rehash

Rebuild the command hash table

$ rehash

$ sudo apt install [package] && rehash

rehash

rehash is a shell builtin that rebuilds the internal hash table of executable commands. The shell maintains a hash table mapping command names to their full paths to avoid repeated $PATH searches. When new programs are installed or removed, this table can become outdated, causing the shell to report "command not found" for newly installed programs or to run stale paths.
Running rehash forces the shell to discard the cached hash table and rediscover all executables in the directories listed in $PATH.
In zsh, rehash also regenerates the completion list so that tab completion reflects newly installed commands.

In bash, the equivalent command is hash -r. Most modern shells handle this automatically when $PATH is modified, but newly installed programs within existing $PATH directories may still require a manual rehash. The command only affects the current shell session.

Command hashing was introduced in the Bourne Shell to improve performance by caching command locations. The rehash builtin appeared in csh (1978) and was adopted by tcsh, zsh, and other shells. Bash uses the hash builtin instead but supports the same concept.

hash(1), which(1), type(1), zsh(1)