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I wrote a bash script but since I'm a self-learner bash rookie, I wanted to ask if I can check the given arguments more efficiently. I also googled about this and checked topics here but the examples I have seen so far are too complicated. In python3, there are a lot easier methods for this but I guess in bash it is a bit more complex.

#!/bin/bash

ERR_MSG="You did not give the argument required"

if [[ ${1?$ERR_MSG} == "a" ]]; then
        echo "ABC"
elif [[ ${1?$ERR_MSG} == "b" ]]; then
        echo "123"
elif [[ ${1?$ERR_MSG} == "c" ]]; then
        echo ".*?"
else
    echo "You did not provide the argument correctly"
    exit 1
fi
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2
  • Can we assume that you want your script to take a single argument that should be either a, b, or c and that you want to output an error message if the argument is something else, if it's missing, or if there are too many arguments? This is not clear from the question. Also, you refer to other examples, but you don't say anything more about these other than that they are too complicated. In what way? They probably show how to do full command line parsing of options etc. Is this what you want to do, or are you happy with your a, b and c arguments? Commented Nov 15, 2021 at 20:19
  • I'm sorry but I didn't understand why my question is not clear to you. Of course, you can assume those. That's why I share the code. But you are right about the examples I mentioned. I can be more clear about it. The examples I have come across were like the one below. Too long for a simple task like this. For example, in Python, argument parsing is just a one-line code. And you can easily make it optional or mandatory etc. But I guess in the Bash we have to write lots of lines. Commented Nov 15, 2021 at 20:36

2 Answers 2

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9

A script that only accepts a single argument that must be either a, b, or c:

#!/bin/bash

if [[ $# -ne 1 ]]; then
    echo 'Too many/few arguments, expecting one' >&2
    exit 1
fi

case $1 in
    a|b|c)  # Ok
        ;;
    *)
        # The wrong first argument.
        echo 'Expected "a", "b", or "c"' >&2
        exit 1
esac

# rest of code here

If you want to do proper option parsing and want to accept -a, -b, or -c as options that do not take arguments, and -d as an option that takes an argument.

#!/bin/bash

# Default values:
opt_a=false
opt_b=false
opt_c=false
opt_d='no value given'

# It's the : after d that signifies that it takes an option argument.

while getopts abcd: opt; do
    case $opt in
        a) opt_a=true ;;
        b) opt_b=true ;;
        c) opt_c=true ;;
        d) opt_d=$OPTARG ;;
        *) echo 'error in command line parsing' >&2
           exit 1
    esac
done

shift "$(( OPTIND - 1 ))"

# Command line parsing is done now.
# The code below acts on the used options.
# This code would typically do sanity checks,
# like emitting errors for incompatible options, 
# missing options etc.

"$opt_a" && echo 'Got the -a option'
"$opt_b" && echo 'Got the -b option'
"$opt_c" && echo 'Got the -c option'

printf 'Option -d: %s\n' "$opt_d"

if [[ $# -gt 0 ]]; then
    echo 'Further operands:'
    printf '\t%s\n' "$@"
fi

# The rest of your code goes here.

Testing:

$ ./script -d 'hello bumblebee' -ac
Got the -a option
Got the -c option
Option -d: hello bumblebee

$ ./script
Option -d: no value given

$ ./script -q
script: illegal option -- q
error in command line parsing

$ ./script -adboo 1 2 3
Got the -a option
Option -d: boo
Further operands:
        1
        2
        3

Option parsing is terminated at the first non-option argument, or at --. Note that since -d takes an argument, -a is taken as that argument in the below example:

$ ./script -d -a -- -c -b
Option -d: -a
Further operands:
        -c
        -b
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1

Copy this code into a file. (Don´t forget to chmod +x) This should do what you need.

#!/bin/bash
VERSION="0.0.1"
ACCEPTED_SERVER="server"
ACCEPTED_USER="user"

usage()
{
    echo " $(basename $0) [-v] [-h] -u user -s server"
    exit
}

version()
{
    echo "Current version: $VERSION"
}

get_opts()
{
    while [[ $# -gt 0 ]]
    do
        key="$1"
        case $key in
            -u|--user)
                shift
                USER="$1"
                shift
                ;;
            -s|--server)
                shift
                SERVER="$1"
                shift
                ;;
            -h|--help)
                usage
                exit
                ;;
            -v|--version)
                version
                exit
            ;;
        esac
    done
}

# echo " arguments: $#"
get_opts $*
if [ $# -gt 4 ] || [ $# -lt 1 ]; then
    usage
fi
echo "$SERVER -- $ACCEPTED_SERVER"
if ! [ "$SERVER" = "$ACCEPTED_SERVER" ]; then
    echo "Wrong server: $SERVER"
    echo "Try with user: $ACCEPTED_SERVER"
    usage
else
    echo "Correct server: $SERVER"

fi
if ! [ "$USER" = "$ACCEPTED_USER" ]; then
    echo "Wrong user: $USER"
    echo "Try with user: $ACCEPTED_USER"
    usage
else
    echo "Correct user: $USER"
fi

echo "Correct! - Server: $SERVER - User: $USER"

Then you can add a function usage, version, etc.

usage()
{
    echo "[-v] [-h] [-u user] [-s server] task"
}
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10
  • This looks cool but what if I wanted to restrict the arguments to specific inputs? For example, I want to accept only 2 options as a USER either "a" or "b"? if "c" is given I want to call usage(). Do I need to write an if condition for that? Also, I didn't understand why you have -p, -m arguments, although you didn't specify them in the get_opts(). Commented Nov 14, 2021 at 21:40
  • @BaranAldemir ignore the other two server options. I had more options in my example. Regarding the verification in the code, let me update the script and add what you need. You are right, you need to add an if condition. Commented Nov 15, 2021 at 18:46
  • I have two more questions. Can we set a default value for an argument? And can we have an optional arguments in this way? Commented Nov 15, 2021 at 18:50
  • In the example I've posted, if one argument is enough, you need to do it also with if-then-else. Commented Nov 15, 2021 at 19:10
  • Omg. I guess it's much better to use python for these kinds of stuff. I can still use this approach in some simple cases tho. Thanks for the help. Commented Nov 15, 2021 at 19:24

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