Ubuntu – The point of the bash Null-operator “:”, colon

bashscriptssyntax

What is the point of the "null" operator in a BASH script? I understand that it is used as a placeholder following an if command when you have nothing to say, but need a command to allow the program to run properly. But what is the overall use for it? When would you use it? When does it make sense to use it?

Best Answer

It's sometimes useful to allow parameter expansions side-effects to occur.

For example, setting a default value

read -p "Enter your name: " name
: ${name:=John Doe}  # if the user entered an empty string
echo "$name"

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

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.

Ubuntu – How to create a bash scrip that calls other scripts depending on commandline swith

#!/bin/bash

function show_help() {
    cat << ENDHELP
-a you can do stuff like that.
-b you can do stuff like that.
-c this is extremely fancy stuff.
ENDHELP
}

case $1 in
    --help)
        show_help
    ;;
    -a)
        sudo /path/to/other/script
    ;;
    -b)
        do_some_stuff
        do_another_stuff
    ;;
    -c)
        do_extremely_fancy_stuff
        do_another_extremely_fancy_stuff
        run_as_many_commands_as_you_want
    ;;
    *)
        echo "Run $0 --help"
    ;;
esac

$0 is the script name.
$1 is the first command line argument, $2 is the second, and so on.

Read bash manpage for reference.

Best Answer

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

Ubuntu – How to create a bash scrip that calls other scripts depending on commandline swith

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