Created SC2307 (markdown)

SC2307.md

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+## 'expr' expects 3+ arguments but sees 1. Make sure each operator/operand is a separate argument, and escape <>&|.
+
+### Problematic code:
+
+```sh
+# | not escaped
+expr 1 | 2
+# > not escaped
+expr "$foo" >= "$bar"
+
+# Missing spaces around +
+expr 1+2
+# Unexpected quoting around an expression
+expr "1 + 2"
+```
+
+### Correct code:
+
+```sh
+expr 16 \| 7
+expr "$foo" \>= "$bar"
+expr 1 + 2
+```
+
+### Rationale:
+
+ShellCheck found an `expr` command with 1 or 2 arguments. `expr` normally expects 3 or more.
+
+Generally, this happens for one of two reasons:
+
+* You are using an operator like `|`, `&`, `>`, `>=`, `<`, `<=`, which needs to be escaped to avoid the shell interpreting it as a pipe, backgrounded command, or redirection.
+* You don't have spaces around operators and operands (or have bad quotes) which causes them not to be separate arguments.
+
+Make sure each operator or operand to `expr` is a separate argument, and that anything containing shell metacharacters is escaped. The correct code shows examples of each.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!