Shell redirection syntax soup

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:

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

• Print and redirect to file:

  Example:

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

Redirect stderr to a file

• Standard way:

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

• Print and redirect stderr to file:

  Example:

Redirect both stdout and stderr to a file

• Common approach:

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

• Print and redirect both to file:

  Example:

• Convenient shorthand:

  Example:

Append instead of overwriting

• Append stdout to a file:

  Example:

• Print and append stdout to file:

  Example:

• Append both stdout and stderr (explicit):

  Example:

• Print and append both stdout and stderr to file:

  Example:

• Convenient shorthand for appending both:

  Example:

Piping output

Pipe stdout to another command

• Basic usage:

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

• Print and redirect piped stdout to file:

  Example:

Pipe both stdout and stderr

• Common way:

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

• Print and redirect both stdout and stderr to file:

  Example:

Shorthand for piping both stdout and stderr (|&)

• Shorthand syntax:

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

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

  Example:

Redirecting file descriptors

Custom file descriptors

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

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

• Print and redirect stdout to custom file descriptor:

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

Redirect stderr to a file descriptor

• Common case:

  Redirects stderr to file descriptor 3. For example:

• Print and redirect stderr to custom file descriptor:

  Example:

Redirect both stdout and stderr to a file descriptor

• Common way:

  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:

Discarding output

Send stdout and stderr to /dev/null

• Common:

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

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

  Example:

• Convenient shorthand:

  Example:

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