What are Redirections in Bash? Example and how-to

About

This article talks about shell redirections in the Bash shell.

Through redirection you can direct the input and output of a command to and from other files and programs, and chain commands together in a pipeline.

Example

Redirect output to stderr for all command that comes next

exec 1>&2

The operator >&2 means redirect the address of file descriptor 1 (stdout) to the address of file descriptor 2 (stderr)

Discard any output

mycommand >/dev/null 2>&1

Operator

>

cat leftFile.txt > rightFile.txt

“Greater than” takes the standard output of the command on the left, and redirects it to the file on the right.

>>

cat myFile.txt >> rightFile.txt

>> takes the standard output of the command on the left and appends (adds) it to the file on the right.

<

cat < rightFile.txt

< takes the standard input from the file on the right and inputs it into the program on the left.

|

cat rightFile.txt | wc

where:

• Shell Data Processing - (WC|Word Count) command (Line count)

| is a pipe. The pipe takes the standard output of the command on the left, and pipes it as standard input to the command on the right.

<<< (3 arrows)

This is a Bash variant, this is an expansion of the here document declaration

The word is expanded and supplied to the command on its standard input.

<<< word

Operators

The fredirection operators may precede or appear anywhere within a simple command or may follow a command.

Order of redirections

Redirections are processed in the order they appear, from left to right.

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

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.

File Descriptor

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close.

Standard stream

In the following descriptions,

• if the file descriptor number is omitted, and the first character of the redirection operator is ‘<’, the redirection refers to the standard input (file descriptor 0).
• If the first character of the redirection operator is ‘>’, the redirection refers to the standard output (file descriptor 1).

Create a symbolic link to redirect a log to stdin/stderr with ln

ln -sf /dev/stdout /var/log/nginx/access.log 
ln -sf /dev/stderr /var/log/nginx/error.log

Redirection Target

The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to:

• brace expansion,
• tilde expansion,
• parameter expansion,
• command substitution,
• arithmetic expansion,
• quote removal,
• filename expansion,
• and word splitting.

If it expands to more than one word, Bash reports an error.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

A failure to open or create a file causes the redirection to fail.

Documentation / Reference

• Redirections