Notes on Linux

Unix Shells and Variables

Shells, sub-shells, and shell scripts

• Shell: A shell is just another program; essentially, a translator between the user (us) and the operating system. Thus, the shell is another program, but one that is modifiable by our commands, and takes its direction from our commands. It uses the commands we type in as directions, which it translates to directions for the operating system. And in simple language, a subshell is a program that can run in the background to execute the commands that you have given it, while you continue to work in the shell.
• Sub-Shell: a subshell is a shell that forks off from the original shell. It can run programs while leaving the parent shell free to do other things. For example, you could call a script using the sh command:

  sh myscript.sh

  It spawns a subshell where the script runs. You can continue to execute commands in the bash shell from which you began.

• Shell Script: A shell script is just what it says, a "script"; i.e., a set of commands that are read by the shell and executed just like they would if you were to type in at the command line manually. You could write a script to modify an environment variable. The beauty of scripts, is you have to write them once, then you can use them just like any other built-in shell command.

There are a number of variables used in Unix when a shell is running

• Global Environment (Shell) Variables: Variables that are available to child processes of the shell. Some programs need environment variables to function correctly. A shell script defines only those environment variables that are needed by the programs that it runs.
• Local (Shell) Variables: Shell variables are similar to environment variables but are local to the shell in which they are defined and not available to other shells or environments. You can set a shell variable in one of two ways:

  set name=value

  Or define them in the .bashrc file which specifies variables for the bash shell. Local variables are variables that are present within the current instance of the shell. They are not available to programs started by the shell. They are set at the command prompt.

• (Shell) Script Variables: Variables defined in shell scripts are local variables to the script. Shell variables will be accessible by any script or process that runs within the shell.

These can be divided into two categories: Local or Global

• Environment variables would be local to the user login shell but global within that shell.
• Shell variables are global across all users of the shell.
• Shell script variables are local to the script in which they are contained.