Difference between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1

Just looking for the difference between

  • 2>&-
  • 2>/dev/null
  • |&
  • &>/dev/null
  • >/dev/null 2>&1

and their portability with non-Bourne shells like tcsh, mksh, etc.

Asked By: Det

||

This is for redirecting the STDERR & STDOUT:

  • 2>/dev/null

    Redirect STDERR to /dev/null (prevent from showing up on console)

  • |&

    Redirect STDERR and STDOUT to STDIN of piped command (cmd1 |& cmd2)

  • &>/dev/null

    Redirect both STDERR & STDOUT to /dev/null (nothing shows up on console)

  • >/dev/null

    Redirect STDOUT to /dev/null (only STDERR shows on console)

  • 2>&-

    Is for closing a file descriptor used with redirection

These are all standard redirection methods for the Bourne shells.

Answered By: BriGuy

For background:

  • a number 1 = standard out (i.e. STDOUT)
  • a number 2 = standard error (i.e. STDERR)
  • if a number isn’t explicitly given, then number 1 is assumed by the shell (bash)

First let’s tackle the function of these. For reference see the Advanced Bash-Scripting Guide.

Functions

2>&-

The general form of this one is M>&-, where “M” is a file descriptor number. This will close output for whichever file descriptor is referenced, i.e. “M”.

2>/dev/null

The general form of this one is M>/dev/null, where “M” is a file descriptor number. This will redirect the file descriptor, “M”, to /dev/null.

2>&1

The general form of this one is M>&N, where “M” & “N” are file descriptor numbers. It combines the output of file descriptors “M” and “N” into a single stream.

|&

This is just an abbreviation for 2>&1 |. It was added in Bash 4.

&>/dev/null

This is just an abbreviation for >/dev/null 2>&1. It redirects file descriptor 2 (STDERR) and descriptor 1 (STDOUT) to /dev/null.

>/dev/null

This is just an abbreviation for 1>/dev/null. It redirects file descriptor 1 (STDOUT) to /dev/null.

Portability to non-bash, tcsh, mksh, etc.

I’ve not dealt much with other shells outside of csh and tcsh. My experience with those 2 compared to bash’s redirection operators, is that bash is superior in that regard. See the tcsh man page for more details.

Of the commands you asked about none are directly supported by csh/tcsh. You’d have to use different syntaxes to construct similar functions.

Answered By: slm

Consider this an addendum to the selected answer.
You may want to know which forms are POSIX and which are not.

Two POSIX forms are involved:

2.7.2 Redirecting Output

The two general formats for redirecting output are:

[n]>word

[n]>|word

where the optional n represents the file descriptor number. If the
number is omitted, the redirection shall refer to standard output
(file descriptor 1).

Output redirection using the ‘>’ format shall fail if the noclobber
option is set (see the description of set -C) and the file named by
the expansion of word exists and is a regular file. Otherwise,
redirection using the ‘>’ or “>|” formats shall cause the file whose
name results from the expansion of word to be created and opened for
output on the designated file descriptor, or standard output if none
is specified. If the file does not exist, it shall be created;
otherwise, it shall be truncated to be an empty file after being
opened.

2.7.6 Duplicating an Output File Descriptor

The redirection operator:

[n]>&word

shall duplicate one output file descriptor from another, or shall
close one. If word evaluates to one or more digits, the file
descriptor denoted by n, or standard output if n is not specified,
shall be made to be a copy of the file descriptor denoted by word; if
the digits in word do not represent a file descriptor already open for
output, a redirection error shall result; see Consequences of Shell
Errors. If word evaluates to ‘-‘, file descriptor n, or standard
output if n is not specified, is closed. Attempts to close a file
descriptor that is not open shall not constitute an error. If word
evaluates to something else, the behavior is unspecified.

Therefore:

Function      POSIX-compat    POSIX 
2>&-          Yes             close 
2>/dev/null   Yes             redir
2>&1          Yes             dup 
|&            No              
&>/dev/null   No
>/dev/null    Yes             redir
>&/dev/null   ?               ?dup

The last line is not in the original question, but it works without complaint in bash. (Also works with /dev/tty substituted for /dev/null).

Answered By: Craig Hicks