simple command
A simple command is a command that you can execute by just giving its name at the prompt.

socket file
A special file for interacting with the network via the UNIX file system.

STDERR
Standard error. A special type of output used for error messages. The file descriptor for STDERR is 2.

STDIN
Standard input. User input is read from STDIN. The file descriptor for STDIN is 0.

STDOUT
Standard output. The output of scripts is usually to STDOUT. The file descriptor for STDOUT is 1.

subdirectory
A directory that is contained within another directory.

subprocess
Process running under the control of another, often referred to as the parent process. See process.

subshell
Shell running under the control of another, often referred to as the parent shell (typically the login shell). See shell.