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16.7. Reading STDERR from a Program
16.7.1. Problem
You want to run a program as you
would with system, backticks, or
open, but you don't want its
STDERR to be sent to your
STDERR. You would like to be able to either ignore
or read the STDERR.
16.7.2. Solution
Use the shell's numeric redirection and duplication syntax for file
descriptors. (We don't check the return value from
open here in order to make the examples easier to
read, but you should always check it in your programs!)To capture a command's STDERR and
STDOUT together:$output = `cmd 2>&1`; # with backticks
# or
$pid = open(PH, "cmd 2>&1 |"); # with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDOUT and discard its
STDERR:$output = `cmd 2>/dev/null`; # with backticks
# or
$pid = open(PH, "cmd 2>/dev/null |"); # with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDERR and discard its
STDOUT:$output = `cmd 2>&1 1>/dev/null`; # with backticks
# or
$pid = open(PH, "cmd 2>&1 1>/dev/null |"); # with an open pipe
while (<PH>) { } # plus a read
To exchange a command's STDOUT and
STDERR, i.e., capture the
STDERR but have its STDOUT come
out on our old STDERR:$output = `cmd 3>&1 1>&2 2>&3 3>&-`; # with backticks
# or
$pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|"); # with an open pipe
while (<PH>) { } # plus a read
To read both a command's STDOUT and its
STDERR separately, it's easiest and safest to
redirect them separately to files, and then read from those files
when the program is done:system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
16.7.3. Discussion
When you launch a command with backticks, a piped
open, or system on a single
string, Perl checks for characters special to the shell. These allow
you to redirect the new program's file descriptors.
STDIN is file descriptor number 0,
STDOUT number 1, and STDERR
number 2. You can then use
2>file to redirect
STDERR to a file. The special notation
&N where N is a file descriptor number is used
to redirect to a file descriptor. Therefore,
2>&1 points STDERR at
STDOUT.Table 16-1 lists some interesting shell file
descriptor redirections.
Table 16-1. Redirections and their meanings
Redirection | Meaning |
|---|---|
0</dev/null | Make STDIN give immediate EOF |
1>/dev/null | Discard STDOUT |
2>/dev/null | Discard STDERR |
2>&1 | Send STDERR to STDOUT instead |
2>&- | Close STDERR (not recommended) |
3<>/dev/tty | Open fd 3 to /dev/tty in read-write mode |
sequences from the Solution:$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
There are four steps here:Step A: 3>&1
Make a new file descriptor, number 3, be a copy of number 1. This
saves the destination of STDOUT in the new file
descriptor we've just opened.Step B: 1>&2
Make STDOUT go wherever STDERR
had been going. We still have the saved destination squirreled away
in descriptor 3.Step C: 2>&3
Make file descriptor 2 a copy of number 3. That means that
STDERR is now going out where
STDOUT originally had been going.Step D: 3>&-
Since we're done moving streams around, keep everything nice and tidy
and close our temporary file descriptor. This avoids file descriptor
leaks.If that's confusing, it might help to think in terms of regular
variables and a sequence of assignment statements, with
$fd1 representing STDOUT and
$fd2 representing STDERR. If
you wanted to exchange the two variables, you'd use a temporary file
to hold one value. That's all we're doing here.$fd3 = $fd1;
$fd1 = $fd2;
$fd2 = $fd3;
$fd3 = undef;
When all's said and done, the string returned from the backticks is
the command's STDERR, and its
STDOUT has been diverted to the original
STDERR.Ordering is important in all of these examples because the shell
processes file descriptor redirections in strictly left to right
order.system("prog args 1>tmpfile 2>&1");
system("prog args 2>&1 1>tmpfile");
The first command sends both standard out and standard error to the
temporary file. The second command sends only the old standard output
there, and the old standard error shows up on the old standard out.
If that's confusing, think in terms of assignments to variables
representing file descriptors. For example:# system ("prog args 1>tmpfile 2>&1");
$fd1 = "tmpfile"; # change stdout destination first
$fd2 = $fd1; # now point stderr there, too
is very different from:# system("prog args 2>&1 1>tmpfile");
$fd2 = $fd1; # stderr same destination as stdout
$fd1 = "tmpfile"; # but change stdout destination
16.7.4. See Also
Your system's sh(1) manpage (if you have one)
for details about file descriptor redirection; the
system function in Chapter 29 of
Programming Perl and in
perlfunc(1)
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| 16.6. Preprocessing Input |  | 16.8. Controlling Input and Output of Another Program |
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