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+## Multiple ! in front of pipelines are a bash/ksh extension. Use only 0 or 1.
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+! ! true
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+true
+```
+### Rationale:
+
+POSIX (and Dash) does not allow multiple `!` pipeline negations in a row. It's also logically unnecessary.
+
+Use either zero or one `!`.
+
+### Exceptions:
+
+Scripts whose shebang declares it will run with Ksh and Bash will not trigger this warning.
+
+If you really want to negate multiple times on POSIX or Dash, e.g. to normalize exit codes to 0 or 1, use `cmd || false` or a command group:
+
+```sh
+! { ! true; } 
+```
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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