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How to parse the usage of Perl command line program, for this problem, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and easy way.

Perl command line program

Command line program

The following Perl parameters make it easy for short Perl programs to run on the command line. -e allows Perl code to be executed directly by the compiler on the command line. For example, we can run the "HelloWorld" program directly on the command line without writing it as a Perl program.

$Perl

-e'print "HelloWorld\ n"'

Multiple-es can also be used at the same time, running in order according to where they appear.

$Perl

-e'print "Hello" $$'- e'print "World\ n"

Like all Perl programs, only the * line of the program does not need to end with;.

Although you can use-e to reference a module,-M makes it easier.

$Perl

-MLWP::Simple-e'printhead "http://www.example.com"'

The-M module name is the same as the use module name. Some modules have default module imports, and if you don't want to import them, you can use-m. The-m module name, like usemodule (), turns off the default import. For example, the following example, because the head function is imported by default and does not execute when-m is used, the result is no output.

$Perl

-mLWP::Simple-e'printhead "http://www.example.com"'

-m and-M have a lot of convenient syntax to help you use them, and you can list the various parameters for use after =.

$Perl

-MCGI=:standard-e'printheader'

Here, CGI.pm 's: standard is introduced, so the header function can be used. Multiple parameters can be introduced by using quotes and commas.

$Perl

-MCGI='header,start_html'-e'printheader,start_html'

Here we introduce the header and start_html functions.

Implicit Loop of Perl Command Line Program

-n and-p add loops so that you can process files one by one.

$Perl

-n-e'somecode'file1

This is the same as the following procedure.

LINE:

While () {

# yourcodegoeshere

}

Note: open the file on the command line and read it line by line. Each line will be saved by default at $_

$Perl

-n-e'print "$.-$_" file

The above line can be written as

LINE:

While () {

Print "$.-$_"

}

Output the current number of lines $. And the current line $_.-p makes the above program easier.-p outputs the contents of $_, like this:

LINE:

While () {

# yourcodegoeshere

} continue {

Printordie "- PdestinationVariations!\ n" $$

}

Continue is here to ensure that print is called in each loop.

Using-p, our print line count program can be changed to

$Perl

-pMuthyeaded rooms = "$.-$_"'

In this case, we do not need to explicitly call the print function, because the-p option has already called it.

Note that the LINE: tag allows us to jump directly to the next input record, no matter how many layers of loop you enter. Use nextLINE.

$Perl

-nmureholder nextLINEunlessracking.

Of course, it can also be written like this:

$Perl

-nMutual printunlessplains nplink'

In more complex situations, nextLINE can make your code easier to understand.

If you want to do some processing before and after the loop, you can use BEGIN or ENDblock. The following line of code calculates the number of words in the text file.

$Perl

-ne'END {print$t} @ wicked / (\ w+) / gposit alternate files. Txt

Put all matching words in each line into the array @ w, and then increment the number of elements of @ w to $t. The total number of words in the print*** output file in ENDblock.

There are two more parameters that can make the program easier. -a turns on automatic detach (split) mode. The space is the default separator. The input is separated according to the separation number and put into the default array @ F. Therefore, we can rewrite the above program as

$Perl

-ane'END {print$x} $x+=@F'file.txt

You can also use-F to change the default separation number to what you want. For example, set the separation number to a non-character:

$Perl

-F'\ W'-ane'END {print$x} $x+=@F'file.txt

Let's introduce a complex example through the Unixpassword file. Unixpassword is a text file, and each line is a user record, separated by a colon. Number one? The line is the user's login shell path. We can figure out how many users are using each different shell path:

$Perl

-F':'-ane'$s {$F [6]} +;'\

>-e'END {print "$_: $s {$_}" forkeys%s}'/ etc/passwd

Although it is not a line now, you can see what problems can be solved by using parameters.

Data delimiters for Perl command line programs

I mentioned $/ and $\-input and output separators in my previous articles. $/ is used to separate data read from the file handle, and the default $/ separator is\ n, so that line by line is read from the file handle each time. $\ is an empty character by default and is automatically added to the end of the data to be print. That's why most of the time print has to add\ n at the end.

$/ and $\ can be used with-nmurp. On the command line, they correspond to-0 (zero) and-l (this is L). -0 can be followed by a hexadecimal or octal value, which is assigned to $/. -0777 turns on paragraph mode, and-00 turns on slurp mode (that is, you can read the entire file in at once), which is the same as setting $/ to empty characters and undef.

Using-l alone has two effects: * * automatically chomp the separator, and the second $/ value is paid to $\ (so that print will automatically add\ n at the end)

Personally, I often use the-l parameter to add\ n to each output. For example

$Perl

-le'print "HelloWorld"'

In-situ editing of Perl command line program

Using the existing parameters, we can write a very effective command line program. Common UnixI/O redirects:

$Perl

-pe'somecode'output.txt

This program reads data from input.txt, then does some processing and then outputs it to output.txt. Of course, you can also redirect the output to the same file.

The above program can be made easier with the-I parameter. -I rename the source file and read it from the renamed source file. * write the processed data to the source file. If-I is followed by another string, the string is combined with the source file name to generate a new file name. This file will be used to store the original file so as not to be overwritten by the-I parameter.

This example replaces all php characters with Perl:

$Perl

-i-pe's/\ bPHP\ b/Perl/g'file.txt

The program reads every line of the file, then replaces characters, and the processed data is rewritten (that is, overwritten) to the source file. If you do not want to overwrite the source file, you can use

$Perl

-i.bak-pe's/\ bPHP\ b/Perl/g'file.txt

The data processed here is written to file.txt,file.txt.bak as a backup of the source file.

This is the answer to the question on how to parse the usage of the Perl command line program. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel for more related knowledge.

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