Cannot understand Shell script with a regular expression

Question

I am new to shell scripting. I have the following snippets and it is really complex. The more I look at it the more I get confused.

while test $# -gt 0; do
        [[ $1 =~ ^-c|--chartrelease$ ]] && { chartRelease="$2"; shift 2; continue; };
        echo "Parameter not recognized: $1, ignored"
        shift
done
: "${chartRelease:="default"}"

Can any provide a simple explanation to clear this mistery? For example what is the purpose of $1 =~ ^-c|--chartrelease$

RavinderSingh13 · Accepted Answer · 2019-09-26 00:55:38Z

2

Could you please go through following once, this is only for explanation purposes.

while test $# -gt 0; do                          ##Starting a while loop which traverse through all arguments provided to script.
[[ $1 =~ ^-c|--chartrelease$ ]] &&               ## Checking condition if  first argument is either starting with letter c OR ending with --chartrelease then only DO next action in {}.
{ chartRelease="$2"; shift 2; continue; };       ##Setting variable chartRelease to 2nd argument  then using shift will remove first 2 arguments and make 3rd argument as new $1, continue will go to next cycle of  while loop skipping all next statements from here.
echo "Parameter not recognized: $1, ignored"     ##Echo is printing that $1 is NOT recognized this will execute when above if condition is NOT TRUE.
        shift  ##Shift will make $2 as new $1
done                                             ##Closing while loop here.
: "${chartRelease:="default"}"                   ##In case variable ChartRelease is null then set its value as string default.

answered Sep 26, 2019 at 0:55

RavinderSingh13

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6 Comments

Learner Over a year ago

in this line=> : "${chartRelease:="default"}" what is the purpose of :?

RavinderSingh13 Over a year ago

@Learner, it sets default value of chartRelease,see this link once stackoverflow.com/questions/4437573/bash-assign-default-value it will help you.

Learner Over a year ago

Thanks. I get the second colon but not sure why we need the first colon though

Learner Over a year ago

Opps sorry just saw the link. Thanks for the nice explanation

RavinderSingh13 Over a year ago

@Learner, your welcome, cheers and have a happy learning on SO.

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