读书笔记之Advanced Bash-Script Guide Chapter 6 Exit and Exit Status

1. The exit command terminates a script. It can also return a value, which is available to the script's parent process.
2. $? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. Following the execution of a pipe, a $? gives the exit status of the last command executed. After a script terminates, a $? from the command-line gives the exit status of the script, that is, the last command executed in the script.
3. The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status.

! true      # Note that the "!" needs a space between it and the command.
echo "exit status of \"! true\" = $?"   # 1