expr, bzip2: add page. iptables: fix typo (#827)

pages/linux/expr.md

@@ -0,0 +1,23 @@
+# expr
+
+> Evaluate expressions and manipulate strings.
+
+- Get string length:
+
+`expr length {{string}}`
+
+- Evaluate logical or math expression with an operator( '+', '-', '*', '&', '|', etc. ). Special symbols should be escaped:
+
+`expr {{ARG1}} {{operator}} {{ARG2}}`
+
+- Get position of the first character in 'string' that matches 'substring':
+
+`echo $(expr index {{string}} {{substring}})`
+
+- Extract part of the string:
+
+`echo $(expr substr {{string}} {{position_to_start}} {{number_of_characters}}`
+
+- Extract part of the string which matches a regular expression:
+
+`echo $(expr {{string}} : '\({{regular_expression}}\)')`