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+## In POSIX sh, `source` in place of `.` is undefined
+
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+source /usr/lib/myfile.sh
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+. /usr/lib/myfile.sh
+```
+
+### Rationale:
+
+Your script uses a shell feature not supported by the shebang. Either rewrite the script to be portable, or change the shebang to explicitly require a shell like Bash.
+
+`source` is a Bash/Ksh specific, readable synonym for `.`.  In this case, you can for the most part easily replace `source` with `.`
+
+Do note that `source myfile` in POSIX sh only searches `$PATH`, while Bash/Ksh will additionally search the current directory.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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