Shell IO Redirection – Differences Between &>, >& and 2>&1

io-redirectionshellzsh

On this SO thread and a few other threads I have seen the following commands for redirecting stdout and stderr to a file.

Are they all equivalent? Is there any difference between them?

command1 >> logfile 2>&1
command &> logfile
command >& logfile

Best Answer

Since you have tagged zsh, let me tell you that all the 3 redirections works exactly the same way. As you might have read in both the duplicate posts (the one in the comment and the one in your post), they all redirect stderr to stdout which inturn is redirected to the file 'logfile' (ie, the logfile will contain both the output and errors).

But their behaviour changes a LOT depending on the shell you are in.

All the three styles of redirections works well in the same way in bash and zsh

But:

Only >& works in csh or tcsh

[soum@server ~]$  ./test.sh > logfile 2>&1
Ambiguous output redirect.
[soum@server ~]$ ./test.sh &> logfile
Invalid null command.
[soum@server ~]$ ./test.sh >& logfile
[soum@server ~]$ echo $SHELL
/bin/tcsh
[soum@server ~]$

In ksh only 2>&1 works. 

$ ./test.sh >& logfile
-ksh: logfile: bad file unit number
$ ./test.sh &> logfile
[1]     23039
$ 1  2  3  4  5  6  logfile  test.sh
ls: cannot access ttr: No such file or directory

[1] +  Done(2)                 ./test.sh &> logfile

I hate ksh. While >& just gave an error, the &> backgrounded a part of the command and emptied the logfile (if non-empty).

Related Question