Created SC2261 (markdown)

SC2261.md

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+## Multiple redirections compete for stdout. Use cat, tee, or pass filenames instead.
+
+(or `stdin`, or `stderr`, or `FD 3`)
+
+### Problematic code:
+
+```sh
+grep foo < input1 < input2 > output1 > output2 > output3
+```
+
+### Correct code:
+
+```sh
+# Merge inputs into a single stream, write outputs individually
+cat input1 input2 | grep foo | tee output1 output2 > output3
+
+# Pass inputs as filenames, write outputs individually
+grep foo input1 input2 | tee output1 output2 > output3
+```
+### Rationale:
+
+A file descriptor, whether stdin, stdout, stderr, or non-standard ones, can only point to a single file/pipe.
+
+For input, many commands support processing multiple filenames. In these cases you can just specify the filenames instead of redirecting. Alternatively, you can use `cat` to merge multiple filenames into a single stream.
+
+For output, you can use `tee` to write to multiple output sinks in parallel. 
+
+### Exceptions:
+
+Zsh will automatically `cat` inputs and `tee` outputs, but none of the shells supported by ShellCheck do.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!