Fix several shellcheck warnings

• Check Check exit code directly [SC2181]
• Double quote to prevent globbing and word splitting. [SC2086]
• In POSIX sh, == in place of = is undefined. [SC3014]
• Declare and assign separately to avoid masking return values. [SC2155]
• In POSIX sh, echo flags are undefined. [SC3037]
• Use $(...) notation instead of legacy backticks .... [SC2006]
• $/${} is unnecessary on arithmetic variables. [SC2004]