CH10 - bash 脚本编程

条件测试1 - test 或 []

[ ]                 test
[ 0 ]               test 0      
[ 1 ]               test 1
[ string ]          test string

[ $UID == 0 ]         比较    [ $UID==0 ]
[ $(id -u) == 0 ]     比较    [ $(id -u)==0 ]

使用命令列表替换 if 语句

if [ ];   then echo T; fi 
if [ 0 ]; then echo T; fi 
if [ 1 ]; then echo T; fi 
if [ string ]; then echo T; fi 

[ ]   && echo T
[ 0 ] && echo T
[ 1 ] && echo T
[ string ] && echo T

if [ ];   then echo T; else echo F; fi 
if [ 0 ]; then echo T; else echo F; fi 
if [ 1 ]; then echo T; else echo F; fi 
if [ string ]; then echo T; else echo F; fi 

[ ]   && echo T || echo F
[ 0 ] && echo T || echo F
[ 1 ] && echo T || echo F
[ string ] && echo T || echo F

使用命令列表替换 if 语句 （续）

[ ]   && echo T
[ 0 ] && echo T
[ 1 ] && echo T
[ string ] && echo T

! [ ]   && echo F
! [ 0 ] && echo F
! [ 1 ] && echo F
! [ string ] && echo F

[ ]   && : || echo F
[ 0 ] && : || echo F
[ 1 ] && : || echo F
[ string ] && : || echo F

~]# help :
:: :
    Null command.

    No effect; the command does nothing.

    Exit Status:
    Always succeeds.

~]# help true false
true: true
    Return a successful result.

    Exit Status:
    Always succeeds.
false: false
    Return an unsuccessful result.

    Exit Status:
    Always fails.

条件测试2 - (())

(( ))                 比较    [ ]
(( 0 ))               比较    [ 0 ]
(( 1 ))               比较    [ 1 ]
(( string ))          比较    [ string ]

条件测试3 - 根据命令结果状态码测试

id -u > /dev/null
echo $?

grep -q '^osmond\>' /etc/passwd || useradd osmond
ping -c1 -W1 8.8.8.8 > /dev/null && elinks --dump http://whatismyip.org || ip r s

检查字符串变量空值与非空值

• 检查空

  [ "$name" = "" ]
  [ -z "$name" ]
  [ ! "$name" ]
  [ "X${name}" = "X" ]
  

• 检查非空

  [ "$name" != "" ]
  [ -n "$name" ]
  [ "$name" ]
  [ "X${name}" != "X" ]
  

检查字符串变量是否为纯数字(正整数)的方法

((n>0)) 2> /dev/null 
[ $n -gt 0 ] 2> /dev/null

[ -n "$n"  -a  -z "$(sed 's/[0-9]//g' <<< "$n")" ]
[ -n "$n"  -a  -z "$(egrep -o '[^0-9]+' <<< "$n")" ]
[ -n "$n"  -a  -z "${n//[0-9]/}" ]
[ -n "$n"  -a  "$n" == "${n//[0-9]/}" ]

[[ -n "$n"  &&  -z "$(sed 's/[0-9]//g' <<< "$n")" ]]
[[ -n "$n"  &&  -z "$(egrep -o '[^0-9]+' <<< "$n")" ]]
[[ -n "$n"  &&  -z "${n//[0-9]/}" ]]
[[ -n "$n"  &&  "$n" == "${n//[0-9]/}" ]]

在 if 、while、until 中使用条件测试

~]# help if while until
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi
    Execute commands based on conditional.

    Exit Status:
    Returns the status of the last command executed.

while: while COMMANDS; do COMMANDS; done
    Execute commands as long as a test succeeds.

    Exit Status:
    Returns the status of the last command executed.

until: until COMMANDS; do COMMANDS; done
    Execute commands as long as a test does not succeed.

    Exit Status:
    Returns the status of the last command executed.

举例：从标准输入读取n的值，直至为全数字的整数为止

n=
until ((n>0)) 2>/dev/null
do
  read -p "Pls input a number: "  n
done
###
n=
while ! ((n>0)) 2>/dev/null
do
  read -p "Pls input a number: "  n
done

n=
while true
do
  read -p "Pls input a number: "  n
  ((n>0)) 2>/dev/null && break
done
###
n=
until false
do
  read -p "Pls input a number: "  n
  ((n>0)) 2>/dev/null && break
done

循环控制语句与 Shell 特性

|while
|until
|for

done|

done<
done<<<
done<<END

done>
done>>

函数

• 定义
  • 是被命名的的 shell 脚本片段
• 显示
  • declare -F
  • declare -f
  • declare -f funName
• 执行
  • 函数与主程序在一个 shell 内执行
  • 主程序与函数中同名变量是一个变量
  • 在函数中使用 local 定义的变量只在此函数及其调用的函数中起作用
• 返回值
  • 状态返回值 exit N
  • 结果返回值 可通过标准输出传递

a=100; b=200

mytest1 () {
  local a
  echo -e "($BASHPID) \t $[a=a+b] \t $a$b"
  mytest2
}
mytest2 () {
  echo -e "($BASHPID) \t $[++a]"
}
declare -f mytest{1,2}

echo $a $b
mytest1
echo $?
echo $a $b
echo $BASHPID