Mirrored here for posterity, and just because it’s damned neat.

HANDY ONE-LINERS FOR RUBY                             November 16, 2005
compiled by David P Thomas 				version 1.0

Latest version of this file can be found at:
    http://www.fepus.net/ruby1line.txt

Last Updated: Wed Nov 16 08:35:02 CST 2005

FILE SPACING:

# double space a file
    $ cat filename | ruby -pe ‘puts’
# triple space a file
    $ cat filename | ruby -pe ‘2.times {puts}’
# undo double-spacing (w/ and w/o whitespace in lines)
    $ cat filename | ruby -lne ‘BEGIN{$/=”nn”}; puts $_’
    $ cat filename | ruby -ne ‘BEGIN{$/=”nn”}; puts $_.chomp’
    $ cat filename | ruby -e ‘puts STDIN.readlines.to_s.gsub(/nn/, “n”)’

NUMBERING:

# number each line of a file (left justified).
    $ cat filename | ruby -ne ‘printf(”%-6s%s”, $., $_)’
# number each line of a file (right justified).
    $ cat filename | ruby -ne ‘printf(”%6s%s”, $., $_)’
# number each line of a file, only print non-blank lines
    $ cat filename | ruby -e ‘while gets; end; puts $.’
# count lines (emulates ‘wc -l’)
    $ cat filename | ruby -ne ‘END {puts $.}’
    $ cat filename | ruby -e ‘while gets; end; puts $.’

TEXT CONVERSION AND SUBSTITUTION:

# convert DOS newlines (CR/LF) to Unix format (LF)
# - strip newline regardless; re-print with unix EOL
    $ cat filename | ruby -ne ‘BEGIN{$=”n”}; print $_.chomp’

# convert Unix newlines (LF) to DOS format (CR/LF)
# - strip newline regardless; re-print with dos EOL
    $ cat filename | ruby -ne ‘BEGIN{$=”rn”}; print $_.chomp’

# delete leading whitespace (spaces/tabs/etc) from beginning of each line
    $ cat filename | ruby -pe ‘gsub(/^s+/, “”)’

# delete trailing whitespace (spaces/tabs/etc) from end of each line
# - strip newline regardless; replace with default platform record separator
    $ cat filename | ruby -pe ‘gsub(/s+$/, $/)’

# delete BOTH leading and trailing whitespace from each line
    $ cat filename | ruby -pe ‘gsub(/^s+/, “”).gsub(/s+$/, $/)’

# insert 5 blank spaces at the beginning of each line (ie. page offset)
    $ cat filename | ruby -pe ‘gsub(/%/, ”   “)’
    FAILS! $ cat filename | ruby -pe ‘gsub(/%/, 5.times{putc ” “})’

# align all text flush right on a 79-column width
    $ cat filename | ruby -ne ‘printf(”%79s”, $_)’

# center all text in middle of 79-column width
    $ cat filename | ruby -ne ‘puts $_.chomp.center(79)’
    $ cat filename | ruby -lne ‘puts $_.center(79)’

# substitute (find and replace) “foo” with “bar” on each line
    $ cat filename | ruby -pe ‘gsub(/foo/, “bar”)’

# substitute “foo” with “bar” ONLY for lines which contain “baz”
    $ cat filename | ruby -pe ‘gsub(/foo/, “bar”) if $_ =~ /baz/’

# substitute “foo” with “bar” EXCEPT for lines which contain “baz”
    $ cat filename | ruby -pe ‘gsub(/foo/, “bar”) unless $_ =~ /baz/’

# substitute “foo” or “bar” or “baz”…. with “baq”
    $ cat filename | ruby -pe ‘gsub(/(foo|bar|baz)/, “baq”)’

# reverse order of lines (emulates ‘tac’) IMPROVE
    $ cat filename | ruby -ne ‘BEGIN{@arr=Array.new}; @arr.push $_; END{puts @arr.reverse}’

# reverse each character on the line (emulates ‘rev’)
    $ cat filename | ruby -ne ‘puts $_.chomp.reverse’
    $ cat filename | ruby -lne ‘puts $_.reverse’

# join pairs of lines side-by-side (like ‘paste’)
    $ cat filename | ruby -pe ‘$_ = $_.chomp + ” ” + gets if $. % 2′

# if a line ends with a backslash, append the next line to it
    $ cat filename | ruby -pe ‘while $_.match(/\$/); $_ = $_.chomp.chop + gets; end’
    $ cat filename | ruby -e ‘puts STDIN.readlines.to_s.gsub(/\n/, “”)’

# if a line begins with an equal sign, append it to the previous line (Unix)
    $ cat filename | ruby -e ‘puts STDIN.readlines.to_s.gsub(/n=/, “”)’

# add a blank line every 5 lines (after lines 5, 10, 15, etc)
    $ cat filename | ruby -pe ‘puts if $. % 6 == 0′

SELECTIVE PRINTING OF CERTAIN LINES

# print first 10 lines of a file (emulate ‘head’)
    $ cat filename | ruby -pe ‘exit if $. > 10′

# print first line of a file (emulate ‘head -1′)
    $ cat filename | ruby -pe ‘puts $_; exit’

# print the last 10 lines of a file (emulate ‘tail’); NOTE reads entire file!
    $ cat filename | ruby -e ‘puts STDIN.readlines.reverse!.slice(0,10).reverse!’

# print the last 2 lines of a file (emulate ‘tail -2′); NOTE reads entire file!
    $ cat filename | ruby -e ‘puts STDIN.readlines.reverse!.slice(0,2).reverse!’

# print the last line of a file (emulates ‘tail -1′)
    $ cat filename | ruby -ne ‘line = $_; END {puts line}’

# print only lines that match a regular expression (emulates ‘grep’)
    $ cat filename | ruby -pe ‘next unless $_ =~ /regexp/’

# print only lines that DO NOT match a regular expression (emulates ‘grep’)
    $ cat filename | ruby -pe ‘next if $_ =~ /regexp/’

# print the line immediately before a regexp, but not the regex matching line
    $ cat filename | ruby -ne ‘puts @prev if $_ =~ /regex/; @prev = $_;’

# print the line immediately after a regexp, but not the regex matching line
    $ cat filename | ruby -ne ‘puts $_ if @prev =~ /regex/; @prev = $_;’

# grep for foo AND bar AND baz (in any order)
    $ cat filename | ruby -pe ‘next unless $_ =~ /foo/ && $_ =~ /bar/ && $_ =~ /baz/’

# grep for foo AND bar AND baz (in order)
    $ cat filename | ruby -pe ‘next unless $_ =~ /foo.*bar.*baz/’

# grep for foo OR bar OR baz
    $ cat filename | ruby -pe ‘next unless $_ =~ /(foo|bar|baz)/’

# print paragraph if it contains regexp; blank lines separate paragraphs
    $ cat filename | ruby -ne ‘BEGIN{$/=”nn”}; print $_ if $_ =~ /regexp/’

# print paragraph if it contains foo AND bar AND baz (in any order); blank lines separate paragraphs
    $ cat filename | ruby -ne ‘BEGIN{$/=”nn”}; print $_ if $_ =~ /foo/ && $_ =~ /bar/ && $_ =~ /baz/’

# print paragraph if it contains foo AND bar AND baz (in order); blank lines separate paragraphs
    $ cat filename | ruby -ne ‘BEGIN{$/=”nn”}; print $_ if $_ =~ /(foo.*bar.*baz)/’

# print paragraph if it contains foo OR bar OR baz; blank lines separate paragraphs
    $ cat filename | ruby -ne ‘BEGIN{$/=”nn”}; print $_ if $_ =~ /(foo|bar|baz)/’

# print only lines of 65 characters or greater
    $ cat filename | ruby -pe ‘next unless $_.chomp.length >= 65′
    $ cat filename | ruby -lpe ‘next unless $_.length >= 65′

# print only lines of 65 characters or less
    $ cat filename | ruby -pe ‘next unless $_.chomp.length < 65'
    $ cat filename | ruby -lpe 'next unless $_.length < 65'

# print section of file from regex to end of file
    $ cat filename | ruby -pe '@found=true if $_ =~ /regex/; next unless @found'

# print section of file based on line numbers (eg. lines 2-7 inclusive)
    $ cat filename | ruby -pe 'next unless $. >= 2 && $. < = 7'

# print line number 52
    $ cat filename | ruby -pe 'next unless $. == 52'

# print every 3rd line starting at line 4
    $ cat filename | ruby -pe 'next unless $. >= 4 && $. % 3 == 0′

# print section of file between two regular expressions, /foo/ and /bar/
    $ cat filename | ruby -ne ‘@found=true if $_ =~ /foo/; next unless @found; puts $_; exit if $_ =~ /bar/’

SELECTIVE DELETION OF CERTAIN LINES

# print all of file except between two regular expressions, /foo/ and /bar/
    $ cat filename | ruby -ne ‘@found = true if $_ =~ /foo/; puts $_ unless @found; @found = false if $_ =~ /bar/’

# print file and remove duplicate, consecutive lines from a file (emulates ‘uniq’)
    $ cat filename | ruby -ne ‘puts $_ unless $_ == @prev; @prev = $_’

# print file and remove duplicate, non-consecutive lines from a file (careful of memory!)
    $ cat filename | ruby -e ‘puts STDIN.readlines.sort.uniq!.to_s’

# print file except for first 10 lines
    $ cat filename | ruby -pe ‘next if $. < = 10'

# print file except for last line
    $ cat filename | ruby -e 'lines=STDIN.readlines; puts lines[0,lines.size-1]'

# print file except for last 2 lines
    $ cat filename | ruby -e 'lines=STDIN.readlines; puts lines[0,lines.size-2]'

# print file except for last 10 lines
    $ cat filename | ruby -e 'lines=STDIN.readlines; puts lines[0,lines.size-10]'

# print file except for every 8th line
    $ cat filename | ruby -pe 'next if $. % 8 == 0'

# print file except for blank lines
    $ cat filename | ruby -pe 'next if $_ =~ /^s*$/'

# delete all consecutive blank lines from a file except the first
    $ cat filename  | ruby -e 'BEGIN{$/=nil}; puts STDIN.readlines.to_s.gsub(/n(n)+/, "nn")'

# delete all consecutive blank lines from a file except for the first 2
    $ cat filename  | ruby -e 'BEGIN{$/=nil}; puts STDIN.readlines.to_s.gsub(/n(n)+/, "nn")'

# delete all leading blank lines at top of file
    $ cat filename | ruby -pe '@lineFound = true if $_ !~ /^s*$/; next if !@lineFound'

If you have any additional scripts to contribute or if you find errors
in this document, please send an e-mail to the compiler.  Indicate the
version of ruby you used, the operating system it was compiled for, and
the nature of the problem.  Various scripts in this file were written or
contributed by:

 David P Thomas 				    # author of this document