A command is composed of zero or more words delimited by whitespace. The
replacement for a substitution is included verbatim in the word. For example, a
space in the replacement is included in the word rather than becoming a
delimiter, and \\ becomes a single backslash in the word. Each word is
processed from left to right and each substitution is performed as soon as it
is complete.
For example, the command
set y [set x 0][incr x][incr x]
is composed of three words, and sets the value of y to 012.
If hash
(“#”)
is the first character of what would otherwise be the first word of a command,
all characters up to the next newline are ignored.