Using '$' in shell

Question

I am new to ubuntu, and tried to use OpenFOAM Installation worked OK; after that I copy-pasted the very first example, worked also OK. but I cannot understand how. There are funny commands like:

mkdir -p $FOAM_RUN 

This creates the folder structure /home/username/OpenFOAM/username-2.3.1/run

Or

cp -r $FOAM_TUTORIALS $FOAM_RUN 

This copies the tutorials in the above mentioned folder

What does the sign $ do?

These commands work when copy-pasted, but I want understanding them and being able to use them later on.

Answer 1

The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is called variable substitution. The $ sign helps us to get the value.

Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains, for example:

We define variable1 with a value 23:

$ variable1 = 23

To print the name of the variable (echo is just like a print function in the terminal):

$ echo variable1
variable1

To print the actual value saved in the variable

$ echo $variable1
23

Answer 2

Variables in your shell are marked with a $. I assume a path has been stored as $FOAM_RUN. You can see all variables in your shell by running:

$ env
XDG_SESSION_ID=2
GDM_LANG=en_US.utf8
XDG_RUNTIME_DIR=/run/user/1000
WINDOWID=41943044
HOME=/home/orangetux
XDG_VTNR=7
SSH_AGENT_PID=1774
XDG_SEAT=seat0
COLORTERM=gnome-terminal
...

You can see the the value of a variable by printing it:

$ echo $HOME
/home/orangetux