On Sat, Apr 16, 2022 at 08:06:23AM +0800, wilson wrote: > > > Greg Wooledge wrote: > > if [ "$1" == on ] > > this sounds strange. why a string doesn't need "" around in shell script?
Shell works by text substitution. If you have foo $bar $baz the shell first replaces $bar and $baz by their values. Suppose bar is `123', baz is `a multi-word string', then the next round is foo 123 a multi-word string Then the shell splits the result in words, goes to look up `foo' in all the places $PATH says it should look (yes, I've taken a HUGE shortcut here, chime in to fill the voids ;-), and the command foo (say /usr/local/bin/foo) gets to see four parameters: `123', `a', `multi-word' and `string'. The quotes hold together things; different rules hold for single quotes (do not substitute $vars) and double quotes (do substitute). So if you want to invoke foo with two params, it is foo $bar "$baz" If you don't know whether bar could contain a space and you also want to have bar in one piece, better safe than sorry: foo "$bar" "$baz" Text substitution systems were a thing in the 1970ies..1990ies. They do have interesting properties, and survive in some odd places, the shells being one (TeX, METAFONT, Tcl come to mind, too). And oh, the C preprocessor. Cheers -- t
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