Perl one-liners

Useful options for Perl one-liners

• -e It allows you to provide the program as an argument rather than in a file.
• -p Places a printing loop around your command so that it acts on each line of standard input.
• -n Places a non-printing loop around your command.
• -i Modifies your input file in-place (making a backup of the original). Handy to modify files without the {copy, delete-original, rename} process.
• -w Activates some warnings. Any good Perl coder will use this.
• -d Runs under the Perl debugger. For debugging your Perl code, obviously.
• -l Octal_number - Sets $\ () to the character represented by that number and also turns on auto-chomping.
• -M Module::Name Use “Module::Name”

Some snipset

1. Replace “this” with “that” in a list of files

   perl -pi -e 's/this/that/g' file1 file2 file3
   

2. Replace “this” with “those” in a list of files, but only on lines that match also “these”.

   perl -pi -e 's/this/those/g if /these/' file
   

   ```

3. Find all repeated lines in a file

   perl -ne 'print if $a{$_}++' file
   

4. Like the command :unix2dos (conversion from Unix to Windows format). The option -i.bak will create backup You can convert Unix file back to Windows format too:

   perl -i.bak -pe 's/\n/\r\n/' filename 
   

5. Print range of lines in file (e.g lines 21-37)

   perl -ne 'print if 21 .. 37'
   

6. Print balance of quotes in each line (useful for finding missing quotes in Perl and other scripts)

   perl -ne '$q=($_=~tr/"//); print"$.\t$q\t$_";' filename
   

7. Print only the lines between “START” and “END”

   perl -i.old -ne 'print if /^START$/ .. /^END$/' foo.txt 
   

8. Compress consecutive empty row instances to a single instance

    perl -00 -pe '' file
   

9. Encod/Decode Base64

    perl -MMIME::Base64 -0777 -ne 'print encode_base64($_)' file
    perl -MMIME::Base64 -le 'print decode_base64("string")'
   

   Wait there is more :-)

   • For URL escape - use URL::escape::uri_escape("$string") and URL::escape::uri_unescape("$esc_string")
   • For HTML encode - use HTML::Entities::encode_entities("<string>") and decode_entities("<string>")

How does it work?

The option -p vs -n

While running:

perl -n -e 'some code' file1

Perl will translate it into the following code:

while (<>) {
        some code
}

The option -l

This option enables automatic line-ending processing. It has two effects:

1. It automatically chops the line terminator when used with -n or -p
2. Assigns $\ to have the value of octal number so that any print statements will have that line terminator added back on. If octal number is omitted, sets $\ to the current value of $/. F For instance, to trim lines to 80 columns:

   perl -lpe 'substr($_, 80) = ""'
   

   Note: The assignment $\ = $/ is done when the switch -l is processed, so the input record separator can be different than the output record separator if the -l`` switch is followed by a-0switch: bash find / -print0 | perl -ln0e 'print "found $_" if -p' `` This sets $\ to newline and then sets $/ to the null character.

The variables: $/ and $\

• $/ - The input record separator, newline by default. This influences Perl’s idea of what a “line” is. Works like awk’s RS variable, including treating empty lines as a terminator if set to the null string (an empty line cannot contain any spaces or tabs).

• $\ - The output record separator for the print operator. If defined, this value is printed after the last of print’s arguments. Default is undef. You cannot call output_record_separator() on a handle, only as a static method. See IO::Handle. You set $\ instead of adding “\n” at the end of the print. Also, it’s just like $/ , but it’s what you get “back” from Perl. ✔