This is part 21 of a series on how to approach bash programming in a way that’s safer and more structured than your basic script.
See part 1 if you want to catch the series from the start.
Last time, we discussed variable scoping and how to protect variables from out-of-scope modification. This time, let’s discuss environment variables.
Variables in bash exist only within the current shell context and within what bash terms subshells. Subshells are sets of commands grouped with parentheses, which run a copy of the parent shell as a subprocess.
A subshell creates a separate process with an exact copy of the parent shell, including all of its variables. When it ends, however, any changes to variables are also gone and the parent shell is (still) in the same state as it was prior to the subshell.
Aside from subshells, however, regular shell variables are not inherited by processes run from the shell. For example, running a program from the command line (even another copy of bash itself) doesn’t allow the child process to receive the regular bash variables defined in the parent shell.
Environment variables are a special kind of shell variable. Unlike regular variables, environment variables are propagated to processes run from the shell. They are, of course, also available to subshells as well.
By convention, environment variables are typically named in all caps, such as PATH, although this is not a requirement. The shell automatically creates a number of environment variables (all with all-caps names), but anyone can create them via export statements, such as in a .bash_profile file:
Environment variables can also be created by prefixing a command with an assignment to a variable name:
Such a declaration creates the environment variable for the subprocess but does not create it in the current shell.
Just because a variable is all caps doesn’t make it an environment variable, however. While that is the convention, bash itself makes several special variables available in all caps which are simply shell variables, not environment variables. When in doubt, check to see whether a variable is in the environment or not with either declare -p or printenv.
Here is a list of special shell variables in bash.
Continue with part 22 - data types