bash Cookbook

Using Multiple Redirects on One Line

Problem

You want to redirect output to several different places.

Solution

Use redirection with file numbers to open all the files that you want to use. For example:

$ divert 3> file.three 4> file.four 5> file.five 6> else.where
$

where divert might be a shell script with various commands whose output you want to send to different places. For example, you might write divert to contain lines like this: echo option $OPTSTR >&5. That is, our divert shell script could direct its output to various different descriptors which the invoking program can send to different destinations.

Similarly, if divert was a C program executable, you could actually write to descriptors 3, 4, 5, and 6 without any need for open() calls.

Discussion

In an earlier recipe we explained that each file descriptor is indicated by a number, starting at 0 (zero). So standard input is 0, out is 1, and error is 2. That means that you could redirect standard output with the slightly more verbose 1> (rather than a simple >) followed by a filename, but there’s no need. The shorthand> is fine. It also means that you can have the shell open up any number of arbitrary file descriptors and have them set to write various files so that the program that the shell then invokes from the command line can use these opened file descriptors without further ado.

While we don’t recommend this technique, it is intriguing.

See Also

• Saving Output to Other Files

• Sending Both Output and Error Messages to Different ...