shell program下的置换command

变量置换
${param:-word}如果param的值为空或未赋值,word取代param,但param的值不变
${param:=word}如果param的值为空或未赋,word的值被赋给param
${param:?msg}如果param的值为空或未赋值将msg信息输出到STDERR,并退出shell
${param:+word}如果param有值,word取代param的值,但param的值不变
example:
if MYFRUIT's value is null, then APPLE's value substition MYFRUIT, MYFRUIT's value was not changed.
$unset MYFRUIT
$FRUIT=${MYRUIT:-APPLE}

命令置换
shell执行命令集合,然后将命令的输出结果置换.
syntax:
`command`
命令置换通常将一个输出赋给一个变量
example:
DATE=`date`
USERS=`who|wc -l`
UP=`date;uptime`
grep `id -un` /etc/passwd

算式置换

syntax
$((exp))
exp abbreviate expression, exp是一个数学表达式,结果是整数.
example1:
echo $((5/2))
2
example2:
echo $((5/2+3*4*(6-2)))
50
运算符有+,-, *, / ()
note:括号之间没有空格