Ubuntu – How to make the argument as optional in bash

bashfunctions

In below function with 9 arguments:

SUM() { 
    echo "The sum is $(($1+$2+$3+$4+$5+$6+$7+$8+$9))"
}

I want to make the second arguments to the next(3..9) become a optional arguments.

When I call the function with 2 arguments I get error:

SUM 3 8
bash: 3+8+++++++: syntax error: operand expected (error token is "+")

Note BOLD: first argument and second argument are force arguments and not optional for function. I only want second arguments to the next is optional and when I call the function less than 2 args the function must return no result.

Best Answer

If you won't pass arguments with spaces:

sum() {  
[[ -n $2 ]] && echo $(( $(tr ' ' '+' <<<"$@") ))
}

Effect:

$ sum 1 2 3
6

Explanation:

<<<"some string" feeds in only "some string" as the input. Think of it as shorthand for echo "some string" |. It is called a Here String.
"$@" expands into all the positional parameters, separated by spaces. It is equivalent to "$1 $2 ...".
Hence, tr ' ' '+' <<<"$@" outputs "$1+$2+$3...", which is evaluated by the outer $(( )).
[[ -n $2 ]] tests if the second parameter is non-empty. You could replace [[ -n $2 ]] && with [[ -z $2 ]] ||.

Another way:

sum() {
[[ -n $2 ]] && (IFS=+; echo $(( $* )))
}

Explanation:

$* is just like $@, except that the parameters are not separated by spaces, but by the first character of the Internal Field Separator (IFS). With IFS=+, it expands to "$1+$2+...". See What is the difference between $* and $@?
We set IFS in a subshell (note the surrounding parentheses) so that the main shell isn't affected. IFS is, by default: \t\n (space, tab, newline). This is an alternative to using local variables.

Now to answer your question:

You can use a default value for any variable or parameter. Either:

SUM() { 
 echo "The sum is $(($1+$2+${3:-0}+${4:-0}+${5:-0}+${6:-0}+${7:-0}+${8:-0}+${9:-0}))" || false
}

Or:

SUM() { 
 echo "The sum is $(($1+$2+${3:=0}+${4:=0}+${5:=0}+${6:=0}+${7:=0}+${8:=0}+${9:=0}))" || false
}

Related Solutions

Ubuntu – How to format “if” statement (conditional) on multiple lines in bash

Run the commands first, then check if at least one of them succeeded.

#!/bin/bash

success=0
do_something arg1 && success=1
do_something arg2 && success=1
do_something arg3 && success=1

if ((success)); then
    printf 'Success! At least one of the three commands succeeded\n'
fi

Ubuntu – bash: populating array with elements containing spaces and special characters

What you're doing is tricky. The normal way is to avoid this and just pass the array values as arguments. In order to have both options, you would have to use eval:

#!/bin/bash
function populate_array () {
    if [ "$#" -gt 0 ] ; then
        # Enter array w/ elements as argument of executable 
        # Note the quotes, they are needed
        array=("$@");
        n=$#
    else
        # Invoke executable with no arg, enter array element later
        # Read a string instead of an array and use eval to make it
        # into an array. That way, you can use tricks like escaping
        # spaces. You also need the -r option to protect the backslashes
        # so that eval will see them. 
        read -r  -p "Enter array elements separated by spaces: " string
        eval array="( $(printf '%s\n' "$string") )"
        n=${#array[@]} 
    fi
    printf "%d array elements \n" "$n"
} 

populate_array "$@"

while (("$n" > 0))   # while [ "$n" -gt 0 ] ALSO WORKS
do
    printf "%s \n" "${array[$n-1]}"
    n=$n-1
done
exit 0

You still need to escape the parentheses if you pass the array values as an argument since ( ) are reserved characters for bash. With that caveat, the script above should work as you expect:

$ foo.sh lkl1239 343.4l 3,344 \(34\) "lklk  lkl" lkaa\ lkc
6 array elements 
lkaa lkc 
lklk  lkl 
(34) 
3,344 
343.4l 
lkl1239

And

$ foo.sh
Enter array elements separated by spaces:  lkl1239 343.4l 3,344 \(34\) "lklk  lkl" lkaa\ lkc 
6 array elements 
lkaa lkc 
lklk  lkl 
(34) 
3,344 
343.4l 
lkl1239

Best Answer

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Ubuntu – How to format “if” statement (conditional) on multiple lines in bash

Ubuntu – bash: populating array with elements containing spaces and special characters

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