Getting "ensure" / "finally" functionality in a shell command (not script)?

Newlines in a script are almost always equivalent to semicolons:

mycmd.sh; ret=$?; rm -rf temp_files; exit $ret

In response to the edit:

Alternatively, you could also use a trap and a subshell:

( trap 'rm -rf temp_files' EXIT; mycmd.sh )

If you're looking for a copy of some languages' try { } finally { }, there is another way: using the trap builtin in bash and other POSIXy shells (see help trap).

#!/bin/bash

# exit with this by default, if it is not set later
exit_code=0  

# the cleanup function will be the exit point
cleanup () {
  # ignore stderr from rm incase the hook is called twice
  rm -rf "temp_files/" &> /dev/null  
  # exit(code)
  exit $exit_code
}

# register the cleanup function for all these signal types (see link below)
trap cleanup EXIT ERR INT TERM

# run your other script
mycmd.sh

# set the exit_code with the real result, used when cleanup is called
exit_code=$?

Read about the trap command's arguments.

Note that cleanup is called:

• if this script is sent SIGINT or SIGTERM or if CTRL-C is pressed (SIGINT)
• if this script exits normally with 0
• if mycmd.sh exits with nonzero status (maybe not what you want -- remove ERR from trap's arguments to disable)

In zsh:

{mycmd.sh} always {rm -rf temp_files}

The always part will be executed even in case of an error like a glob with no match or runtime syntax error that would exit the script.