How hard is it to write your own shell? I'm trying to find out!

Not sure you want to do that :) Anyway:

go install github.com/arnodel/meshell

$ meshell gives you a repl

$ meshell script.sh runs a shell script. You can also do $ meshell <script.sh.

• cd builtin
• exit builtin
• shift builtin
• simple commands (ls -a)
• pipelines (ls | grep foo)
• and, or lists (touch foo || echo ouch)
• command lists (sleep 10; echo "Wake up!")
• redirects to files (ls >my-files, echo onions >>shopping.txt, go build . 2> build_errors)
• redirect stdin (cat <foo >bar)
• redirect to fd (./myscript.sh 2>&1 >script_output.txt)
• command groups ({echo "my files"; ls})
• subshells ((a=12; echo $a))
• env variable substitutions (echo $PATH)
• tilde expansion (PATH=$PATH:~/bin) - hard to know what the rule is! (use $HOME for now)
• simple parameter substitution (echo ${var})
• general parameter expansion (echo ${PATH:stuff}) - that's a rabbit hole
• command substitution (ls $(go env GOROOT))
• shell variables (a=hello; echo "$a, $a!")
• functions with return (function foo() {echo $2; return; echo $1}; foo hello there )
• local variables
• if then else if cond; then echo foo; elif cond2; then echo bar; else exit; fi
• while loops while [ $# -gt 0 ]; do echo $1; shift; done
• for loops
• export (export a=10)
• arguments (echo $1 ${2})
• arg list (echo $@ ${@})
• arg count (echo $# ${#})
• status code (mycommand; echo $?)
• PID (echo $$)
• expressions [[ x = y ]]
• arithmetic (( x = y+1 ))
• comments echo no comment # Print "no comment"
• add more to the list