Created Optional (markdown)

Optional.md

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+## Optional Checks
+
+ShellCheck supports additional checks that are not enabled by default. These are generally subjective or stylistic.
+
+### Enabling an optional check
+
+Checks can be enabled with flags or an `enable` [[directive]] in the file or `.shellcheckrc`:
+
+```
+#!/bin/sh
+# shellcheck enable=require-variable-braces
+echo "$RANDOM"  # Will emit a suggestion to use `${RANDOM}`
+```
+
+or put `enable=require-variable-braces` in a `.shellcheckrc` in your home directory or project root.
+
+They can also be enabled on the command line with `shellcheck -o require-variable-braces myscript`
+
+
+### Enabling all checks
+
+It's a good idea to enable all warnings with `-Wall` in C, but this is not the case in ShellCheck. Optional checks are more subjective rather than more comprehensive, and may conflict with each other.
+
+However, if you for debugging or evaluation purposes want to see what's available, you can enable them with `-o all` or `enable=all` as above.
+
+
+### Available checks
+
+To see which checks are available in your version of ShellCheck, use the `--list-optional` flag. This list is not necessarily up to date:
+
+```
+$ shellcheck --list-optional
+
+name:    add-default-case
+desc:    Suggest adding a default case in `case` statements
+example: case $? in 0) echo 'Success';; esac
+fix:     case $? in 0) echo 'Success';; *) echo 'Fail' ;; esac
+
+name:    avoid-nullary-conditions
+desc:    Suggest explicitly using -n in `[ $var ]`
+example: [ "$var" ]
+fix:     [ -n "$var" ]
+
+name:    check-unassigned-uppercase
+desc:    Warn when uppercase variables are unassigned
+example: echo $VAR
+fix:     VAR=hello; echo $VAR
+
+name:    deprecate-which
+desc:    Suggest 'command -v' instead of 'which'
+example: which javac
+fix:     command -v javac
+
+name:    quote-safe-variables
+desc:    Suggest quoting variables without metacharacters
+example: var=hello; echo $var
+fix:     var=hello; echo "$var"
+
+name:    require-double-brackets
+desc:    Require [[ and warn about [ in Bash/Ksh
+example: [ -e /etc/issue ]
+fix:     [[ -e /etc/issue ]]
+
+name:    require-variable-braces
+desc:    Suggest putting braces around all variable references
+example: var=hello; echo $var
+fix:     var=hello; echo ${var}
+```