Shell Programming and Scripting

Hello,

I recently started going in depth with the shell, so I started learning from Linux Shell Scripting CookBook, 2nd edition. I am at the first chapter atm, and the author tells to define a function in the ~/.bashrc.

The function is below.

Code:

prepend() { [ -d "$2" ] && eval $1=\"$2':'\$$1\" && export $1; }

And another given version is

Code:

prepend() { [ -d "$2" ] && eval $1=\"$2\$\{$1:+':'\$$1\}\" && export $1 ;}

The function, can be called like:

Code:

#prepend PATH /opt/myapp/bin

I managed to define it in bashrc and it works, however I want to understand step by step what it does(basically what each part of the code does and what it means) and unfortunately the author doesn't explain.

I know that basically when called, the function adds, the given path, into the variable that is mentioned. If you say prepend PATH /home/usr it will add /home/usr, to the existing content in the $PATH at the end of what is already in the variable.

Can anybody help with how it works step by step? Sorry if this is a very n00b question, but I don't know whom to ask.

Kind Regards,
George

Code:

prepend() {
    [ -d "$2" ] &&
    eval $1=\"$2\$\{$1:+':'\$$1\}\" &&
    export $1
}

well [ is a command for tests. it's about the same as test.
looking at help test we see -d checks to see if $2 exists as a directory before doing anything else.

&& joins the statements. in A && B, B is only performed if A was true. similarly A || B, B is only performed if A was false. (Like short-circuit evaluation in C) Read more in the bash reference manual, section 3.2.3 Lists of Commands

A clearer version would perhaps be

Code:

prepend() {
    if [ -d "$2" ]; then
        eval $1=\"$2\$\{$1:+':'\$$1\}\" &&
        export $1
    fi
}

eval is rather bad practice, unless absolutely necessary. I personally wouldn't use this function.. So much can go wrong executing user-defined strings as code. But moving past that. It does that: executes the line again... So placing our variables into there we'll see

Code:

prepend PATH /opt/myapp/bin

become as

Code:

eval PATH="/opt/myapp/bin:$PATH"

the single quotes around the colon there isn't very helpful... But neither is this particular function.