Running shell (operating system) commands from within Ruby
Ruby is an excellent systems admin / scripting tool, allowing other shells and processes to be run from within a Ruby program in various ways.
Using backtics or the %x notation, commeand output on STDOUT is routed back into a variable - for example:
first = `grep -c option247 ac_20160516 ac_20160517`
second = %x!grep -c option247 ac_20160516 ac_20160517!
and you can embed a whole shell script this way using a here document - see [here]
function allows you to run a command with STDOUT routes to Ruby's STDOUT, and have the job status returned
third = system("grep -c option247 ac_20160516 ac_20160517")
and the exec
syste function transfers control to the job you're running terminating your rubys process!
exec "df -h"
To control STDIN and STDERR as well as STDOUT, you can use shell redirects, for example
fourth = %x!grep -c option247 ac_20160516 ac_20160517 2>&1!
and to run a shell command returning data in parallel while that command is still running, you can use a pipe, for example
handle = IO.popen("grep -c option247 ac_20160516 ac_20160517")
For ultimate flexibility, the open3
module provides yu with control over all three standard input/outputs:
h_in,h_out,h_err = Open3.popen3("grep -c option247 ac_20160516 ac_20160517")
though you need to be very careful of pipe buffering if you're using this.
All the examples above in complete programs (and with sample output) [here]
. Examples from our Ruby course material
- further example [here]
to show setting of environment variables to be passed into shell commands. (written 2016-05-18)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesR113 - Ruby - Further Input and Output 
Expect with Ruby - a training example to get you started - (2016-05-18) 
RUby - loading, using, changing, storing JSON format data - (2015-10-23) 
Reading and parsing a JSON object in Ruby - (2015-06-01) 
Reading and checking user inputs - first lessons - Ruby - (2013-02-17)R115 - Using Ruby with Other Languages 
Rake - a build system using code written in Ruby - (2011-02-03)
Some other Articles
Processing data line by line - iterator in Ruby with yieldReading in XML in Ruby with xmlsimpleSwindon, Chippenham and Melksham - day trips by train to Weymouth start for 2016Running shell (operating system) commands from within RubyExceptions in Ruby - throwing, catching and usingAlternating valuses / flip-flop / toggle - example in RubySeparating detailed data code from the main application - Ruby exampleAnnual review of Melksham Matters for Chamber of Commerce AGMRuby training, half a world away
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