Perl One Liner Construction

Perl one liners require skill and imagination to construct. Consider the requirement to replace an arbitrary number of lines between a start and an end line with the output of a command. First, suitable input data must be generated: there is no better test than to run actual data through the devised solution. Spare your audience the untested guesses, and when ready, include the test code and data in your release, so that others can confirm the results, or think of cases your tests may have missed. The input data should include several lines of data to be replaced, as well as lines before and after the start and end tags to confirm the solution does not affect data it should not:

$ < input first start replacethis and this too end last

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Portions of the data will not be printed; therefore, the -n switch should be used to loop without printing.

$ first start replacethis and this too end last

The arbitrary lines between start and end suggest the range operator, and otherwise to print lines outside this range. Memorizing the various operators and functions and knowing how to apply them to various tasks is an essential skill for Perl programming. The Perl Cookbook can help bridge a lack of skill, as it offers specific examples and solutions to those problems.

$ -e '{ ; } else { print }' first last

Next, the shell command. Backticks suffice here, as the command output can be passed directly to print:

$ -e '{ print `pwd` } else { print }' first /home/jmates/tmp /home/jmates/tmp /home/jmates/tmp /home/jmates/tmp last

This reveals a bug, though, as the shell command is being executed for each line between and including the start to end lines. Since the command should be executed only once, the logical solution is a state variable that only allows the command to run once:

$ -e '{ $d++ || print `pwd` }' \ -e 'else { print }' first /home/jmates/tmp last

This assignment relies on autovivication, where $d, false by default, first fails the check, and allows the shell command to run. After the first check fails, $d increments to a true value, and will therefore no longer allow the command to run. A full solution would also print the start and end tags, as the requirement called for only the text between the tags to be replaced. That code is left as an exercise.

A robust solution would check the shell command for errors, and perhaps also handle standard error output. The Perl one liners article includes a section on converting one liners to proper scripts.

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Perl One Liner Construction