判断一个命令是否存在

1、很多系统的which并不设置退出时的返回值，即使要查找的命令不存在，which也返回0。

# which ls
/usr/bin/ls
# echo $?
0
# which aaa
no aaa in /usr/bin /bin /usr/sbin /sbin /usr/local/bin /usr/local/bin /usr/local/sbin /usr/ccs/bin /usr/openwin/bin /usr/dt/bin
# echo $?
0

2、许多系统的which实现，都偷偷摸摸干了一些“不足为外人道也”的事情。所以，不要用which，可以使用下面的方法：

$ command -v foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
$ type foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
$ hash foo 2>/dev/null || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }