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Executing_external_commands.md

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Executing external commands

Table of Contents


Issuing commands

  • Ruby has various ways to execute external commands
  • Use system if the program should wait for the issued command to finish and continue executing the Ruby script
  • entire command can be passed as a string
    • the command will be interpreted by shell before executing just like on a commandline (/bin/sh is the default on Unix-like systems)
    • do not use this way to execute arbitrary user input, that'd be a security risk
  • or command name and arguments can be separated out
    • no shell interpretation, user has to perform necessary expansion before passing the command (methods like Dir.glob help in that aspect)
    • this way is preferred when accepting arbitrary user input
#!/usr/bin/env ruby

system('clear')
system('echo Hello $USER')

Running the above script

$ ./screen_clear.rb
# screen clears followed by below output

Hello learnbyexample
  • Some more examples
$ irb --simple-prompt
>> system('echo $HOME')
/home/learnbyexample
=> true

>> system('seq', '-s,', '10')
1,2,3,4,5,6,7,8,9,10
=> true

>> system('seq -s, 10 > out.txt')
=> true

>> system('ls *.txt')
out.txt
=> true
>> exit

$ cat out.txt
1,2,3,4,5,6,7,8,9,10

Exit status

  • use return value of system
    • true indicates zero exit status
    • false indicates non-zero exit status
    • nil indicates command execution failure
  • or special variable $? to know details of the command issued like PID/exit status/etc
#!/usr/bin/env ruby

cmd1 = 'ls out.txt'
cmd2 = 'grep "foo" out.txt'
cmd3 = 'xyz'

for cmd in [cmd1, cmd2, cmd3]
  puts "Command: #{cmd}"
  rv = system(cmd)

  # inspect method will display human readable representation of object
  puts "system return value: #{rv.inspect}"
  puts "Command exit status: #{$?.exitstatus}"
  puts '-' * 30
end

Running the above script

$ ls out.txt; echo $?
out.txt
0
$ grep "foo" out.txt; echo $?
1
$ xyz; echo $?
xyz: command not found
127

$ ./exit_status.rb
Command: ls out.txt
out.txt
system return value: true
Command exit status: 0
------------------------------
Command: grep "foo" out.txt
system return value: false
Command exit status: 1
------------------------------
Command: xyz
system return value: nil
Command exit status: 127
------------------------------

Further Reading


Saving command output

  • to save stdout of external command in a variable, place the command within backticks
  • or use percent string %x, which allows to use different delimiters
  • use this only if the command is known, if not use Open3 which allows separating out command and arguments like system method
>> working_dir = `pwd`
=> "/home/learnbyexample/ruby_programs\n"
>> working_dir.chomp
=> "/home/learnbyexample/ruby_programs"

>> nums = %x/seq 3/
=> "1\n2\n3\n"
>> puts nums
1
2
3
=> nil

>> foo = %x(echo `seq 2`).chomp
=> "1 2"

# only stdout is saved
>> files = %x{ls *.txt *.log}
ls: cannot access '*.log': No such file or directory
=> "out.txt\n"

Further Reading