The shell is a command programming language that provides an interface to the UNIX† operating system. Its features include control-flow primitives, parameter passing, vari- ables and string substitution. Constructs such as while, if then else, case and for are avail- able. Two-way communication is possible between the shell and commands. String-val- ued parameters, typically file names or flags, may be passed to a command. A return code is set by commands that may be used to determine control-flow, and the standard output from a command may be used as shell input.

The shell can modify the environment in which commands run. Input and output can be redirected to files, and processes that communicate through ‘pipes’ can be invoked. Commands are found by searching directories in the file system in a sequence that can be defined by the user. Commands can be read either from the terminal or from a file, which allows command procedures to be stored for later use.

Read in full here:

https://cscie26.dce.harvard.edu/~dce-lib113/reference/unix/bourne_shell.pdf

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