Thu 14 Jul 2016 — Tue 20 Mar 2018


Based on Digital Ocean's intro to Linux I/O Redirection.

Streams are numbered:

  • stdin (0) < or <<
  • stdout (1) > or >>
  • stderr (2) 2> or 2>>
  • both stdout and stderr &>

Redirection makes both streams point to the same place.

<<, >> and 2>> append, <, > and 2> overwrite.

Targetting other streams.

If you want to target one stream to another instead of a file, refer to it using & and the number of the stream.

e.g. 2>&1 redirects stderr to stdout, and >&2 redirects stout to stderr.

If using a file redirect as well, these stream redirects must come first, e.g. 2> err.log >&2.

I'm not sure what the name of this bit is.

Not clear if you can redirect things to &0, or why you'd want to.

Special files

/dev/null is discard

Process Substitution

This creates a pipe.

## Command inside the <() goes to stdin
wc <(cat /usr/share/dict/linux.words)

## Command inside the >() gets a stream which would have been written to a file otherwise.


a | b sends output from a to input of b.


The tee command sends its output to a file as well as the output stream.

Named Pipes

Use the mkinfo command to create named pipes.

They make nice temporary files, but remember to rm them after.

Process substitution is usually better.


Takes some standard input and turns it into command arguments. xargs behaves a little like backquote or the $(some-command) expansion.

echo some inputs | xargs some-program

xargs breaks input on spaces or newlines. You can use the xargs -0 on null characters instead, or xargs --delimiter=some-char to specific a character. xargs doesn't support multibyte (for example, unicode) characters.

You can also read your inputs from a file directly:

xargs -a input-file some-program

The xargs -p flag puts you into interactive mode, so you can choose whether or not to execute each command.

xargs placeholder substitution

The xargs -I placeholder argument lets you select where in a command your inputs go. Input is no longer broken on blank spaces with this flag.

echo input text | xargs -I PLACEHOLDER echo oy PLACEHOLDER oy PLACEHOLDER savaloy

Parallel xargs

echo some inputs | xargs -P 10 some-program

This is a nice, easy way to run some commands in parallel, but xargs doesn't handle shared access to resources. This means that your standard output stream is likely to get out of order.

The usual way to handle this is to have each command output to a different file using placeholder substitution, and then collect them up at the end.