Famous Perl One-Liners Explained, Part VI: Selective printing and deleting of lines

Posted: 27 Apr 2011 10:13 AM PDT

This is the sixth part of a nine-part article on famous Perl one-liners. In this part I will create various one-liners for selective printing and deleting of certain lines. See part one for introduction of the series.

Famous Perl one-liners is my attempt to create "perl1line.txt" that is similar to "awk1line.txt" and "sed1line.txt" that have been so popular among Awk and Sed programmers and Linux sysadmins.

The article on famous Perl one-liners will consist of nine parts:

Part I: File spacing.
Part II: Line numbering.
Part III: Calculations.
Part IV: String creation and array creation.
Part V: Text conversion and substitution.
Part VI: Selective printing and deleting of certain lines (this part).
Part VII: Handy regular expressions.
Part VIII: Release of perl1line.txt.
Part IX: Release of Perl One-Liners e-book.

The selective printing and selective deleting of certain lines is actually the same process. If you wish to delete certain lines, you just print the lines you're interested in. Or the other way around! For example, to delete all lines with even line numbers, print the odd lines, and to delete odd lines print the even lines.

After I am done with the 8 parts of the article, I will release the whole article series as a pdf e-book! Please subscribe to my blog to be the first to get it!

Here are today's one-liners:

82. Print the first line of a file (emulate head -1).

 perl -ne 'print; exit'

This is the simplest one-liner so far. Here Perl reads in the first line into $_ variable thanks to -n option, then it calls print statement that prints the contents of the $_ variable. And then it just exists. That's it. The first line got printed and that's all we wanted.

83. Print the first 10 lines of a file (emulate head -10).

 perl -ne 'print if $. <= 10'

This one-liner uses the $. special variable. This variable stands for "current line number." Each time Perl reads in the next line, it increments $. by one. Therefore it's very simple to understand what this one-liner does, it prints the line if the line number is equal to or less than 10.

This one liner can also be written the other way around without use of if statement,

 perl -ne '$. <= 10 && print'

Here the print statement gets called only if $. <= 10 boolean expression is true, and it's true only if current line number is less than or equal to 10.

84. Print the last line of a file (emulate tail -1).

 perl -ne '$last = $_; END { print $last }'

Printing the last line of the file is a bit tricker, because you always have to maintain the previous line in memory. In this one-liner we always save the current line in $_ to $last variable. When Perl program ends, it always executes code in the END block. Now just before exiting it read in the last line, so when it quits, we print $last that prints the last line.

85. Print the last 10 lines of a file (emulate tail -10).

 perl -ne 'push @a, $_; @a = @a[@a-10..$#a]; END { print @a }'

Now this is tricky. Here we push each line to the @a array, and then we replace with a slice of itself. We do @a = @a[@a-10..$#a], which means, replace @a with last 10 elements of a. @a-10 is evaluated in scalar context here and it returns number of elements in the array minus 10. #$a is the last index in the @a array. And @a[@a-10..$#a] takes the last 10 elements of the array, so @a always contains just 10 last elements.

Here is an example. Suppose @a contains ("line1", "line2", "line3", "line4"). And let's say we want to print last 4 lines of the file. Now when we read the 5th line, the array becomes ("line1", "line2", "line3", "line4", "line5"). At this moment @a-4 is 1, because @a in scalar context is 5. The $#a however is 4 because that's the last index in the array. Now taking the slice, @a[@a-4..$#a] is @a[1..4], which drops the front element from the array and the @a array becomes ("line2", "line3", "line4", "line5").

86. Print only lines that match a regular expression.

 perl -ne '/regex/ && print'

Here /regex/ is short for $_ =~ /regex/. Since the -n operator puts every line in $_ variable the /regex/ returns true on all lines that matched the regex. If that happened, print prints the line.

87. Print only lines that do not match a regular expression.

 perl -ne '!/regex/ && print'

This is the same as the previous one-liner, except the regular expression match has been negated. So all the lines that don't match the regex get printed.

88. Print the line before a line that matches a regular expression.

 perl -ne '/regex/ && $last && print $last; $last = $_'

In this one-liner every line gets saved to $last variable. Now when the next line matches /regex/ and there has been a previous line $last, then it print $last prints the last line, and then it assigns the current line to the last line variable via $last = $_.

89. Print the line after a line that matches a regular expression.

 perl -ne 'if ($p) { print; $p = 0 } $p++ if /regex/'

Here we set the variable $p if the line matches a regex. It indicates that the next line should be printed. Now when the next line is read in and $p is set, then that line gets printed and $p gets set to 0 again to reset the state.

90. Print lines that match regex AAA and regex BBB in any order.

 perl -ne '/AAA/ && /BBB/ && print'

This one-liner is basically the same as one-liner #86 above. Here we test if a line matches two regular expressions instead of line. If a line matches both regexes, then it gets printed.

91. Print lines that don't match match regexes AAA and BBB.

 perl -ne '!/AAA/ && !/BBB/ && print'

This one-liner is almost the same as one-liner #87. Here we test if a line doesn't match two regular expressions in any order. If it doesn't match /AAA/ and it doesn't match /BBB/, then we print it.

92. Print lines that match regex AAA followed by regex BBB followed by CCC.

 perl -ne '/AAA.*BBB.*CCC/ && print'

Here we simply chain regexes AAA, BBB and CCC with .*, which stands for match anything or nothing at all. If AAA is followed by BBB and that is followed by CCC then we print the line. It also matches AAABBBCCC with nothing in between the regexes.

93. Print lines that are 80 chars or longer.

 perl -ne 'print if length >= 80'

This one-liner prints all lines that are 80 chars or longer. In Perl you can sometimes omit the brackets () for function calls. In this one we omitted brackets for length function call. In fact, length, length()<./code> and length($_) are the same.94. Print lines that are less than 80 chars in length.

 perl -ne 'print if length < 80'

This is the opposite of previous one-liner. It checks if the length of a line is less than 80 characters.

95. Print only line 13.

 perl -ne '$. == 13 && print && exit'

As I explained in one-liner #83, the $. special variable stands for "current line number". So if $. has value 13, then we print the line and exit.

96. Print all lines except line 27.

 perl -ne '$. != 27 && print'

Just like in previous one-liner, we check if the current line is line 27, if it's not then we print it, otherwise we skip it.

Another way to write the same is to reverse print and $. != 27 and use if statement,

 perl -ne 'print if $. != 27'

97. Print only lines 13, 19 and 67.

 perl -ne 'print if $. == 13 || $. == 19 || $. == 67'

If you have Perl 5.10 or later then you can use the ~~ smart match operator,

 perl -ne 'print if int($.) ~~ (13, 19, 67)'

The smart matching operator ~~ appeared only in Perl 5.10. You can do all kinds of smart matching with it, for example, check if two arrays are the same, if an array contains an element, and many other use cases (see perldoc perlsyn). In this particular one-liner we use int($.) ~~ (13, 19, 67) that determines if numeric value $. is in the list (13, 19, 69). It's basically short for, grep { $_ == int($._) } (13, 19, 67). If the check succeeds the line gets printed.

98. Print all lines between two regexes (including lines that match regex).

 perl -ne 'print if /regex1/../regex2/'

This one-liner uses the flip-flop operator, which becomes true when a line matches regex1 and becomes false after another line matches regex2. Therefore this one-liner prints all lines between (and including) lines that match regex1 and regex2.

99. Print all lines from line 17 to line 30.

 perl -ne 'print if $. >= 17 && $. <= 30'

This one-liner is very simple to understand. The $. variable stands for the current line number, so it checks if the current line number is greater than or equal to 17 and less than or equal to 30.

I just thought of another way to write it,

 perl -ne 'print if int($.) ~~ (17..30)'

This is one-liner uses the Perl 5.10 (and later) smart matching operator ~~. It basically says, is the current line number in the list (17, 18, 19, ..., 30). If it is, the smart match succeeds and the line gets printed.

You can write the same idea in older Perls as following,

 perl -ne 'print if grep { $_ == $. } 17..30'

What happens here is grep checks if the current line number is in the list (17, 18, ..., 30). If it is, it returns a list of just one element, and a list of one element is true, and the line gets printed. Otherwise grep returns the empty list, which is false, and nothing gets printed.

100. Print the longest line.

 perl -ne '$l = $_ if length($_) > length($l); END { print $l }'

This one-liner keeps the longest line seen so far in the $l variable. In case the current line $_ exceeds the length of currently longest line, it gets replaced. Just before exiting, the END block is executed and it prints the longest line $l.

101. Print the shortest line.

 perl -ne '$s = $_ if $. == 1; $s = $_ if length($_) < length($s); END { print $s }'

This one-liner is the opposite of the previous one. But as we're finding the minimum and $s is not defined for the first line, we have to set it to first line explicitly. Otherwise it's the same just with the length check reversed length($_) < length($s).

102. Print all lines that contain a number.

 perl -ne 'print if /\d/'

This one-liner uses a regular expression \d that stands for "match a number" and checks if a line contains one. If it does, the check succeeds, and the line gets printed.

103. Find all lines that contain only a number.

 perl -ne 'print if /^\d+$/'

This one-liner is very similar to the previous one, but instead of matching a number anywhere on the line, it anchors the match to the beginning of the line, and to the end of the line. The regular expression ^\d+$ means "match one or more numbers that start at the beginning of line and end at the end of the line".

104. Print all lines that contain only characters.

 perl -ne 'print if /^[[:alpha:]]$/

This one-liner checks if the line contains only characters and if it does, it prints it. Here the [[:alpha:]] stands for "match all characters". You could also write the same as [a-zA-Z] (if you live in ASCII world).

105. Print every second line.

 perl -ne 'print if $. % 2'

This one-liner prints first, third, 5th, 7th, etc, line. It does so because $. % 2 is true when the current line number is odd, and it's false when the current line number is even.

106. Print every second line, starting the second line.

 perl -ne 'print if $. % 2 == 0'

This one-liner is very similar to the previous one but except printing 1st, 3rd, 5th, etc, lines, it prints 2nd, 4th, 6th, etc, lines. It prints them because $. % 2 == 0 is true when the current line number is 2, 4, 6, ....

107. Print all lines that repeat.

 perl -ne 'print if ++$a{$_} == 2'

This one-liner keeps track of the lines it has seen so far and it also keeps the count of how many times it has seen the line before. If it sees the line the 2nd time, it prints it out because ++$a{$_} == 2 is true. If it sees the line more than 2 times, it just does nothing because the count for this line has gone beyond 2 and the result of the print check is false.

108. Print all unique lines.

 perl -ne 'print unless $a{$_}++'

Here the lines get printed only if the hash value $a{$_} for the line is 0. Every time Perl reads in a line, it increments the value for the line and that makes sure that only never before seen lines get printed.

Have Fun!

Thanks for reading the article! The next part is going to be about various interesting, intriguing, silly and crazy regular expressions, because Perl is all about regular expressions.