Added alternatives.

SC2039.md

@@ -1,4 +1,4 @@
-## #!/bin/sh was specified, but _____ is not standard.
+## `#!/bin/sh` was specified, but _____ is not standard.
 
 The shebang indicates that the script works with `/bin/sh`, but you are using non-standard features that may not be supported. 
 
@@ -7,3 +7,103 @@ It may currently work for you, but it can or will fail on other OS, the same OS
 Either declare a specific shell like `#!/usr/bin/env bash` to make sure this shell is always used, or rewrite the script in a portable way.
 
 For help with rewrites, the Ubuntu wiki has [a list of portability issues](https://wiki.ubuntu.com/DashAsBinSh) that broke people's `#!/bin/sh` scripts when Ubuntu switched from Bash to Dash. ShellCheck may not warn about all these issues.
+
+### `$'c-style-escapes'`
+
+Bashism:
+
+```Bash
+a=$' \t\n'
+```
+
+POSIX:
+
+```sh
+a="$(printf '%b_' ' \t\n')"; a="${a%_}" # protect trailing \n
+```
+
+### `$"msgid"`
+
+Bashism:
+
+```Bash
+echo $"foo $(bar) baz"
+```
+
+POSIX:
+
+```sh
+. gettext.sh # GNU Gettext sh library
+# ...
+barout=$(bar)
+eval_gettext 'foo $barout baz' # GNU Gettext automatically echos. See their doc for more info.
+```
+
+Or you can change them to normal double quotes so you go without `gettext`.
+
+### Arithmetic `for` loops
+
+Bash:
+
+```Bash
+for ((init; test; next)); do foo; done
+```
+
+POSIX:
+
+```sh
+: $((init))
+while [ $((test)) -ne 0 ]; do foo; : $((next)); done
+```
+
+### standalone `((..))`
+
+Bash:
+
+```Bash
+((a=c+d))
+((d)) && echo d is true.
+```
+
+POSIX:
+
+```sh
+: $((a=c+d))
+[ $((d)) -ne 0 ] && echo d is true.
+```
+
+### `select` loops
+
+It takes extra care over terminal columns to make select loop look like bash's, which generates a list with multiple items on one line, or like `ls`.
+
+It is, however, still possible to make a naive translation for `select foo in bar baz; do eat; done`:
+
+```sh
+while
+  _i=0 _foo= foo=
+  for _name in bar baz; do echo "$((_i+=1))) $_name"; done
+  printf '$# '; read _foo
+do
+  [ "$foo" -le "$_i" ]] || continue
+  case _foo in 1) foo=bar;; 2) foo=baz;; esac
+  eat
+done
+```
+
+### Here-strings
+
+Bash:
+
+```Bash
+grep aaa <<< "$g"
+```
+
+POSIX:
+
+```sh
+printf '%s' "$g" | grep aaa # since we want to avoid `echo`
+```
+
+### echo flags
+
+See koalaman/shellcheck#461.