overwriting files, as tee overwrites without confirmation unless -a is used.

    The tee command enhances command-line workflows by providing a simple way to capture, inspect, and process output without disrupting its flow.