. (source or dot operator)

Run a command script in the current shell context.

SYNTAX
      . filename [arguments]

      source filename [arguments]

A period `.' is a synonym for `source'

When a script is run using `source' it runs within the existing shell, any variables created or modified by the script will remain available after the script completes. In contrast if the script is run just as filename, then a separate subshell (with a completely separate set of variables) would be spawned to run the script.

There is a subtle difference between executing a script by running .ss64script (dot ss64script) and . ss64script (dot space ss64script)
the first is running a file thats been hidden from the 'ls' command, (although ls -a will show hidden files) the second option will execute ss64script even if it has not been set as an executable with chmod.

Unless you provide an exact path to filename then bash will look first via the PATH variable and then in the current directory (only if filename is not found in $PATH.) If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged.

The return status is the exit status of the last command executed, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. `source' is a bourne shell builtin and a POSIX `special' builtin

Related bash commands:

command - Run a command - ignoring shell functions
builtin - Run a shell builtin
crontab - Schedule a command to run at a later time
chroot - Run a command with a different root directory
exec - Execute a command
if - Conditionally perform a command
nohup - Run a command immune to hangups
su - Run a command with substitute user and group id
type - Describe a command
watch - Execute/display a program periodically

Equivalent Windows commands:

Batch files all share the same environment unless SETLOCAL is specified.