shell script里面的美元符号什么时候该用

Shell Script

#!/bin/sh
foo=Hello
echo $foo World

This will say “Hello World”. But did you see what happened with the dollars? To set a variable, no dollar. To read it, use the dollar.

This is particularly confusing to many users of the shell. I can’t even provide a good reason as to why it should work this way, whether historically or pragmatically.

Similarly, when reading variable contents, do not use the dollar:

#!/bin/sh
echo "What do you want to tell the world?"
read msg
echo $msg World

This will tell your message to the world… if you say “Hello”, then it will say “Hello World”. If you say “Goodbye Cruel”, then it will say “Goodbye Cruel World”.

Notice the dollars… whilst strange, it is at least consistent. You use the dollar symbol to quote the content of the variable; otherwise, leave it out.

For more in-depth stuff about variables, check out http://steve-parker.org/sh/variables1.shtml, http://steve-parker.org/sh/variables2.shtml
and http://steve-parker.org/sh/variables3.shtml.