Created SC2094 (markdown)

SC2094.md

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+# SC2094 Make sure not to read and write the same file in the same pipeline.
+
+### Problematic code:
+
+    grep foo file.txt | sed -e 's/foo/bar/g' > file.txt
+
+### Correct code:
+
+    grep foo file.txt  | sed -e 's/foo/bar/g' > tmpfile && mv tmpfile file.txt
+
+### Rationale:
+
+Each step in a pipeline runs in parallel. 
+
+In this case, `grep foo file.txt` will immediately try to read `file.txt` while `sed .. > file.txt` will immediately try to truncate it. 
+
+This is a race condition, and results in the file being partially or (far more likely) entirely truncated. 
+
+### Contraindications
+
+You can ignore this error if:
+
+* The file is a device or named pipe. These files don't truncate in the same way.
+* The command mentions the filename but doesn't read/write it, such as `echo log.txt > log.txt`.