The tc shell is an enhanced but completely compatible version of the Berkeley C shell. Its two most common uses are as an interactive run time environment and as an interpreter of shell scripts.
The term “login shell” is used to distinguish the shell invoked by the OS for a user at login time from a shell invoked from the command line of an existing login session or perhaps as part of the execution of a shell script.
Sourcing vs. Execution
An important distinction in shell scripting are the concepts of sourcing and execution. When you execute a process (such as a shell script) the shell spawns a child process to run the commands contained therein. When the child process terminates, any changes to environment variables are lost. ...