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leroykayanda
leroykayanda

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Passing arguments to a bash script

Short Options

./test.sh -a -b -c charlie -d river lorem ipsum PARSED_ARGUMENTS is -a -b -c=charlie -d=river -- 'lorem' ALPHA : 1 BETA : 1 CHARLIE : charlie DELTA : river Parameters remaining are: lorem
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Long Options

./test.sh --alpha --beta --charlie=charlie --delta=river lorem PARSED_ARGUMENTS is --alpha --beta --charlie 'charlie' --delta 'river' -- 'lorem' ALPHA : 1 BETA : 1 CHARLIE : charlie DELTA : river Parameters remaining are: lorem
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The bash script is in github.

-a and -b are called switches because they do not take any parameters while -c and -d to take parameters.

In the script, we first set some default values.

ALPHA=unset
BETA=unset
CHARLIE=unset
DELTA=unset

The usage function will be used to show a user the expected inputs if they specify incorrect parameters.

usage() { echo "Usage: k8s-setup [ -a | --alpha ] [ -b | --beta ] [ -c | --charlie CHARLIE ] [ -d | --delta DELTA ] filename(s)" exit 2 }
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We use getopt to get the parameters a user specifies.

PARSED_ARGUMENTS=$(getopt -a -n k8s-setup -o abc:d: --long alpha,beta,charlie:,delta: -- "$@")

We use a colon (:) to indicate that an argument expects a value e.g --charlie=charlie rather than just --charlie.

We have passed the -a option to getopt so that it accepts the single hyphen form of a switch i.e -charlie as well as --charlie are all accepted. With -n k8s-setup, k8s-setup is the script name. This is displayed if user input causes an error.

./test.sh --alpha --betas --charlie=charlie --delta river lorem k8s-setup: unrecognized option '--betas' Usage: k8s-setup [ -a | --alpha ] [ -b | --beta ] [ -c | --charlie CHARLIE ] [ -d | --delta DELTA ] filename(s)
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This is more user friendly than.

./test.sh --alpha --betas --charlie=charlie --delta river lorem getopt: unrecognized option '--betas' Usage: k8s-setup [ -a | --alpha ] [ -b | --beta ] [ -c | --charlie CHARLIE ] [ -d | --delta DELTA ] filename(s)
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If getopt accepts all inputs, it returns a status code of 0. This is how we catch errors.

VALID_ARGUMENTS=$? if [ "$VALID_ARGUMENTS" != "0" ]; then usage fi
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The set command takes any arguments after the options (here “--” signals the end of the options) and assigns them to the positional parameters ($0.. $n)

eval set -- "$PARSED_ARGUMENTS"
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while : do case "$1" in -a | --alpha) ALPHA=1 ; shift ;; -b | --beta) BETA=1 ; shift ;; -c | --charlie) CHARLIE="$2" ; shift 2 ;; -d | --delta) DELTA="$2" ; shift 2 ;; # -- means the end of the arguments; drop this, and break out of the while loop --) shift; break ;; # If invalid options were passed, then getopt should have reported an error, # which we checked as VALID_ARGUMENTS when getopt was called... *) echo "Unexpected argument: $1 " usage ;; esac done
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Now we loop through the $1 variable. Each time we have processed a parameter, we use shift to push the parameter off the stack so that the next parameter is not at position $1. If a parameter has a value we use shift 2 so that both the parameter and its argument are pushed off the stack. -- signifies that there are no more parameters and we break out of the loop. * captures any unexpected input and calls the usage function. Any parameters left on the stack are captured by $@.

References

1. https://www.shellscript.sh/tips/getopt/index.html

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