我们经常会为某个命令设置 alias，比如我在 zsh 中设置了
alias diff="icdiff"(icdiff 是一个加强版的 diff)
➜ which diff
diff: aliased to icdiff
➜ diff
Usage: icdiff [options] left_file right_file
Show differences between files in a two column view.
Options:
--version show program's version number and exit
……如果想暂时忽略 alias，使用系统自带的 diff，则可以在前面加上 command 命令：
➜ command diff --help
Usage: diff [OPTION]... FILES
Compare files line by line.
-i --ignore-case Ignore case differences in file contents.
--ignore-file-name-case Ignore case when comparing file names.
……