Bash: Data Streams

Created: April 10, 2020

https://www.gnu.org/software/bash/manual/bash.html#Redirections

Streams

Standard input (stdin), standard output (stdout), and standard error (stderr).

File Descriptors

File Descriptor	Name
0	stdin
1	stdout
2	stderr

Redirecting stdout and stderr

Description	Command
Redirect stdout to `FILE`.	`COMMAND > FILE`
Same as above.	`COMMAND 1> FILE`
Redirect stderr to `FILE`.	`COMMAND 2> FILE`
Redirect stdout to `FILE1` and stderr to `FILE2`.	`COMMAND 1> FILE1 2> FILE2`
Redirect stdout and stderr to the same `FILE`.	`COMMAND &> FILE`
`2>&1` instructs the shell to redirect stderr (2) to the same destination as stdout (1). `2>&1` must come after FILE. `&>` is a Bash-ism.	`COMMAND > FILE 2>&1`
Input streams can be piped to `/dev/null` to get rid of them.	`COMMAND > /dev/null`
Persistently re-direct stderr to FILE until the shell or script closes, or the command is overridden.	`exec 2> FILE`

Appending stdout and stderr

In all the commands above, a single > can become >> to trigger an append to FILE, rather than an overwrite. 2>&1 remains the same; only the preceding > needs to be changed to >>.

Taking input from sources other than stdin

COMMAND < FILE

< causes COMMAND to take input from FILE rather than stdin.

COMMAND «< WORD

WORD undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Path name expansion and word splitting are not performed. This operator is called Here Strings.

Here Documents

Redirect multiple lines to a command.

COMMAND « TERMINATOR

After pressing enter, the shell will allow extra lines to be input. Every line will be redirected to COMMAND until a line contains TERMINATOR. TERMINATOR lines are not redirected to COMMAND.

COMMAND «- TERMINATOR

Same as above except leading tab characters are stripped from input lines.

Explanation

If the initial TERMINATOR is not quoted, all the lines input will be subject to parameter expansion, command substitution, and arthimetic expansion. \, $, and \must be escaped with`, and new lines character are ignored.

If the initial TERMINATOR is quoted, no expansion will occur. The second occurence of TERMINATOR should not be quoted.

noclobber

By default, standard distros ship with an option called noclobber off. With noclobber on, the > operator will not overwrite existing files and will instead return an error. The >| operator can be used to override this behaviour and force an overwrite. noclobber has no impact on the >> operator.

Description	Command
Turn on `noclobber`.	`set -o noclobber`
Turn off `noclobber`.	`set +o noclobber`

Gotchas

COMMAND >& FILE:
Another alternative to COMMAND &> FILE for redirecting both stdout a nd stderr. Bad practice. FILE may not expand to a number or -.

To do

Custom file descriptors.
Moving file descriptors.
Opening file descriptors for reading and writing.