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Tags: programming bash shell shopt aliases context
Clipped on: 2012-11-01

OSX: This works from the command line: alias ruby="/opt/local/bin/ruby1.9"

but in side a shell script, it has no effect. I want to write a script that will switch between ruby 1.8 and ruby 1.9, so this needs to be a script - not in my profile.

It appears "source" works, but "./". Why is this? How can I replicate this in my script?


asked Feb 4 '10 at 5:16
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4 Answers

up vote 18 down vote accepted

./ will be executed in a sub-shell and the changes made apply only the to sub-shell. Once the command terminates, the sub-shell goes and so do the changes.

sourcing the file using . ./ or source ./ will read and execute commands from the file-name argument in the current shell context, that is when a script is run using source it runs within the existing shell, any variables created or modified by the script will remain available after the script completes.

answered Feb 4 '10 at 5:23
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Nice, I didn't know this command source, thank you :) – javano Sep 17 at 8:35
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You can write a function in your .profile to switch the aliases

function toggle-ruby() {
  if [ "$1" == "1.9" ]; then
    alias ruby=/opt/local/bin/ruby1.9
    alias ruby=/opt/local/bin/ruby1.8

then run you can run:

toggle-ruby 1.9


toggle-ruby 1.8

to switch back and forth.

answered Feb 4 '10 at 5:44
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this is a good approach. thanks! – phil swenson Feb 4 '10 at 14:25
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The simple answer for you is that scripts create non-interactive shells and, by default, the expand_aliases option is often disabled.

You can fix this very simply by just adding the following line to the top of your script to enable the alias expansion:

shopt -s expand_aliases

This problem has been bugging me, so I did research and then wrote a blog post once I figured out how to fix it for myself: Post about using alias from within Linux shell scripts.

Of course, right after I figured out that part, I found that, while it works for what you need, it will not work if you have a subshell within a a subshell. I am still looking into the fix for that problem, that is how I just came across your question. On the blog post, I mention a cheap hack that I use to grab the alias in a shell script. It isn't elegant, but it actually works even in this multiple subshell problem I have.

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answered Jul 28 '10 at 16:11
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If you need to switch Ruby versions, try rvm.

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answered May 26 at 2:32
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One million upvotes. – Ziggy Jul 7 at 23:34
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