Redirecting Standard Input and Standard Output to and from the Standard File Handles

As well as redirecting the stadard output, you can also redirect other streams, for example to send the standard error data to standard output. In Bash the >, < and >> operators we've already discussed can also be prefixed by the file descriptor (remeber the numbers 0, 1 and 2 in the table earlier) to redirect that stream. If the number is omitted it is assumed to be 1 for the standard output which is why the commands we've used so far work.

This command executes program1 and send's any data it writes to standard error to file1.

program1 2> file1

executes program1, directing the standard error stream to file1.

Here's an example program you can use to test it. Save it as redirect1.py:

import sys
while 1:
    try:
        input = sys.stdin.readline()
        if input:
            sys.stdout.write('Echo to stdout: %s'%input)
            sys.stderr.write('Echo to stderr: %s'%input)
    except KeyboardError:
         sys.exit()

This program constantly polls stdin and echos any message it recieves to both stdout and stdout.

In shells derived from csh (the C shell), the syntax instead appends the & character to the redirect characters, thus achieving a similar result.

Another useful capability is to redirect one standard file handle to another. The most popular variation is to merge standard error into standard output so error messages can be processed together with (or alternately to) the usual output. Example:

find / -name .profile > results 2>&1

will try to find all files named .profile. You need the & character even in Bash this time. Executed without redirection, it will output hits to stdout and errors (e.g. for lack of privilege to traverse protected directories) to stderr. If standard output is directed to file results, error messages appear on the console. To see both hits and error messages in file results, merge stderr (handle 2) into stdout (handle 1) using 2>&1 .

It's possible use 2>&1 before ">" but it doesn't work. In fact, when the interpreter reads 2>&1, it doesn't know yet where standard output is redirected and then standard error isn't merged.

If the merged output is to be piped into another program, the file merge sequence 2>&1 must precede the pipe symbol, thus:

find / -name .profile 2>&1 | less

In Bash a simplified form of the command:

command > file 2>&1

is:

command &>file

or:

command >&file

but don't use these shortcuts or you might get confused. Better to be more verbose and explicit.

The &> operator redirects standard output (stdout) and standard error (stderr) at the same time. This is simpler to type than the Bourne shell equivalent command > file 2>&1.