How do the environments of a standard Terminal command-line and a bash script differ?

Question

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    

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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.

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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  

Answer 1

Assuming that you're using the same shell in your script and in your terminal, the main difference is that ~/.bashrc is read by interactive shells but not by script-interpreting shells. The reason is that scripts are usually meant to be somewhat portable, and for that you don't want to depend on the user's shell customizations.

Many things in typical .bashrc files only apply to the command line anyway (prompt settings, key bindings). Some, like function definitions, make sense in scripts as well; typically you'd put them into a separate file and source it both from ~/.bashrc and scripts. Sometimes you want some shell options for interactive use but not in scripts, because they lead to shorter (good on the command line) but more cryptic or less portable (bad in scripts) code. There are a few things that bash does differently, e.g. history expansion (!!, etc.) is on by default only in interactive shells.

Here you're using the +(…) construct, which is a ksh extended globbing pattern. Bash doesn't enable them by default (for backward compatibility) but you can turn them on with shopt -s extglob. Apparently you have this in your ~/.bashrc.

You can put shopt -s extglob in your script, or use a portable way of truncating leading 0s from a variable such as

var=${var#${var%%[!0]*}}

Answer 2

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.