一個是外部命令一個不是：
$ type env
env 是 /usr/bin/env第二個用法適用於 shell 內建命令：
brlin$ help for
for: for 名稱 [in 字詞 … ] ; do 命令 ; done
為列表中的每個成員執行命令。
「for」迴圈為列表中的每個成員執行一系列的命令。如果沒有
「in WORDS ...;」則假定使用「in "$@"」。對於 WORDS 中的每
個元素，NAME 被設定為該元素，並且執行 COMMANDS 命令。
退出狀態：
回傳最後執行命令的狀態。
brlin$ LANG=C help for
for: for NAME [in WORDS ... ] ; do COMMANDS; done
Execute commands for each member in a list.
The `for' loop executes a sequence of commands for each member in a
list of items. If `in WORDS ...;' is not present, then `in "$@"' is
assumed. For each element in WORDS, NAME is set to that element, and
the COMMANDS are executed.
Exit Status:
Returns the status of the last command executed.
brlin$ env LANG=C help for
env: ‘help’: 沒有此一檔案或目錄