I always struggle with the syntax for redirecting multiple streams to another command or a file. LLMs do help, but beyond the most obvious cases, it takes a few prompts to get the syntax right. When I know exactly what I’m after, scanning a quick post is much faster than wrestling with a non-deterministic kraken. So, here’s a list of the redirection and piping syntax I use the most, with real examples.

Redirecting stdout and stderr

Redirect stdout to a file

  • Standard way:

    command > file

    This replaces the content of file with the stdout of command. For example:

    echo "Hello, world!" > hello.txt

  • Print and redirect to file:

    command | tee file

    Example:

    echo "Hello, world!" | tee hello.txt

    This prints “Hello, world!” to the terminal and also writes it to hello.txt.

Redirect stderr to a file

  • Standard way:

    command 2> file

    Sends all errors (stderr) to file. For example:

    ls non_existing_file 2> error.log

  • Print and redirect stderr to file:

    command 2> >(tee file)

    Example:

    ls non_existing_file 2> >(tee error.log)

Redirect both stdout and stderr to a file

  • Common approach:

    command > file 2>&1

    Combines stdout and stderr into one stream and saves them to file. For example:

    ls non_existing_file existing_file > output.log 2>&1

  • Print and redirect both to file:

    command 2>&1 | tee file

    Example:

    ls non_existing_file existing_file 2>&1 | tee output.log

  • Convenient shorthand:

    command &> file

    Example:

    ls non_existing_file existing_file &> output.log

Append instead of overwriting

  • Append stdout to a file:

    command >> file

    Example:

    echo "Appending line" >> hello.txt

  • Print and append stdout to file:

    command | tee -a file

    Example:

    echo "Appending line" | tee -a hello.txt

  • Append both stdout and stderr (explicit):

    command >> file 2>&1

    Example:

    ls non_existing_file existing_file >> output.log 2>&1

  • Print and append both stdout and stderr to file:

    command 2>&1 | tee -a file

    Example:

    ls non_existing_file existing_file 2>&1 | tee -a output.log

  • Convenient shorthand for appending both:

    command &>> file

    Example:

    ls non_existing_file existing_file &>> output.log

Piping output

Pipe stdout to another command

  • Basic usage:

    command1 | command2

    This sends the stdout of command1 to the input of command2. For example:

    echo "Hello, world!" | grep "Hello"

  • Print and redirect piped stdout to file:

    command1 | tee file | command2

    Example:

    echo "Hello, world!" | tee output.txt | grep "Hello"

Pipe both stdout and stderr

  • Common way:

    command1 2>&1 | command2

    Combines stdout and stderr, then pipes the combined stream to command2. For example:

    ls non_existing_file existing_file 2>&1 | grep "No"

  • Print and redirect both stdout and stderr to file:

    command1 2>&1 | tee file | command2

    Example:

    ls non_existing_file existing_file 2>&1 | tee output.txt | grep "No"

Shorthand for piping both stdout and stderr (|&)

  • Shorthand syntax:

    command1 |& command2

    This is equivalent to command1 2>&1 | command2, combining stdout and stderr. For example:

    ls non_existing_file existing_file |& grep "No"

  • Print and redirect both stdout and stderr using |&:

    command1 |& tee file | command2

    Example:

    ls non_existing_file existing_file |& tee output.txt | grep "No"

Redirecting file descriptors

Custom file descriptors

  • Create a new file descriptor (e.g., 3) and redirect stdout to it:

    exec 3> outputfile
command >&3

    This sends the stdout of command to file descriptor 3, which points to outputfile. For example:

    exec 3> custom_output.txt
echo "Using FD 3" >&3

  • Print and redirect stdout to custom file descriptor:

    exec 3> custom_output.txt
echo "Using FD 3" | tee /dev/tty > /dev/fd/3

    This prints “Using FD 3” to the terminal and simultaneously writes it to custom_output.txt.

Redirect stderr to a file descriptor

  • Common case:

    command 2>&3

    Redirects stderr to file descriptor 3. For example:

    exec 3> error_output.txt
ls non_existing_file 2>&3

  • Print and redirect stderr to custom file descriptor:

    command 2> >(tee >(cat > /dev/fd/3))

    Example:

    ls non_existing_file 2> >(tee >(cat > /dev/fd/3))

Redirect both stdout and stderr to a file descriptor

  • Common way:

    command > /dev/fd/3 2>&1

    Combines stdout and stderr, and redirects them to file descriptor 3.

    Note: There’s no shorthand equivalent for redirecting both stdout and stderr to a file descriptor. You need to use the full syntax. For example:

    exec 3> combined_output.txt
ls non_existing_file existing_file > /dev/fd/3 2>&1

Discarding output

Send stdout and stderr to /dev/null

  • Common:

    command > /dev/null 2>&1

    Silences all output (stdout and stderr). For example:

    ls non_existing_file > /dev/null 2>&1

  • Print and discard stdout and stderr (not sure why you’d ever need this):

    command | tee /dev/null

    Example:

    ls non_existing_file | tee /dev/null

  • Convenient shorthand:

    command &>/dev/null

    Example:

    ls non_existing_file &>/dev/null

At a glance

  • Redirect stdout: command > file

  • Redirect stderr: command 2> file

  • Redirect both stdout and stderr:

    • Standard: command > file 2>&1
    • Shorthand: command &> file

  • Append stdout: command >> file

  • Append both stdout and stderr:

    • Standard: command >> file 2>&1
    • Shorthand: command &>> file

  • Pipe stdout: command1 | command2

  • Pipe both stdout and stderr:

    • Standard: command1 2>&1 | command2
    • Shorthand: command1 |& command2

  • Custom file descriptors:

    • Create and redirect stdout: exec 3> file; command >&3
    • Redirect stderr: command 2>&3
    • Redirect both stdout and stderr: command > /dev/fd/3 2>&1 (no shorthand available)

  • Discard stdout and stderr:

    • Standard: command > /dev/null 2>&1
    • Shorthand: command &>/dev/null

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