Shell variables


Variables are local to the shell or "environment" variables
Environment variables are process attributes, at OS level
Local variables are not inherited by child processes
The shell command "export" is used to promote a variable to the environment
The shell expands an unexistent variable into an empty string

Special constructs (good in scripts):
Default value: ${var:-val}
Default assignment: ${var:=val}
Error if missing: ${var:?errmsg}

Some variables carry a system-wide special meaning
$PATH, for example. is known to execvp(3) and to the shell itself
Many commands can be configured through environment variables
The environment is a simple tool for parameter passing