Ubuntu – Using ‘$’ in shell

command line

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

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.

Best Answer

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

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SFTP – Setting Up SFTP with Filezilla to Access Files Outside Home Directory

Here I'm assuming you are able to ssh/sftp to your user's home directory successfully and you want to edit (with your user) files and folders under /var/www that are owned by user and :group - www-data:www-data (without changing their ownership).

Here I'm assuming also the topic How to avoid using sudo when working in /var/www? doesn't cover you needs. For example you don't want to change the permissions under /var/www.

I think the most easiest and clear way to solve this task is to mount /var/www (or certain directory inside) into your user's home directory and change the owner to your user and :group. This could be achieved by the tool bindfs:

sudo apt update && sudo apt install bindfs

Here we will mount the entire directory /var/www in a directory called also www/ and located in your user's home directory.

mkdir "$HOME/www"
sudo bindfs -u $(id -u) -g $(id -g) --create-for-user=www-data --create-for-group=www-data /var/www "$HOME/www"

The command substitutions $(id -u) and $(id -g) will return the UID and GID of the current user.
If you want to execute the above command for another user use $(id -u <user>) and $(id -g <user>). Where <user> is an actual username.
For more details about the arguments used with bindfs read its manual page - man bindfs.
If you want to un-mount ~/www ($HOME/www) use the command:
```
sudo fusermount -u ~/www
```

To mount /var/www in ~/www automatically during the system startup add the following line into the bottom of /etc/fstab:

bindfs#/var/www /home/<user>/www fuse force-user=<uid>,force-group=<gid>,create-for-user=www-data,create-for-group=www-data 0 0

Note: you should replace <user> with the actual username; also should replace <uid> and <gid> with the actual UID and GID of the <user>, you can find them by the commands: id -u <user> and id -u <user>.

To see the result reboot the system or execute:

sudo mount -a    # maybe you should execute `sudo fusermount -u ~/www` first

Here is animated demo how this works:

Update:

The only limitation of this approach that I found is when you change the ownership of the bind directory this will change the ownership also for the source directory. For example the next command is not a good idea:

chown -R $(id -u):$(id -g) $HOME/www

Maybe there is a suitable option for the bindfs command that will prevent this to happen, but I can't tell that at the moment.

Notes:

Best Answer

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