creating dir with mkdir in debian -

i trying create directory using mkdir in debian problem directory created in root directory. problem comes trying install openfoam , suspect because "$" used create it.

following instructions in openfoam web site, says create folder

$home/openfoam/user-2.2.1

and then, create within folder called "run" using:

mkdir -p /foam_run

the problem final directory (run) should have following form:

/home/openfoam/user-2.2.1/run

but using debian cannot create dir "sudo", have root , when go inside directory /home/openfoam/user-2.2.1 , mkdir -p /foam_run , created folder is:

/root/openfoam/root-2.2.1/run

and need created in /home directory , not in /root directory. had created folder using

mkdir -p /home/openfoam/user-2.2.1/run

but when try execute openfoam orders doesn't work.

my questions are:

why should mkdir $foam_run create run directory , if "mkdir run", doesn't work?

why "$" symbol there when use mkdir? searching , (for example) mkdir manual says nothing it.

excuse me, not expert in linux.

the instructions straightforward, seem have missed 1 of notes. reiterate point making, need source ~/.bashrc after updating it. if echo "$foam_run" should see variable set /home/luser/openfoam/luser-2.2.1/run.

if prefer override hand, , assuming user name luser , openfoam version 2.2.1, this:

foam_run=$home/openfoam/$user-2.2.1/run mkdir -p "$foam_run" # note pedantic quoting 

but if .bashrc contains code instruct add, should not necessary.