Reversal using functions in Bash

There are two big problems with your code:

1. When assigning to mydir, you probably want to copy the whole array. But you assign $arr, which is the same as assigning only the first element, ${arr[0]}. Quoting Arrays section of man bash:

   Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0.

   You should use mydir=( "${arr[@]}" ) to copy the array and then a similar construct to pass the whole array to the reverse function. But you don’t have to copy the array at all, this main is sufficient:

   function main(){
       arr=( tiger lion bear )
       reverse "${arr[@]}"
   }
   

2. Your implementation of reverse reverses letters in its first parameter, i.e. the first item of arr array, as it is called now. You should reverse the array instead. It will be stored in parameters of the function, the @ variable. You can get all the parameters of your function as a params array via params=( "$@" ).

_{The second point strongly hints me that you have no idea what you are doing and you just copy-pasted parts of the script from somewhere. I will not write the code for you. It is really easy to do if you know the basics of Bash. If you don’t, go learn them. You will learn nothing by copy-pasting ad-hoc snippets from Stack Overflow or other sites. Next time, you would come to ask virtually the same question again. We are not here to make your homework for you, we are here to teach you.}

There are also several minor issues, but still pretty severe in terms of functionality.

• You must not have spaces around the = in variable assignment in shell. You have them in main when assigning to mydir. This is why Bash probably prints an error saying that is cannot find any command named mydir. Each shell splits the command line into words and treats the first word as command name.

• Your reverse function copies input variable needlessly – unless you really want to print the original value at the end as you do now, of course. But then, I would rather copy the content to a variable named original and work on the input variable, because then the intent would be easier to understand.

• Everywhere you expand a variable in shell, you should expand it inside double quotes, unless you have a good reason to do otherwise and you know what you are doing. If you expand it outside double quotes, you are going to get into trouble with escaping errors. Double quotes protect most special characters inside from being interpreted by the shell, only allowing variable, command, arithmetic and history expansion.

• In the loop in reverse, i is supposed to contain a number. It is nonsense to test if for string equality to [. That condition is always false.

• You never initialize var variable, but you use its value in the echo statement at the end of reverse body.

  You also do not specify value of rev variable before you first use it. As all variables are empty by default and you use reverse only once, this is not an issue, but it still is not a good practice.

  Another variable without initialization is reverse at the last line of main. Function call in shell returns only status, pretty much like any other command. You can modify a (global) variable inside the function, but you don’t do that. The echo command at the end of main is thus useless.

I already told you enough to figure out the correct implementation of reverse even if you are a shell beginner. If you read the Arrays section of Bash manual again, everything should be clear. If it is not, comment and I’ll try to give you more guidance.