Programming Perl, 3rd Edition

Input Operators

There are several input operators we'll discuss here because they parse as terms. Sometimes we call them pseudoliterals because they act like quoted strings in many ways. (Output operators like print parse as list operators and are discussed in Chapter 29.)

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 = `finger $user`;

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 modeled 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 28 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 $user in our finger example above ...