How can I run an external command and capture its output in Perl?

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Solution 1

You can use the backtics to execute your external program and capture its stdout and stderr.

By default the backticks discard the stderr and return only the stdout of the external program.So

$output = `cmd`;

Will capture the stdout of the program cmd and discard stderr.

To capture only stderr you can use the shell's file descriptors as:

$output = `cmd 2>&1 1>/dev/null`;

To capture both stdout and stderr you can do:

$output = `cmd 2>&1`;

Using the above you'll not be able to differenciate stderr from stdout. To separte stdout from stderr can redirect both to a separate file and read the files:

`cmd 1>stdout.txt 2>stderr.txt`;

Solution 2

In most cases you can use the qx// operator (or backticks). It interpolates strings and executes them with the shell, so you can use redirections.

  • To capture a command's STDOUT (STDERR is unaffected):

    $output = `cmd`;
    
  • To capture a command's STDERR and STDOUT together:

    $output = `cmd 2>&1`;
    
  • To capture a command's STDERR but discard its STDOUT (ordering is important here):

    $output = `cmd 2>&1 1>/dev/null`;
    
  • To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR:

    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
    
  • To read both a command's STDOUT and its STDERR separately, it's easiest to redirect them separately to files, and then read from those files when the program is done:

    system("program args 1>program.stdout 2>program.stderr");
    

Solution 3

You can use IPC::Open3 or IPC::Run. Also, read How can I capture STDERR from an external command from perlfaq8.

Solution 4

Beware about the answer of Eugene (can't comment on his answer), just above, that the syntax to exchange SDTOUT and STDERR is valid on Unixes (Unixen-like shells such as ksh, or bash I guess) but not under Windows CMD (error: 3>& was unexpected at this time.).

The appropriate syntax under Windows CMD and Perl on Windows is:

perl -e "$r=qx{nslookup 255.255.255.255 2>&1 1>&3 3>&2};

Note that the command:

nslookup 255.255.255.255

will produce (something like) on STDOUT:

Server:  mymodem.lan
Address:  fd37:c01e:a880::1

and on STDERR:

*** mymodem.lan can't find 255.255.255.255: Non-existent domain

You can test that this syntax works with the following CMD/Perl syntax:

First:

perl -e "$r=qx{nslookup 255.255.255.255 2>&1 1>&3 3>&2}; $r=~s/[\n\r]//eg; print qq{on STDOUT qx result=[$r]};"

you get: Server: mymodem.lan Address: fd37:c01e:a880::1 on STDOUT qx result=[*** mymodem.lan can't find 255.255.255.255: Non-existent domain]

Then

perl -e "$r=qx{nslookup 255.255.255.255 2>&1 1>&3 3>&2}; $r=~s/[\n\r]//eg; print STDOUT qq{on STDOUT qx result=[$r]};" 2>&1 1>NUL:

you get: Server: mymodem.lan Address: fd37:c01e:a880::1

QED [fr:CQFD]

Note that it is not possible to get BOTH stderr and stdout as returned string for a qx or backticks command. If you know for sure that the err text returned by your spawned command is of length N lines, you can still redirect STDERR to STDOUT like describe by Eugene and others but capture your qx returned text in an array instead of as scalar string. The STDERR flow will be returned into the array before the STDOUT so that the N first lines of your array are the SDTERR lines. Like:

@r=qx{nslookup 255.255.255.255 2>&1};
$r[0] is "*** mymodem.lan can't find 255.255.255.255: Non-existent domain"

But of course you must be sure that there is an err text on STDERR and of strictly N lines (stored in @r[0..N-1]). If not, the only solution is using temp files as described above.

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Updated on July 09, 2022

Comments

  • gameover
    gameover over 1 year

    I'm new to Perl and want to know of a way to run an external command (call it prg) in the following scenarios:

    1. Run prg, get its stdout only.
    2. Run prg, get its stderr only.
    3. Run prg, get its stdout and stderr, separately.
    • FMc
      FMc over 13 years
      Do yourself a big favor and spend some time on perldoc.perl.org -- for starters the "Tutorials" and "FAQs" areas. Also run this to familiarize yourself with Perl's system for accessing help info on the command-line: perldoc --help. The answer to your specific question could have been found directly by command-line searches such as these: perldoc -q capture or perldoc -q external.
  • dave4420
    dave4420 over 13 years
    Another way to read stdout and stderr separately, without using temporary files, is to use IPC::Open3.
  • William Pursell
    William Pursell over 13 years
    Every time backticks are used, a kitten dies. In shell, use $(). In perl, use qx.
  • Severun
    Severun over 9 years
    Thanks for that, I killed 20 kittens before I used qx ;)
  • L__
    L__ over 7 years
    The second solution cmd 2>&1 1>/dev/null kills both stdout and stderr. Somehow the shell doesn't identify that the two redirections are separate!!
  • x-yuri
    x-yuri about 5 years
    How do you pass arguments to cmd with backticks, ensuring proper escaping?
  • x-yuri
    x-yuri about 5 years
    Fails: perl -e 'my $f = ".../a b"; print(qx/file $f/);'. Succeeds: perl -e 'use IPC::Open2; my $f = ".../a b"; open2(my $out, my $in, "file", $f); print(readline($out))'
  • x-yuri
    x-yuri about 5 years
    It'd be great to have an example here. You can use my comment for inspiration.
  • Hi-Angel
    Hi-Angel over 3 years
    It is worth adding that the command return code is stored at $?, in case you want to peek at it before making use of the command output.

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