Created SC2270 (markdown)

SC2270.md

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+## To assign positional parameters, use 'set -- first second ..' (or use [ ] to compare).
+
+### Problematic code:
+
+```sh
+if [ -z "$1" ]
+then
+  $1="help"
+fi
+```
+
+or
+
+```sh
+if $1="help"
+then
+  echo "Usage: $0 filename"
+fi
+```
+
+### Correct code:
+
+```sh
+if [ -z "$1" ]
+then
+  set -- "help"
+fi
+```
+
+or
+
+```sh
+if [ $1 = "help" ]
+then
+  echo "Usage: $0 filename"
+fi
+```
+### Rationale:
+
+You have a command on the form `$2=value`.
+
+If the goal is to assign a new value to the positional parameters, use the `set` builtin: `set -- one two ..` will cause `$1` to be "one" and `$2` to be "two".
+
+If you instead want to compare the value, use `[ ]` and add spaces: `[ "$1" = "foo" ]`  
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!