Created SC2103 (markdown)

SC2103.md

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+# Consider using ( subshell ), 'cd foo||exit', or pushd/popd instead.
+
+### Problematic code:
+
+    for dir in */
+    do
+      cd "$dir"
+      convert index.png index.jpg
+      cd ..
+    done
+
+
+### Correct code:
+
+    for dir in */
+    do
+      (
+      cd "$dir" || exit
+      convert index.png index.jpg
+      )
+    done
+
+or
+
+    for dir in */
+    do
+      cd "$dir" || continue
+      convert index.png index.jpg
+      cd ..
+    done
+
+### Rationale:
+
+When doing `cd dir; somestuff; cd ..`, `cd dir` can fail when permissions are lacking, if the dir was deleted, or if `dir` is actually a file. 
+
+In this case, `somestuff` will run in the wrong directory and `cd ..` will take you to an even more wrong directory. In a loop, this will likely cause the next `cd` to fail as well, propagating this error and running these commands far away from the intended directories.
+
+Check `cd`s exit status and/or use subshells to limit the effects of `cd`. 
+
+### Contraindications
+
+None known.