Redirecting output

How to provide standard input to commands, and how to direct output and error messages to the desired destinations. Example:

echo 'hello world' > /dev/null

Commands have three file descriptors associated with them: stdin (0), stdout (1) and stderr (2).

From the source: The GNU bash manual, Redirections.

Redirection theory

On paper, this is how redirection works:

[n]<file: Indicates which file stdin should be taken from.
[n]>file: Indicates which file stdout should be written to. Unless the noclobber shell option is enabled ( set -o noclobber ), this overwrites the file, even if the command does not write anything to file n.
[n]>|file: Like >, but overwrites the file even if the noclobber shell option is enabled.
[n]>>file: If the file does not exist, this works like >. If the file exists, the command output is appended to it.
[n]&>file (and [n]>&file): Redirects both stdout and stderr to the indicated file. This is equivalent to 1>file 2>&1.
[n]&>>file: Like &>file, but appends to the file. This is equivalent to 1>>file 2>&1.

A couple of notes to these operators:

For < operators, n defaults to 0 ( stdin).
For > operators, n defaults to 1 ( stdout).
Rather than specifying a file, a file descriptor can be given, prefixed by &, like this:
```
command 2>&1
```
a common mistake is to leave out the & (as in: command 2>1), which will simply redirect stderr to the file ${PWD}/1.
Whitespace is allowed between the operator and the filename ( > file), but not between the file descriptor id and the operator ( 2>file).

Redirection location in commands

Redirections can occur pretty much anywhere on the commandline - the following three - are equivalent. The middle one should probably be avoided, particularly for more complex commands:

>file ls .
ls >file .
ls . >file

The order of redirections is significant

If the same file descriptor is redirected several times, only the last (right-most) takes effect:

ls . >file1 >file2   # is equivalent to
ls . >file2

More subtly, when one file descriptor is redirected into another, the order makes a big difference:

cat ~/.emacs 2>&1 1>file     # stderr ends up on stdout,
stdout goes to the file

This is not the same as:

cat ~/.emacs 1>file 2>&1     # stdout and stderr both go
in the file

The difference being whether 2 is directed to 1 before or after 1 is directed to file.

Writing a message to stderr

Use echo and redirect stdout to stderr:

echo "Wrong input - you are invalid" >&2

Forcing a command into silence ( /dev/null)

Sometimes commands will insist (in the sense that they do not provide a --quiet option) on telling you things you do not care about. For example, cp will complain if a file to be copied does not exist, but if in that case, you want to do nothing, this will silence it:

cp a b &> /dev/null

Make sure there really are no possible errors that you care about.

Pipelines

See the GNU bash manual: Pipelines (operators | and |&)