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+## Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.
+
+### Problematic code:
+
+```sh
+make mytarget
+
+if [ $? -ne 0 ]
+then
+  echo "Build failed"
+fi
+```
+
+### Correct code:
+
+```sh
+if ! make mytarget
+then
+  echo "Build failed"
+fi
+```
+### Rationale:
+
+Running a command and then checking its exit status `$?` 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 $?"; fi`).
+
+To check that a command returns success, use `if mycommand; then ...`.
+
+To check that a command returns failure, use `if ! mycommand; then ...`. Notice that `!` will overwrite `$?` value.
+
+To additionally capture output with command substitution: `if output=$(mycommand); then ...`
+
+This also applies to `while`/`until` loops.
+
+### Exceptions:
+
+The default Solaris 10 bourne shell does not support '!' outside of the test command (`if ! mycommand; then ...` returns `!: not found`)
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