sh command: exec 2>&1

Question

What will this command do?

exec 2>&1 

Answer 1

One quite useful application of exec 2>&1 that I have come across is when you want to merge stderr and stdout for several commands separated by semicolons. My particular example happened when I was sending more than one command to popen in PHP and I wanted to see the errors interleaved like you would see if you typed the commands at a shell prompt:

$ echo hi ; yikes ; echo there
hi
-bash: yikes: command not found
there
$ 

The following does not merge stderr and stdout except for the last echo (which is pointless because the yikes causes the error):

echo hi ; yikes ; echo there 2>&1

I can get the merged output the "hard way" as follows:

echo hi 2>&1; yikes 2>&1; echo there 2>&1

It looks a lot cleaner and less error prone if you use exec:

exec 2>&1 ; echo hi; echo yikes; echo there

You get the stdout and stderr output nicely interleaved exactly as you would see on the terminal if you executed the three commands separated by a semicolon.