In my Mind

Hi everyone,

I recently wrote a script and I needed to use arrays.

Unfortunately, there are no arrays in the POSIX shell specification. But, after some research I found a way to use pseudo-arrays.

The first trick is to use the set command which permits to modify the values of the positional parameters. You will therefor be able to get the number of elements ($#), access any items (for the n-th item: $n) or iterate through them. The only thing I don't know is how to change the value of one of them.

The second trick is to properly quote the various elements you want to put in it. To do that I wrote a small function (which I won't describe here) which is able substitute a substring by another one in a string. I used this function to replace each occurrence of a single quote by “a single quote + an escaped single quote + a single quote” in my items and I enclosed them between two single quotes.

Here is an example of how using it.

#!/bin/sh
 
. jfsh.sh # Contains the quote function (and subst which is used by it)
 
print_arrays()
{
    local array entry
 
    for array
    do
        eval "set -- $array" # Loads the array contained in $1
 
        printf '%d elements\n' $#
        for entry
        do
            printf '  “%s”\n' "$entry"
        done
    done
}
 
reverse_array()
{
    local entry result
 
    eval "set -- $1"
 
    for entry
    do
        result="$(quote "$entry") $result"
    done
 
    printf '%s\n' "$result"
}
 
array1="$(quote 1 "second element" "third one")"
 
 # This array will contains the filenames of every files in this directory.
array2="$(quote *)"
 
# We can apply a function on array1.
array3="$(reverse_array "$array1")"
 
# Let's print the various arrays.
print_arrays "$array1" "$array2" "$array3"

The file “jfsh.sh” is available on github.