Created SC2243 (markdown)

SC2243.md

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+## Prefer explicit -n to check for output (or run command without [/[[ to check for success)
+
+### Problematic code:
+
+```sh
+if [ "$(mycommand --myflags)" ]
+then
+  echo "True"
+fi
+```
+
+### Correct code:
+
+```sh
+# Check that the command outputs something on stdout
+if [ -n "$(mycommand --myflags)" ]
+then
+  echo "The command had output on stdout"
+fi
+
+# Check instead that the command succeeded (exit code = 0)
+if mycommand --myflags
+then
+  echo "The command reported success"
+fi
+```
+
+(if the command instead outputs "0" or "false", see [[SC2244]] for integer and "boolean" comparisons)
+
+### Rationale:
+
+`[ "$(mycommand)" ]` is equivalent to `[ -n "$(mycommand)" ]` and checks whether the command's output on stdout was non-empty.
+
+Users more familiar with other languages are often surprised to learn that it is nothing like e.g. `if (myfunction())`, since it does not care about what the command/function `return`s.
+
+Using an explicit `-n` helps clarify that this is purely a string operation. And of course, if the intention was to check whether the command ran successfully, now would be a good time to fix it as in the alternate example.
+
+### Exceptions:
+
+If you are familiar with the semantics of `[`, you can [[ignore]] this suggestion with no ill effects.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!