13.3 Send (only) Standard Error Down a Pipe

A vertical bar character ( | ) on a command line pipes the standard output of a process to another process. How can you pipe the standard error, not the standard output? You might want to put a long-running cruncher command in the background, save the output to a file, and mail yourself a copy of the errors. In the C shell, run the command in a subshell ( 13.7 ) . The standard output of the command is redirected inside the subshell. All that's left outside the subshell is the standard error; the |& operator ( 13.5 ) redirects it (along with the empty standard output) to the mail ( 1.33 ) program:

% 

(cruncher > 



outputfile



) |& mail 



yourname

 & [1] 12345

Of course, you don't need to put that job in the background ( 1.26 ) . If you want the standard output to go to your terminal instead of a text file, use /dev/tty ( 45.20 ) as the outputfile .

The Bourne shell gives you a lot more flexibility and lets you do just what you need. The disadvantage is the more complicated syntax ( 45.21 ) . Here's how to run your cruncher program, route the stderr through a pipe to the mail program, and leave stdout going to your screen:

$ 

(cruncher 3>&1 1>&2 2>&3 3>&-) | mail 



yourname



 &

 12345

To redirect stdout to an output file and send stderr down a pipe, try this:

$ 

(cruncher 3>&1 >



outputfile



 2>&3 3>&-) | mail 



yourname



 &

 12345