Shell Programming and Scripting

Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution.

This is called partial quoting, sometimes referred to as weak quoting.

Using single quotes (' ... ') causes the variable name to be used literally, and no substitution will take place.

This is full quoting, sometimes referred to as strong quoting.

Yes, 100% of the time, '$var' is interpreted as the literal four characters, the dollar sign retained, and "$var" is interpreted as a shell variable. The following examples show this:

Code:

$ x=100
$ echo "$x"
100
$ echo '$x'
$x
$ echo "$y"
[ blank line ]

In the final example, the shell tries to interpret $y as a variable. Since there is no such variable, the shell substitutes "nothing", and a blank line results.

This distinction between "single quotes" and 'double quotes' is really an important concept, so it's good you kept asking.

---------- Post updated at 02:53 PM ---------- Previous update was at 02:47 PM ----------

Just FYI, there is one additional case you probably already know about, but here goes anyway. You can use the \ "backslash" to "escape" the dollar sign, and prevent the shell from interpreting $var as a variable, as follows.

Code:

$ echo "\$x"
$x

Important to note that this substitution happens right now. If you change the value of one of the variables used later, the other string won't alter with it.