Reserved Words (Bash Reference Manual)

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3.2.1 Reserved Words

Reserved words are words that have special meaning to the shell. They are used to begin and end the shell’s compound commands.

The following words are recognized as reserved when unquoted and the first word of a command (see below for exceptions):

if then elif else fi time
for in until while do done
case esac coproc select function
{ } [[../ ]] !

in is recognized as a reserved word if it is the third word of a case or select command. in and do are recognized as reserved words if they are the third word in a for command.