Grabbing the output of a command and inserting it into a bash script

Question

I have a fairly simple bash script which I need to make a bit more complicated, but I'm not certain of how to do it. The script, so far, is like so:

    #!bin/bash

    if rails -v | grep -q "3.2"
     then
    echo "Rails 3.2 installed. Uninstalling and adding Rails 3.1.4."
    gem uninstall rails -v=$version
    gem install rails -v=3.1.4
     else
    echo "Rails 3.2 not installed. Exiting."
    exit 1
 fi

As you can see, it's not complete and pretty basic. The overall goal of this script is to see if said server has rails 3.2.x installed and, if so, to then uninstall it and install rails 3.1.4. I've got everything covered but the actual insertion of the required version into the "gem uninstall" portion. The $version part should be replaced with the version number output by rails -v on the server.

Any assistance with this is appreciated.

Thanks.

chepner · Accepted Answer · 2012-05-31 20:06:43Z

1

Something like

gem uninstall rails -v=$(rails -v | grep -o "3.2.*")

Tailor your regular expression to match just the version number, and grep will output just that.

answered May 31, 2012 at 20:06

chepner

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1 Comment

Dennis Williamson Over a year ago

Or, more generally, grep -o '[0-9]\+\..*'

William Pursell · Accepted Answer · 2012-05-31 19:17:11Z

0

 gem uninstall rails -v=$( rails -v )

answered May 31, 2012 at 19:17

William Pursell

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5 Comments

William Pursell Over a year ago

What is the output of rails -v?

Dennis Williamson Over a year ago

@Striketh: Try gem uninstall rails -v=$( rails -v | sed 's/.* //' ) (or other ways to extract only the version number).

Striketh Over a year ago

@William - It's always going to be "Rails x.x.x" with the x's representing the version number.

Striketh Over a year ago

@DennisWilliamson: This works, but it's not 100%. When the command is executed it goes into rails manual prompt requiring a Y or N about uninstalling rails, and I don't want any manual input. Is there a way to set the variable earlier in with something like version=( rails -v | sed 's/.* //' ) and then simply have the $version variable inserted into the gem uninstall line to bypass this prompt? When entered directly as gem uninstall rails -v=3.2.3 for example, this prompt is bypassed.

Striketh Over a year ago

Disregard - just needed the -I flag in the rails command. Works perfectly with that.

Infinite Recursion · Accepted Answer · 2015-03-27 12:23:50Z

0

Try surrounding it with quotes, like this: gem uninstall rails -v="$( rails -v | sed 's/.* //' )".

edited Mar 27, 2015 at 12:23

Infinite Recursion

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answered May 31, 2012 at 20:16

vergenzt

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