Create path.md

Author: jAniceto

linux/path.md

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+# Set `$PATH` variable in Ubuntu
+
+The `$PATH` variable is one of the default environment variable in linux (ubuntu). It is used by the shell to look for executable files or commands.
+
+One way to permanently add a directory to `$PATH` environment variable is to use the `~/.profile` file. Add the following to the `~/.profile` file to add `myNewDir` to `$PATH`
+
+```
+$ export PATH=$PATH:/myNewDir
+$ source ~/.profile
+```
+
+## The `source` command
+
+`source` is a bash shell built-in command that executes the content of the file passed as argument, **in the current shell**. It has a synonym in `.` (period). 
+
+Note that `./` and `source` are not quite the same: 
+
+- `./script` runs the script as an executable file, launching a new shell to run it 
+- `source some_script` reads and executes commands from filename in the current shell environment
+- `source some_script` is the same as `. some_script`