leejunkim

Shell

3 min read

  • One of the primary ways to interact with your computer
  • Shell VS Bash
  • The Shell as a Programming Environment
    • The shell is more than just a command prompt; it’s a complete programming environment, similar to Python or Ruby. It includes features like variables (e.g., $PATH), conditionals, loops, or functions!
    • When you type a command, you are writing a small piece of code for the shell to interpret.

How the Shell Finds and Executes Programs

Environment variable

  • things that are set whenever you start your shell
    • bunch of things that’s already set
  • Example: $PATH

Build in / External Commands

  • Bash commands
  • built-in
    • already part of bash so no need to search the filesystem for a program
    • Ex) cd, alias, export, source
    • NOT UNDER $PATH
  • External (Not built in)
    • Separate programs on your computer. → Each one is its own executable file living in a directory like /bin or /usr/bin
    • Ex) ls, grep, find, date, cat, echo
    • Uses $PATH <<<<<
  • Note: echo exists as both a shell built-in and an external command (/bin/echo)
  • You can check if its built in or not: type <COMMAND>, like type echo

$PATH

  • How does the shell know where to find programs that are external?
    • The shell consults an environment variable called $PATH
    • $PATH contains a list of directories, separated by colons (:), where the shell should look for executable programs.
    • Essentially, $PATH is the roadmap the shell uses to locate commands.
missing:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
missing:~$ which echo
/bin/echo
missing:~$ /bin/echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin # same as above
  • built in commands are NOT HERE
    • If a command is built in, the shell finds and executes it before it even considers looking at $PATH
  • which
    • tells you the exact location (the full path) of the program that the shell will execute
    • ex) /bin/echo is where the echo program is
  • You can run a program without relying on $PATH by providing its full, direct path. This guarantees you are running that specific executable
    • /bin/echo "hello world"
    • But typically you usually don’t need to type /bin/echo because the echo program’s location is already listed in your $PATH variable

What happens

  1. Check for an alias: It first checks if the command is an alias (a nickname you’ve set for a longer command).
  2. Check for a built-in command: If it’s not an alias, it checks if the command is one of its built-in functions. If it is, it runs it immediately without using $PATH.
  3. Search $PATH: If the command is not a built-in, the shell then searches through the directories listed in $PATH to find a matching external program.
  4. Error: If it can’t find the command after all these steps, it will give you the command not found error.

Graph View

Backlinks