Bash Input Redirection

May 17th, 2008 by Mitch Frazier in

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If you use the shell you surely know about redirection: 

  # echo 'hello world' >output
  # cat <output
The first line writes "hello world" to the file "output", the second reads it back and writes it to standard output (normally the terminal).

Then there are "here" documents: 

  # cat <<EOF
  > hello
  > world
  > EOF
A "here" document is essentially a temporary, nameless file that is used as input to a command, here the "cat" command.

A less commonly seen form of here document is the "here" string: 

  # cat <<<'hello world'
In this form the string following the "<<<" becomes the content of the "here" document.

Another less commonly seen form of redirection is redirecting to a specific file descriptor: 

  # echo 'Error: oops' >&2
This redirects the output of the "echo" command to file descriptor 2, aka standard error. This is useful if you want to keep the error output of your scripts from contaminating the normal output when the output of your script is redirected.

These features work in bash and may not be available in other shells.

__________________________

Mitch Frazier is the System Administrator at Linux Journal.

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It's worth mentioning some

On May 20th, 2008 Adam Backstrom (not verified) says:

It's worth mentioning some more examples of input redirection. For a file matched_files containing a list of files, you could grep the list:

grep mp3 < matched_files

Or do something to each file in the list:

while read line ; do
  flac "$line" && rm -f "$line"
done < matched_files

Lots more great examples are on the Useless Use of Cat Award page.

swap STDOUT and STDERR

On May 17th, 2008 Serge van Ginderachter (not verified) says:

swap STDOUT and STDERR: "3>&1 1>&2 2>&3"

as in:

(/usr/bin/$COMMAND $PARAM 3>&1 1>&2 2>&3 | grep -v $uninteresting_error ) 3>&1 1>&2 2>&3

Grep stderr

On May 17th, 2008 Mitch Frazier says:

In a bit more detail:

  • 3>&1 - moves file descriptor 1 (aka stdout) to file descriptor 3.
  • 1>&2 - moves file descriptor 2 (aka stderr) to file descriptor 1.
  • 2>&3 - moves file descriptor 3 to file descriptor 2 (aka stderr).
Similar to: 
   tmp    = stdout
   stdout = stderr
   stderr = tmp
and thereby swapping the standard out and the standard error. Which then allows the stderr (rather than stdout) to be piped into grep.

__________________________

Mitch Frazier is the System Administrator at Linux Journal.

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