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Theodor Zlatanov collection of Perl one-liners


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Theodor Zlatanov collection

In 2003 Theodor Zlatanov, who authored a series of interesting articles about Perl on IBM DeveloperWorks site published an article Cultured Perl One-liners 102. Among other interesting one-liners he provided a collection of useful one-liners of using ranges in Perl one-lines as well as in place editing:

Listing 3: Printing a range of lines
# 1. just lines 15 to 17
perl -ne 'print if 15 .. 17'

# 2. just lines NOT between line 10 and 20
perl -ne 'print unless 10 .. 20'

# 3. lines between START and END
perl -ne 'print if /^START$/ .. /^END$/'

# 4. lines NOT between START and END
perl -ne 'print unless /^START$/ .. /^END$/'

A problem with the first one-liner in Listing 3 is that it will go through the whole  file, even if the necessary range has already been covered. The third one-liner does not  have that problem, because it will print all the lines between the START  and END  markers. If there are eight sets of START/END  markers, the third one-liner will print the lines inside all eight sets.

Preventing the inefficiency of the first one-liner is easy: just use the $.  variable, which tells you the current line. Start printing if $.  is over 15 and exit if $.  is greater than 17.

... ... ...

Listing 5: In-place editing
# 1. in-place edit of *.c files changing all foo to bar
perl -p -i.bak -e 's/\bfoo\b/bar/g' *.c

# 2. delete first 10 lines
perl -i.old -ne 'print unless 1 .. 10' foo.txt

# 3. change all the isolated oldvar occurrences to newvar
perl -i.old -pe 's{\boldvar\b}{newvar}g' *.[chy]

# 4. increment all numbers found in these files
perl -i.tiny -pe 's/(\d+)/ 1 + $1 /ge' file1 file2 ....

# 5. delete all but lines between START and END
perl -i.old -ne 'print unless /^START$/ .. /^END$/' foo.txt

# 6. binary edit (careful!)
perl -i.bak -pe 's/Mozilla/Slopoke/g' /usr/local/bin/netscape

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Cultured Perl One-liners 101

find . -name "*.mp3" | perl -pe 's/.\/\w+-(\w+)-.*/$1/' | sort | uniq

perl -pi -e'$_ = sprintf "%04d %s", $., $_' test # inserting numbers in the file

find . -name "*.jpg" | perl -ne'chomp; $name = $_; $quote = chr(39); s/[$quote\\!]/_/ ; print "mv \"$name\" \"$_\"\n"'

Cultured Perl One-liners 102

Grep-style operations

#grep abba foo

perl -en 'print if /abba/' foo

Working with columns

Cultured Perl One-liners 102

# add first and penultimate columns
# NOTE the equivalent awk script:
# awk '{i = NF - 1; print $1 + $i}'
perl -lane 'print $F[0] + $F[-2]'

Sed-style operations

Practical uses include omitting lines matching a regular expression, printing a range of lines, inplace editing of multiple files, etc

Printing of the range of lines:

$ (echo a; echo b) | perl -nle 'print unless /b/'

$ (echo a; echo b) | perl -nle 'print unless $. == 1'

Any time the $. line number variable is being used with multiple files, the eof function may need to be used to reset the current line number counter. The following one-liners demonstrate this feature by reading the file input twice, and resetting the line number counter in the second case.

# print the range of lines 5 to 50
perl -ne 'print if $. >= 5; exit if $. >= 50;'

Add a line to a file

Appending data to existing files is easy. So is inserting data into arbitrary locations in a file, such as prepending a new first line to a set of files. In the following case, #!/usr/bin/perl will be added as the first line of all *.pl files in the current directory.

$ perl -i -ple 'print q{#!/usr/bin/perl} if $. == 1; close ARGV if eof' *.pl

If a recursive replace is needed, either investigate the use of the modules File::Find or IO::All, or simply have the unix shell pull in the required files as arguments to perl. While the second example is longer, it will work properly if filenames have spaces in their names, due to find -print0 and xargs -0 using the NUL character to delimit filenames instead of spaces.

$ perl -i -ple 'print q{#!/usr/bin/perl} if $. == 1; close ARGV if eof' \
`find . -type f -name "*.pl"`

$ find . -type f -name "*.pl" -print0 | \
xargs -0 perl -i -ple 'print q{#!/usr/bin/perl} if $. == 1; close ARGV if eof'

The following trick shows how to replace the second line of a file with some text, but only if that line is blank.

$ perl -ple 's/^$/some text/ if $. == 2; close ARGV if eof'

Home on the range

To skip ranges of text, use the .. operator. This operator is documented in perlop. The following one-liners illustrate different ways of collapsing runs of newlines. The first example eliminates all blank lines.

$ cat input

$ perl -ne 'print unless /^$/../^$/' input

The unless statement is equivalent to if not, but is different from if ! due to the associativity and precedence rules covered in perlop. A benefit of this behavior allows the reduction of runs of blank lines to a single blank line.

$ perl -ne 'print if ! /^$/../^$/' input


Line numbers can also be used with the range operator, for instance to remove the first four lines of a file.

$ perl -nle 'print unless 1 .. 4' input

Altering record parsing

Perl uses the -0 option to allow changing the input record separator. The two main uses of this option are -00 to operate in paragraph mode, and -0777 to read all input into $_ at once. The paragraphs file contains the -0 documentation from perlrun, and is used in the following example to extract just the paragraph with the word special in it.

$ perl -00 -ne 'print if /special/' paragraphs

The special value 00 will cause Perl to slurp files in paragraph mode. The value 0777 will cause Perl to slurp files whole because there is no legal byte with that value.

Parsing the entire input file as a single line can be used to alter the newlines that otherwise require a range operator to deal with, as shown above. The following is a different way to remove runs of newlines from a file: by treating the entire file as a single line, a repeating s///g expression can be used to replace newlines as needed.

$ cat input

$ perl -0777 -pe 's/\n+/\n/g' input

Custom Quoting

Shell quoting may cause problems when writing expressions on the command line. Single quotes are usually used to delimit Perl expressions, to prevent shell interpolation of the code. To use a literal single quote inside such a single quoted string, the awkward '\'' syntax will need to be used, to end the single quoted string, include a literal quote, then restart the quoted string.

$ perl -le 'print "'\'' is a single quote"'
' is a single quote

Alternatives include using an octal escape code instead; see ascii(1) for a listing of codes.

$ perl -le 'print "\047 is a single quote"'
' is a single quote

Perl also allows different quoting operators, see the "Quote and Quote-like Operators" section under perlop for more information on these.

$ perl -le 'print q{single quoted: $$} . qq{ interpolated: $$}'
single quoted: $$ interpolated: 11506

Output to Multiple Files

To split output among multiple files, change where standard output points at based on some test. For example, the following will split a standard unix mailbox file inbox into multiple files named out.*, incrementing a number for each message in the mailbox.

$ perl -pe 'BEGIN { $n=1 } open STDOUT, ">out.$n" and $n++ if /^From /' inbox

Converting One Liners

One liners may be used as quick example code, or could be found in someone's shell history. The following section demonstrates how to convert such one liners to full Perl scripts.

In-place editing

# 1. in-place edit of *.c files changing all foo to bar
perl -p -i.bak -e 's/\bfoo\b/bar/g' *.c

# 2. delete first 10 lines
perl -i.old -ne 'print unless 1 .. 10' foo.txt

# 3. change all the isolated oldvar occurrences to newvar
perl -i.old -pe 's{\boldvar\b}{newvar}g' *.[chy]

# 4. increment all numbers found in these files
perl -i.tiny -pe 's/(\d+)/ 1 + $1 /ge' file1 file2 ....

# 5. delete all but lines between START and END
perl -i.old -ne 'print unless /^START$/ .. /^END$/' foo.txt

# 6. binary edit (careful!)
perl -i.bak -pe 's/Mozilla/Slopoke/g' /usr/local/bin/netscape 



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