See past commands with history and which

When you forget the exact command you ran five minutes ago, Bash already stored it. history prints that list. which answers a different question: which executable will run if you type a name.

Run history with no extra arguments. Each line starts with a number, then the command.

Read the session log:

The numbers are useful later with !n replay, but the first skill is simply reading the list so you can copy a command that already worked.

which echo prints the path of the echo program the shell will run. That tells you whether you are using a builtin, a system binary, or a custom script that shadowed the name.

which echo

Typical output is /usr/bin/echo or a similar path. If which prints nothing, the name is not on your PATH.

Use history to remember what you did. Use which when two tools share a name and you need to know which one actually runs.