perl:tldr:d4051

The given command performs a specific task using the cat command, perl interpreter, and regular expressions. Here's a breakdown of the command:

1. cat ${filename}: The cat command is used to concatenate and display the contents of the file specified by ${filename} variable. The ${filename} represents a placeholder for an actual filename or path that should be provided when executing the command.

2. |: The pipe symbol (|) is used to redirect the output of the previous command (cat) as input for the next command (perl) in the command pipeline.

3. perl -n -l -e 'print if /${regular_expression}/': This part executes the perl interpreter with a specific command-line options (-n -l -e). The options are explained below:

   • -n option: Specifies that perl should run a loop around the code provided by the following -e option.
   • -l option: Enables automatic line-ending processing. It removes the newline character from the input and appends it while printing.
   • -e option: Allows to specify a Perl program on the command line.

   The Perl program provided in single quotes ('...') is: print if /${regular_expression}/. Here's what it does:

   • print if /${regular_expression}/: It checks if the current line (provided by -n option) matches the regular expression ${regular_expression}. If it does, the line is printed to the output. Otherwise, it is skipped.

So, in summary, the command reads the contents of the ${filename} file, one line at a time. It then checks each line against the ${regular_expression} and prints only the lines that match the regular expression.