Fixed string interpolation in system commands, closes #81

docs/syntax/system-commands.md

@@ -42,6 +42,14 @@ x = $(cat /proc/$file)
 echo(x) # processor: 0\nvendor_id: GenuineIntel...
 ```
 
+and if you need `$` literals in your command, you
+simply need to escape them with a `\`:
+
+``` bash
+$(echo $PWD) # "" since the ABS variable PWD doesn't exist
+$(echo \$PWD) # "/go/src/github.com/abs-lang/abs"
+```
+
 Currently, commands need to be on their own line, meaning
 that you will not be able to have additional code
 on the same line. This will throw an error:

evaluator/evaluator.go

@@ -874,8 +874,24 @@ func evalHashIndexExpression(hash, index object.Object) object.Object {
 }
 
 func evalCommandExpression(cmd string, env *object.Environment) object.Object {
-	re := regexp.MustCompile("\\$([a-zA-Z_]{1,})")
+	// Match all strings preceded by
+	// a $ or a \$
+	re := regexp.MustCompile("(\\\\)?\\$([a-zA-Z_]{1,})")
 	cmd = re.ReplaceAllStringFunc(cmd, func(m string) string {
+		// If the string starts with a backslash,
+		// that's an escape, so we should replace
+		// it with the remaining portion of the match.
+		// \$VAR becomes $VAR
+		if string(m[0]) == "\\" {
+			return m[1:]
+		}
+
+		// If the string starts with $, then
+		// it's an interpolation. Let's
+		// replace $VAR with the variable
+		// named VAR in the ABS' environment.
+		// If the variable is not found, we
+		// just dump an empty string
 		v, ok := env.Get(m[1:])
 
 		if !ok {

evaluator/evaluator_test.go

@@ -1037,6 +1037,7 @@ func TestCommand(t *testing.T) {
 		{`$(echo -n "123")`, "123"},
 		{`$(echo -n hello world)`, "hello world"},
 		{`$(echo hello world | xargs echo -n)`, "hello world"},
+		{`$(echo \$CONTEXT)`, "abs"},
 	}
 
 	for _, tt := range tests {