How to pass array literal to a bash function-CodePudding

First I've read about passing arrays in general -- all examples I saw first created temporary variable for array and then passed it. Taken from https://stackoverflow.com/a/26443029/210342

show_value () # array index
{
    local -n myarray=$1
    local idx=$2
    echo "${myarray[$idx]}"
}

shadock=(ga bu zo meu)
show_value shadock 2

Is there a way to pass array directly as literal, i.e. without creating temporary variable?

I tried naive approach simply substituting the name with data, but I syntax error on "(".

Update:

I use openSUSE Leap 15.3 with bash 4.4. The above code of course works, but I would like to change the call into:

show_value (ga bu zo meu) 2

i.e. pass array data directly (without using extra variable).

CodePudding user response：

If you want to change the order of the arguments:

show_value () #  index array_element [...]
{
    local idx=$1
    local -a myarray=("${@:2}")
    echo "${myarray[$idx]}"
}

then

shadock=(ga bu zo meu)
show_value 2 "${shadock[@]}"   # => zo

If you want to keep the index as the last argument, then

show_value () #  array_element [...] index
{
    local -a myarray=("${@:1:$#-1}")
    local idx=${!#}
    echo "${myarray[$idx]}"
}

show_value "${shadock[@]}" 2   # => zo

local -n myarray=$1 is certainly much tidier than all that, isn't it? It will also be faster and more memory efficient -- you don't have to copy all the data.

CodePudding user response：

Function arguments are not structured enough to handle nested data structures like arrays. Arguments are a flat list of strings, that's it.

You can expand the array inline:

show_value "${shadock[@]}" 2

However the delimiters are lost. There's no way to know where the array starts and end since it expands to:

show_value ga bu zo meu 2

You'll have to figure out the array bounds yourself. Options include:

If the command has only a single array parameter, make it the last one. This is what many traditional UNIX tools that take multiple file names do. Examples: ls <file>..., chmod <mode> <file>....
You can put the array in the middle if there's a fixed number of arguments preceding/following it such that you can unambiguously determine where the array is. Example: cp <file>... <dir>.
If you have multiple arrays, you can ask users to separate them with --. Many of git's more complicated subcommands do this.

I'd caution against taking an array name as an argument. That's a very shell-specific technique. It would make it hard to turn the function into a full-fledged script or binary down the road.

CodePudding user response：

Can you consider this syntax :

show_value () # array index
{
    local -a myarray=$1
    local idx=$2
    echo "${myarray[$idx]}"
}

show_value "(ga bu zo meu)" 2

For security reasons, you need to be sure of the contents of the array you pass into the function.