Bash Scripts – Differences Between Terminal Command-Line and Bash Script Environments

bashcommand linescripts

I know there is something different about the environment of the Terminal command-line and the environment in a bash script, but I don't know what that difference is…

Here is the example which finally led me to ask this quesiton; it may flush out some of the differences.

I am trying to strip leading '0's from a number, with this command.

  • var="000123"; var="${var##+(0)}" ; echo $var

When I run this command from the Terminal's command-line, I get: 123

However, when I run it from within a script, it doesn't work; I get: 000123

I'm using Ubuntu 10.04, and tried all the following with the sam results:

  • GNOME Terminal 2.30.2
  • Konsole 2.4.5
  • #!/bin/bash
  • #!/bin/sh

My bash version 4 1 5 1 release i486-pc-linux-gnu (in terminals and scripts)

'shopt' differences between CLI and Script: 

    CLI   Script  
    on     off    checkwinsize    
    on     off    expand_aliases  
    on     off    extglob         
    on     off    histappend      
    off    on     hostcomplete

What is causing this difference?

Even if some upgrade will make it work in scripts…
I am trying to find out the what and why,
so in future, I'll know what to look out for.

Here are 3 bash ways to strip leading '0's. (I knew only one when I first asked this queston) 

var=0001230; var="$[10#$var]";           echo $var   # fastest= 1.00  integers only
var=0001230; var="${var##+(0)}";         echo $var   # slower x 1.25  works with strings, but requires shopt -s extglob
var=0001230; var="${var#${var%%[!0]*}}"; echo $var   # slower x 1.61  works with strings

Best Answer

Your question cannot be answered quite generally. Some comments though. On my version of bash (3.1.17(1)-release), your command doesn't have the desired output even when run from the command line; the same with zsh. So presumably something is fishy about your command. I don't know what "##+(0)" is suppoed to accomplish, but "#0" succeeds in removing one leading zero. This show a way to remove arbitrary numbers of zeros.

If there really is a difference between the behavior on the command line and from a script, then most likely the script uses a different interpreter (different bash version, bash instead of zsh) or different shell options (try running shopt). The latter difference might be the result of your interactive shell sourcing $HOME/.bashrc and $HOME/.profile whereas scripts generally don't. That shouldn't affect environment variables, as they're inherited if exported, but it should affect shell options, which need to be set in every shell.

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