Updated SC2181 (markdown)

SC2181.md

@@ -4,6 +4,7 @@
 
 ```sh
 make mytarget
+
 if [ $? -ne 0 ]
 then
   echo "Build failed"
@@ -24,6 +25,12 @@ Running a command and then checking its exit status `$?` against 0 is redundant.
 
 Instead of just checking the exit code of a command, it checks the exit code of a command (e.g. `[`) that checks the exit code of a command.
 
+Apart from the redundancy, there are other reasons to avoid this pattern:
+
+* Since the command and its status test are decoupled, inserting an innocent command like `echo "make finished"` after `make` will cause the `if` statement to silently start comparing `echo`'s status instead.
+* Scripts that run or are called with `set -e ` aka `errexit` will exit immediately if the command fails, even though they're followed by a clause that handles failure. 
+* The value of `$?` is overwritten by `[`/`[[`, so you can't get the original value in the relevant then/else block (e.g. `if mycmd; then echo "Success"; else echo "Failed with $?"; fi`).
+
 To check that a command returns success, use `if mycommand; then ...`.
 
 To check that a command returns failure, use `if ! mycommand; then ...`.