How can I print the contents of a directory in reverse just using Bash arrays?

Question

Currently, I'm working on this homework assignment:

Write a Bash script, reverse.sh, which reverses the contents of a directory passed as a parameter. Assuming /my/dir contains cache cron games lib log run tmp, your program reverse.sh /my/dir will return tmp run log lib games cron cache. Use an array and two functions: main() and reverse(). You will manually manipulate the list as we did today. DO NOT use the built-in command sort -r.

For now, I've decided to use one function to get proper output first, that's not my problem. This is my script so far:

function main(){
    p=$1
    cd $p
    reverse $p
}

function reverse(){
    local p2=$1
    local ARRAY=()
    local count=0
    for entry in $p2*
    do
     ((count++))
     ARRAY+=($entry)
    done
    while [ $count -gt -1 ]; do
     echo ${ARRAY[$count]}
     ((count--))
    done
}
main

However, I get the same output every time, no matter what directory I add as a parameter when running the script.

Answer 1

p=$1 will not work because main is not being called with any arguments (the args are not automatically passed from the command line to the function). Fix is to pass all the command line args to the function:

    main $@

Answer 2

Unshift Positional Parameters in Bash

I'm sure your homework assignment is trying to teach you something in particular, but this is taking the long way around even if you're supposed to be simulating a stack with a Bash array. In Bash, you can do this with a single local variable by leveraging positional parameters and the shift builtin. For example:

# Function to sort names by emulating unshifting onto a stack.
reverse () {
    local -a result
    for dir in "$@"; do
        result=("$1" "${result[@]}") # unshift $1 onto stack
        shift                        # remove  $1 from positional params
    done
    echo "${result[@]}"
}

reverse cache cron games lib log run tmp

tmp run log lib games cron cache

For purposes of demonstration, this function assumes it is being passed a list of names to sort; it makes no assumptions about how you collect those names, because bike-shedding. However, on systems with GNU ls, you might consider reverse $(ls -Q /path/to/dir) (do not quote the command substitution) as a pragmatic solution that will handle most common use cases.

Bike-Shedding

A professional-quality script will require a great deal more bullet-proofing than can be provided in a reasonably-sized example. Considerations include:

• Handling spaces, newlines, and other legal (but unusual) characters in filenames.
• Handling nullglob expansions.
• Testing for directory attributes of globs and arguments.
• Dealing with symlinks, absolute/relative paths, and various expansion validations.
• Dealing with globs that exceed getconf ARG_MAX or the hardcoded value of MAX_ARG_STRLEN.
• Et cetera, ad nauseum.

My advice is to write code that is "good enough" for your known data set, rather than trying to cope with every possible edge case. It's always important to know your data and your environment so that you don't over-engineer a problem.

Answer 3

If your script is supposed to take /my/dir as a parameter, you would need

for entry in $p2/*

As is, it works for me with main /my/dir/. Except that the directory name is included in each of the array elements. You could fix that with basename or cd.

But if your call to main in the script is supposed to pass the command line argument, you would need to end your script

main "$1"