Programming Perl, 4th Edition

Command Input (Backtick) Operator

First of all, we have the command input operator, also known as the backtick operator, because it looks like this:

$info = `perldoc $module`;

A string enclosed by backticks (grave accents, technically) first undergoes variable interpolation just like a double-quoted string. The result is then interpreted as a command line by the system, and the output of that command becomes the value of the pseudoliteral. (This is modelled after a similar operator in Unix shells.) In scalar context, a single string consisting of all the output is returned. In list context, a list of values is returned, one for each line of output. (You can set $/ to use a different line terminator.)

The command is executed each time the pseudoliteral is evaluated. The numeric status value of the command is saved in $? (see Chapter 25 for the interpretation of $?, also known as $CHILD_ERROR). Unlike the csh version of this command, no translation is done on the return data—newlines remain newlines. Unlike in any of the shells, single quotes in Perl do not hide variable names in the command from interpretation. To pass a $ through to the shell, you need to hide it with a backslash. The $module in our perldoc example above is interpolated by Perl, not by the shell. (Because the command undergoes shell processing, see Chapter 20 for security concerns.)

The generalized form of backticks is qx// (for “quoted execution”), but the operator works exactly the same way as ordinary backticks. ...