qx/STRING/
`STRING`

A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).

Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell supports this) if you care to address this. To capture a command's STDERR and STDOUT together:

$output = `cmd 2>&1`;

To capture a command's STDOUT but discard its STDERR:

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

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 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");

Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead:

$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$

Note that how the string gets evaluated is entirely subject to the command interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as it's unclear how to escape which characters. See the perlsec manpage for a clean and safe example of a manual fork() and exec() to emulate backticks safely.

On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell supports that (e.g. ; on many Unix shells; & on the Windows NT cmd shell).

Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings don't exceed this limit after any necessary interpolations. See the platform-specific release notes for more details about your particular environment.

Using this operator can lead to programs that are difficult to port, because the shell commands called vary between systems, and may in fact not be present at all. As one example, the type command under the POSIX shell is very different from the type command under DOS. That doesn't mean you should go out of your way to avoid backticks when they're the right way to get something done. Perl was made to be a glue language, and one of the things it glues together is commands. Just understand what you're getting yourself into.

See I/O Operators for more discussion.

Originally Posted by satimis

Hi folks,

Perl
===

Instead of using following command printing output to a file;

$ perl AAA.pl > /pathto/name_of_file

is it possible adding "print file function" to the script

1) If YES
Please advise how to make it.

2) Can I add following bash syntax to the script
user=$(whoami);
now=$(date +%Y.%m.%d.%R);
File="/tmp/satimis/comparison_${user}_${now}.txt";

so that the printout file will carry name of creator, date and time, etc.

If possible please shed me some light to re-edit the script.

TIA

B.R.
satimis