Perl One-liners学习笔记2

ponh

http://www.catonmat.net/blog/perl-one-liners-explained-part-two/
Line Numbering
9. Number all lines in a file.
perl -pe '$_ = "$. $_"'
As I explained in the first one-liner, "-p" causes Perl to assume a loop around the program (specified by "-e") that reads each line of input into the " $_ " variable, executes the program and then prints the " $_ " variable.
In this one-liner I simply modify " $_ " and prepend the " $. " variable to it. The special variable " $. " contains the current line number of input.
The result is that each line gets its line number prepended.
10. Number only non-empty lines in a file.
perl -pe '$_ = ++$a." $_" if /./'
Here we employ the "action if condition" statement that executes "action" only if "condition" is true. In this case the condition is a regular expression "/./", which matches any character except newline (that is, it matches a non-empty line); and the action is " $_ = ++$a." $_" ", which prepends variable " $a " incremented by one to the current line. As we didn't use strict pragma, $a was created automatically.
The result is that at each non-empty line " $a " gets incremented by one and prepended to that line. And at each empty line nothing gets modified and the empty line gets printed as is.
11. Number and print only non-empty lines in a file (drop empty lines).
perl -ne 'print ++$a." $_" if /./'
This one-liner uses the "-n" program argument that places the line in " $_ " variable and then executes the program specified by "-e". Unlike "-p", it does not print the line after executing code in "-e", so we have to call "print" explicitly to get it printed.
The one-liner calls "print" only on lines that have at least one character in them. And exactly like in the previous one-liner, it increments the line number in variable " $a " by one for each non-empty line.
The empty lines simply get ignored and never get printed.
12. Number all lines but print line numbers only non-empty lines.
perl -pe '$_ = "$. $_" if /./'
This one-liner is similar to one-liner #10. Here I modify the " $_ " variable that holds the entire line only if the line has at least one character. All other lines (empty ones) get printed without line numbers.
13. Number only lines that match a pattern, print others unmodified.
perl -pe '$_ = ++$a." $_" if /regex/'
Here we again use the "action if condition" statement but the condition in this case is a pattern (regular expression) "/regex/". The action is the same as in one-liner #10. I don't want to repeat, see #10 for explanation.
14. Number and print only lines that match a pattern.
perl -ne 'print ++$a." $_" if /regex/'
This one-liner is almost exactly like #11. The only difference is that it prints numbered lines that match only "/regex/".
15. Number all lines, but print line numbers only for lines that match a pattern.
perl -pe '$_ = "$. $_" if /regex/'
This one-liner is similar to the previous one-liner and to one-liner #12. Here the line gets its line number prepended if it matches a /regex/, otherwise it just gets printed without a line number.
16. Number all lines in a file using a custom format (emulate cat -n).
perl -ne 'printf "%-5d %s", $., $_'
This one-liner uses the formatted print "printf" function to print the line number together with line. In this particular example the line numbers are left aligned on 5 char boundary.
Some other nice format strings are "%5d" that right-aligns line numbers on 5 char boundary and "%05d" that zero-fills and right-justifies the line numbers.
Here my Perl printf cheat sheet might come handy that lists all the possible format specifiers.
17. Print the total number of lines in a file (emulate wc -l).
perl -lne 'END { print $. }'
This one-liner uses the "END" block that Perl probably took as a feature from Awk language. The END block gets executed after the program has executed. In this case the program is the hidden loop over the input that was created by the "-n" argument. After it has looped over the input, the special variable " $. " contains the number of lines there was in the input. The END block prints this variable. The " -l " parameter sets the output record separator for "print" to a newline (so that we didn't have to print "$.\n").
Another way to do the same is:
perl -le 'print $n=()=<>'
This is a tricky one, but easy to understand if you know about Perl contexts. In this one-liner the " ()=<> " part causes the <> operator (the diamond operator) to evaluate in list context, that causes the diamond operator to read the whole file in a list. Next, " $n " gets evaluated in scalar context. Evaluating a list in a scalar context returns the number of elements in the list. Thus the " $n=()=<> " construction is equal to the number of lines in the input, that is number of lines in the file. The print statement prints this number out. The " -l " argument makes sure a newline gets added after printing out this number.
This is the same as writing the following, except longer:
perl -le 'print scalar(()=<>)'
And completely obvious version:
perl -le 'print scalar(@foo=<>)'
Yet another way to do it:
perl -ne '}{print $.'
This one-liner uses the eskimo operator "}{" in conjunction with "-n" command line argument. As I explained in one-liner #11, the "-n" argument forces Perl to assume a " while(<>) { } " loop around the program. The eskimo operator forces Perl to escape the loop, and the program turns out to be:
while (<>) {
}{                    # eskimo operator here
print $.;
}
It's easy to see that this program just loops over all the input and after it's done doing so, it prints the " $. ", which is the number of lines in the input.
18. Print the number of non-empty lines in a file.
perl -le 'print scalar(grep{/./}<>)'
This one-liner uses the "grep" function that is similar to the grep Unix command. Given a list of values, " grep {condition} " returns only those values that match condition. In this case the condition is a regular expression that matches at least one character, so the input gets filtered and the "grep{/./}" returns all lines that were non empty. To get the number of characters we evaluate the list in scalar context and print the result. (As I mentioned in the previous one-liner list in scalar context evaluates to number of elements in the list).
A golfer's version of this one-liner would be to replace "scalar()" with " ~~ " (double bitwise negate), thus it can be shortened:
perl -le 'print ~~grep{/./}<>'
This can be made even shorter:
perl -le 'print~~grep/./,<>'
19. Print the number of empty lines in a file.
perl -lne '$a++ if /^$/; END {print $a+0}'
Here I use variable $a to count how many empty lines have I encountered. Once I have finished looping over all the lines, I print the value of $a in the END block. I use " $a+0 " construction to make sure " 0 " gets output if no lines were empty.
I could have also modified the previous one-liner:
perl -le 'print scalar(grep{/^$/}<>)'
Or written it with " ~~ ":
perl -le 'print ~~grep{/^$/}<>'
These last two versions are not as effective, as they would read the whole file in memory. Where as the first one would do it line by line.
20. Print the number of lines in a file that match a pattern (emulate grep -c).
perl -lne '$a++ if /regex/; END {print $a+0}'
This one-liner is basically the same as the previous one, except it increments the line counter $a by one in case a line matches a regular expression /regex/.