[SOLVED] Meaning of parameters in bash function: eval, $@, $#

gacanepa · 04-10-2013, 02:24 PM

The following function is used to create a directory and cd into it. It must be added to the .bashrc file inside your home directory.

Code:

function mkdircd () { mkdir -p "$@" && eval cd "\"\$$#\"";
}

I am having trouble understanding what this

Code:

"$@" && eval cd "\"\$$#\""

is used for.
I know that
-$@: All arguments as separate words.
-$#: Number of arguments.
And I believe that the backward slashes (\) are used as escape sentences, but I'm not really sure.
Any comments will be more than welcome.

millgates · 04-10-2013, 04:35 PM

The first command,

Code:

mkdir -p "$@"

creates directories specified by the arguments supplied to the function. The "$@" ensures that names containing spaces will be treated correctly. The other command,

Code:

eval cd "\"\$$#\""

cd's to the last directory specified. The entire point here is to get the last argument of the function. The example uses eval to make the command be evaluated twice. So, during the first pass \" will become ", \$ will become $ and $# will be replaced by its value. Then the result will be executed as a command.
So, asuming that the function is called as, for example:

Code:

mkdircd a b c d

Directories a, b, c and d will be created. Then, eval will force the other part to be evaluated. So,

Code:

cd "\"\$$#\""

will become

Code:

cd "$4"

because the function was called with 4 arguments.
However, keep in mind that eval is a tool that is best left alone.
Instead, I would write

Code:

mkdircd () { mkdir -p "$@" && cd "${!#}"; }

which IMHO is much nicer.

Also, please note that a function definition in bash is either

Code:

function foo { ... }

or

Code:

foo() { ... }

Using of

Code:

function foo() { ... }

is not recommended.

gacanepa · 04-10-2013, 07:19 PM

Thanks a lot! For taking the time to write and also for sharing your knowledge.
I added to your reputation and will mark this thread as solved.
Just one more thing. Would you mind very much explaining why this

Code:

function foo() { ... }

is not recommended?

chrism01 · 04-10-2013, 08:33 PM

See pt 25 here http://mywiki.wooledge.org/BashPitfa...tion_foo.28.29

gacanepa · 04-10-2013, 09:39 PM

Quote:

Originally Posted by chrism01

See pt 25 here http://mywiki.wooledge.org/BashPitfa...tion_foo.28.29

Thanks a lot chrism01! I will add that page to my bookmarks.

David the H. · 04-11-2013, 04:34 PM

Here are two other ways to get the directory name:

Code:

mkdircd() { mkdir -p "$@" && cd "${@: -1}" ;}    #or "${@:(-1)}"

Since "$@" is a kind of array, you can use bash's array expansion patterns with it. Either a space or parentheses must be included when using negative values, to avoid conflict with the default value parameter substitution.

Code:

mkdircd() { mkdir -p "$@" && cd "$_" ;}

"$_" expands to the last argument of the most recent command run.

gacanepa · 04-11-2013, 05:56 PM

Quote:

Originally Posted by David the H.

Here are two other ways to get the directory name:

Code:

mkdircd() { mkdir -p "$@" && cd "${@: -1}" ;}    #or "${@:(-1)}"

Since "$@" is a kind of array, you can use bash's array expansion patterns with it. Either a space or parentheses must be included when using negative values, to avoid conflict with the default value parameter substitution.

Code:

mkdircd() { mkdir -p "$@" && cd "$_" ;}

"$_" expands to the last argument of the most recent command run.

Thank you David the H!