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Learning Perl, Second Edition by Randal L. Schwartz, Tom Christiansen

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14.2. Using Backquotes

Another way to launch a process is to put a /bin/sh shell command line between backquotes. Like the shell, this fires off a command and waits for its completion, capturing the standard output as it goes along:

$now = "the time is now ".`date`; # gets text and date output

The value of $now winds up with the text the time is now along with the result of the date (1) command (including the trailing newline), so it looks something like this:

the time is now Fri Aug 13 23:59:59 PDT 1993

If the backquoted command is used in a list context rather than a scalar context, you get a list of strings, each one being a line (terminated in a newline[2]) from the command's output. For the date example, we'd have just one element because it generated only one line of text. The output of who looks like this:

merlyn     tty42    Dec  7 19:41
fred       tty1A    Aug 31 07:02
barney     tty1F    Sep  1 09:22

[2] Or whatever you've set $/ to.

Here's how to grab this output in a list context:

foreach $_ (`who`) { # once per text line from who
    ($who,$where,$when) = /(\S+)\s+(\S+)\s+(.*)/;
    print "$who on $where at $when\n";
}

Each pass through the loop works on a separate line of the output of who, because the backquoted command is evaluated within a list context.

The standard input and standard error of the command within backquotes are inherited from the Perl process.[3] This means that you normally get just the standard output of the commands within the backquotes as the value of the backquoted-string. ...

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