Notes on Redirection in Bash

Motivation

What does the following command mean? I don’t fully understand the redirection before reading docs.

exec chroot . sh <dev/console >/dev/console 2>&1

Read the Docs

In bash(1), general introduction:

Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. Redirection may also be used to modify file handles in the current shell execution environment.

Redirecting Input and Output

The following docs explains < in our example.

`Redirecting Input Redirection``: of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.The general format for redirecting input is: [n]<word

The following docs explains > in our example.

Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.The general format for redirecting output is:[n]>word

Note that the order of redirections is significant. For example, the command

              ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist, while the command

              ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist.

Duplicating File Descriptor

The redirection operator

              [n]<&word

is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to -, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.

The operator

              [n]>&word

is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used.If the digits in word do not specify a file descriptor open for output, a redirection error occurs. If word evaluates to -, file descriptor n is closed. As a special case, if n is omitted, and word does not expand to one or more digits or -, the standard output and standard error are redirected as described previously.

Some simplication can be made.

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms, the first is preferred. This is semantically equivalent to >word 2>&1

Opening File Descriptors for Reading and Writing

The redirection operator [n]<>word causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.