Improve sanity check

* Use 'git diff --name-only HEAD~1' to list changed files as
  "git show --name-only --format=''" doesn't work for pull requests.
* Add instructions on the installation of a specific version shellcheck.
* Add descriptions for the skipped pycodestyle and shellcheck codes.
* Skip the following shellcheck as 'which' is widely used and supported
  and busybox doesn't support 'command'.

  SC2230: which is non-standard. Use builtin 'command -v' instead.

Signed-off-by: Chase Qi <[email protected]>

.travis.yml

@@ -4,7 +4,9 @@ python:
   - "3.6"
 
 # Pre-installed packages.
-# shellcheck 0.5.0
+# shellcheck v0.5.0
+# Refer to the following link to install specific version shellcheck binary.
+# https://github.com/koalaman/shellcheck#installing-the-shellcheck-binary
 # php 7.2
 install:
   - pip install pyyaml pycodestyle

sanity-check.sh

@@ -5,4 +5,14 @@ python3 validate.py \
     -g \
     -r build-error.txt \
     -p E501 \
-    -s SC1091
+    -s SC1091 SC2230
+
+# pycodestyle checks skipped:
+# E510: line too long
+
+# Shellchecks skipped:
+# SC1091: not following
+
+# Reason: 'which' is widely used and supported. And 'command' applets isn't
+# available in busybox, refer to https://busybox.net/downloads/BusyBox.html
+# SC2230: which is non-standard. Use builtin 'command -v' instead.

validate.py

@@ -236,7 +236,7 @@ def main(args):
     if args.git_latest:
         # check if git exists
         git_status, git_result = subprocess.getstatusoutput(
-            "git show --name-only --format=''")
+            "git diff --name-only HEAD~1")
         if git_status == 0:
             filelist = git_result.split()
             exitcode = run_unit_tests(args, filelist)