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Taken from http://www.fepus.net/ruby1line.txt

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
    $  ruby -pe 'puts' < file.txt
# triple space a file
    $  ruby -pe '2.times {puts}' < file.txt
# undo double-spacing (w/ and w/o whitespace in lines)
    $  ruby -lne 'BEGIN{$/="\n\n"}; puts $_' < file.txt
    $  ruby -ne 'BEGIN{$/="\n\n"}; puts $_.chomp' < file.txt
    $  ruby -e 'puts STDIN.readlines.to_s.gsub(/\n\n/, "\n")' < file.txt

NUMBERING:

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

TEXT CONVERSION AND SUBSTITUTION:

# convert DOS newlines (CR/LF) to Unix format (LF)
# - strip newline regardless; re-print with unix EOL
    $  ruby -ne 'BEGIN{$\="\n"}; print $_.chomp' < file.txt

# convert Unix newlines (LF) to DOS format (CR/LF)
# - strip newline regardless; re-print with dos EOL
    $  ruby -ne 'BEGIN{$\="\r\n"}; print $_.chomp' < file.txt

# delete leading whitespace (spaces/tabs/etc) from beginning of each line
    $  ruby -pe 'gsub(/^\s+/, "")' < file.txt

# delete trailing whitespace (spaces/tabs/etc) from end of each line
# - strip newline regardless; replace with default platform record separator
    $  ruby -pe 'gsub(/\s+$/, $/)' < file.txt

# delete BOTH leading and trailing whitespace from each line
    $  ruby -pe 'gsub(/^\s+/, "").gsub(/\s+$/, $/)' < file.txt

# insert 5 blank spaces at the beginning of each line (ie. page offset)
    $  ruby -pe 'gsub(/%/, "   ")' < file.txt
    FAILS! $  ruby -pe 'gsub(/%/, 5.times{putc " "})' < file.txt

# align all text flush right on a 79-column width
    $  ruby -ne 'printf("%79s", $_)' < file.txt

# center all text in middle of 79-column width
    $  ruby -ne 'puts $_.chomp.center(79)' < file.txt
    $  ruby -lne 'puts $_.center(79)' < file.txt

# substitute (find and replace) "foo" with "bar" on each line
    $  ruby -pe 'gsub(/foo/, "bar")' < file.txt

# substitute "foo" with "bar" ONLY for lines which contain "baz"
    $  ruby -pe 'gsub(/foo/, "bar") if $_ =~ /baz/' < file.txt

# substitute "foo" with "bar" EXCEPT for lines which contain "baz"
    $  ruby -pe 'gsub(/foo/, "bar") unless $_ =~ /baz/' < file.txt

# substitute "foo" or "bar" or "baz".... with "baq"
    $  ruby -pe 'gsub(/(foo|bar|baz)/, "baq")' < file.txt

# reverse order of lines (emulates 'tac') IMPROVE
    $  ruby -ne 'BEGIN{@arr=Array.new}; @arr.push $_; END{puts @arr.reverse}' < file.txt

# reverse each character on the line (emulates 'rev')
    $  ruby -ne 'puts $_.chomp.reverse' < file.txt
    $  ruby -lne 'puts $_.reverse' < file.txt

# join pairs of lines side-by-side (like 'paste')
    $  ruby -pe '$_ = $_.chomp + " " + gets if $. % 2' < file.txt

# if a line ends with a backslash, append the next line to it
    $  ruby -pe 'while $_.match(/\\$/); $_ = $_.chomp.chop + gets; end' < file.txt
    $  ruby -e 'puts STDIN.readlines.to_s.gsub(/\\\n/, "")' < file.txt

# if a line begins with an equal sign, append it to the previous line (Unix)
    $  ruby -e 'puts STDIN.readlines.to_s.gsub(/\n=/, "")' < file.txt

# add a blank line every 5 lines (after lines 5, 10, 15, etc)
    $  ruby -pe 'puts if $. % 6 == 0' < file.txt

SELECTIVE PRINTING OF CERTAIN LINES

# print first 10 lines of a file (emulate 'head')
    $  ruby -pe 'exit if $. > 10' < file.txt

# print first line of a file (emulate 'head -1')
    $  ruby -pe 'puts $_; exit' < file.txt

# print the last 10 lines of a file (emulate 'tail'); NOTE reads entire file!
    $  ruby -e 'puts STDIN.readlines.reverse!.slice(0,10).reverse!' < file.txt

# print the last 2 lines of a file (emulate 'tail -2'); NOTE reads entire file!
    $  ruby -e 'puts STDIN.readlines.reverse!.slice(0,2).reverse!' < file.txt

# print the last line of a file (emulates 'tail -1')
    $  ruby -ne 'line = $_; END {puts line}' < file.txt

# print only lines that match a regular expression (emulates 'grep')
    $  ruby -pe 'next unless $_ =~ /regexp/' < file.txt

# print only lines that DO NOT match a regular expression (emulates 'grep')
    $  ruby -pe 'next if $_ =~ /regexp/' < file.txt

# print the line immediately before a regexp, but not the regex matching line
    $  ruby -ne 'puts @prev if $_ =~ /regex/; @prev = $_;' < file.txt

# print the line immediately after a regexp, but not the regex matching line
    $  ruby -ne 'puts $_ if @prev =~ /regex/; @prev = $_;' < file.txt

# grep for foo AND bar AND baz (in any order)
    $  ruby -pe 'next unless $_ =~ /foo/ && $_ =~ /bar/ && $_ =~ /baz/' < file.txt

# grep for foo AND bar AND baz (in order)
    $  ruby -pe 'next unless $_ =~ /foo.*bar.*baz/' < file.txt

# grep for foo OR bar OR baz
    $  ruby -pe 'next unless $_ =~ /(foo|bar|baz)/' < file.txt

# print paragraph if it contains regexp; blank lines separate paragraphs
    $  ruby -ne 'BEGIN{$/="\n\n"}; print $_ if $_ =~ /regexp/' < file.txt

# print paragraph if it contains foo AND bar AND baz (in any order); blank lines separate paragraphs
    $  ruby -ne 'BEGIN{$/="\n\n"}; print $_ if $_ =~ /foo/ && $_ =~ /bar/ && $_ =~ /baz/' < file.txt

# print paragraph if it contains foo AND bar AND baz (in order); blank lines separate paragraphs
    $  ruby -ne 'BEGIN{$/="\n\n"}; print $_ if $_ =~ /(foo.*bar.*baz)/' < file.txt

# print paragraph if it contains foo OR bar OR baz; blank lines separate paragraphs
    $  ruby -ne 'BEGIN{$/="\n\n"}; print $_ if $_ =~ /(foo|bar|baz)/' < file.txt

# print only lines of 65 characters or greater
    $  ruby -pe 'next unless $_.chomp.length >= 65' < file.txt
    $  ruby -lpe 'next unless $_.length >= 65' < file.txt

# print only lines of 65 characters or less
    $  ruby -pe 'next unless $_.chomp.length < 65' < file.txt
    $  ruby -lpe 'next unless $_.length < 65' < file.txt

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

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

# print line number 52
    $  ruby -pe 'next unless $. == 52' < file.txt

# print every 3rd line starting at line 4
    $  ruby -pe 'next unless $. >= 4 && $. % 3 == 0' < file.txt

# print section of file between two regular expressions, /foo/ and /bar/
    $  ruby -ne '@found=true if $_ =~ /foo/; next unless @found; puts $_; exit if $_ =~ /bar/' < file.txt

SELECTIVE DELETION OF CERTAIN LINES

# print all of file except between two regular expressions, /foo/ and /bar/
    $  ruby -ne '@found = true if $_ =~ /foo/; puts $_ unless @found; @found = false if $_ =~ /bar/' < file.txt

# print file and remove duplicate, consecutive lines from a file (emulates 'uniq')
    $  ruby -ne 'puts $_ unless $_ == @prev; @prev = $_' < file.txt

# print file and remove duplicate, non-consecutive lines from a file (careful of memory!)
    $  ruby -e 'puts STDIN.readlines.sort.uniq!.to_s' < file.txt

# print file except for first 10 lines
    $  ruby -pe 'next if $. <= 10' < file.txt

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

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

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

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

# print file except for blank lines
    $  ruby -pe 'next if $_ =~ /^\s*$/' < file.txt

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

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

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

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

Tue Jun 26 18:17:36 CDT 2007
 * Thanks to Taylor Carpenter  for feedback on improving redirection format.

I was recently trying to get a Ruby script working on Ubuntu. This script required Hpricot and using Ruby Gems to install Hpricot always resulted in an error:

 gem install hpricot --remote
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /var/lib/gems/1.8 directory.
gabriel@windsor-telecom-2874:~/Music$ sudo gem install hpricot --remote
Building native extensions.  This could take a while...
ERROR:  Error installing hpricot:
	ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb install hpricot --remote
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
	from extconf.rb:1

Gem files will remain installed in /var/lib/gems/1.8/gems/hpricot-0.7 for inspection.
Results logged to /var/lib/gems/1.8/gems/hpricot-0.7/ext/hpricot_scan/gem_make.out

The answer was simple:
sudo aptitude install ruby-dev

Just install the Ruby Dev package, this will also allow you to install lots of other gems such as SQLite3, etc.

There was a time when I would recursively grep the contents of literally thousands of files at a time to search for particular occurances of characters. The usual starting place was something using grep, which we can time for crude benchmarking:

user@localhost$ time grep -rn 'hello' *
templates/temp.tpl:1:hello

real 1m56.190s
user 0m1.400s
sys 0m0.940s


Using Ruby to write a script, which I named Grope, I made the search process 450 times faster, reducing an operation that took more than a minute to taking a blink of an eye…

user@localhost$ time grope 'hello'
templates/festival/06/temp.tpl: 1
hello...

real 0m0.181s
user 0m0.100s
sys 0m0.070s


A trivial Ruby script for replacing any number of tabs in input.csv with a single comma and then writing it as output.csv.

The first version works from the command line and makes use of concatenating a file and piping it to a ruby command which then diverts the output in to a file, a one liner:
cat input.csv | ruby -pe 'gsub( /\t+/, "," )' > output.csv

This can be further enhanced by using the in-place-edit switch of the Ruby interpreter, editing the input.csv file and leaving a pristine copy with .bak extension.
ruby -i.bak -pe 'gsub( /\t+/, "," )' input.csv


The final version is what you might put in to a Ruby file. Although the input and output files are hard-coded, it would be trivial to allow them to be specified on the command line.

#!/usr/bin/env ruby

# Initialise a new file object that is writable
file = File.new( "output.csv", "w" )

# Open the existing tab delimited file and read each line
File.open( "input.csv", "r" ).each do |line|
  # Perform a global substitution of tabs with commas and
  # put that in the new file
  file.puts line.gsub( /\t+/, "," )
end

# Tidy up by closing the newly written output file.
file.close

Recently, I had to setup a production environment for Rails applications at work. I encountered many difficulties, including problems with permissions on client/server computers, and not least because the documentation I had was out of date (Agile web dev and The Rails Way). Online tutorials were helpful but incomplete. I have compiled all that I discovered here. (more…)