shell实例(七) --参数/字符串替换

1.说明

${parameter-default} --------如果变量parameter没被声明，那么就使用默认值

${parameter:-default} -------如果变量parameter没被设置，那么就使用默认值

filename=${1:-$DEFAULT_FILENAME}---------如果参数1没有设置，那么就取默认的DEFAULT_FILENAME

${parameter=default} ----如果变量parameter没声明，那么就把它的值设为default.

${parameter:=default} -----如果变量parameter没设置，那么就把它的值设为default. 

${parameter+alt_value} ----如果变量parameter被声明，那么就是用alt_value,否则使用null字符串.

${parameter:+alt_value} -----如果变量parameter被设置，那么就是用alt_value,否则使用null字符串. 

${parameter?err_msg} -----如果parameter已经被声明，那么就使用设置的值，否则打印err_msg错误消息.

${parameter:?err_msg} ----如果parameter已经被设置，那么就使用设置的值，否则打印err_msg错误消息.

 

 

冒号在linux中的使用

1、注释
　　: this is comment

　　: ' this is a multiple
　　lines comment.'

　　将注释作为参数传递给冒号命令， 因为没有输出所以可以当作注释阿，够巧妙吧。

　　2、清空文件
　　: > file

2.使用参数替换和错误消息

 

#! /bin/sh
: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo

ThisVariable=Value-of-ThisVariable
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable."
echo
echo

: ${ZZXy23AB?"ZZXy23AB has not been set."}

echo "You will not seen this message,because script already terminated."
HERE=0
exit $HERE

 

3.变量长度

${#array[*]}和${#array[@]}表示数组中元素的个数

 

#! /bin/sh
E_NO_ARGS=65

if [ $# -eq 0 ]
then
  echo "Please invoke this script with one or two command-line arguments."
  exit $E_NO_ARGS
fi

var01=abcdEFGH28ij
echo "var01 = ${var01}"
echo "Length of var01 = ${#var01}"

var02="abcd EFGH28ij"
echo "var01 = ${var02}"
echo "Length of var02 = ${#var02}"

echo "Number of command-line arguments passed to script = ${#@}" -----参数个数
echo "Number of command-line arguments passed to script = ${#*}"  -----参数个数

exit 0

 

 

4.参数替换中的模式匹配

#! /bin/sh
var1=abcd12345abc6789
pattern1=a*c

echo
echo "var1 = $var1"
echo "var1 = ${var1}"

echo "Number of characters in ${var1} = ${#var1}"
echo

echo "pattern1 = $pattern1"
echo "---------1--------"
echo '${var1#$pattern1} = ' "${var1#$pattern1}"
echo '${var##$pattern1} = ' "${var1##$pattern1}"

echo;echo;echo

pattern2=b*9
echo "var1 = $var1"
echo
echo "pattern2 = $pattern2"
echo "-------------------"
echo '${var1%pattern2} = ' "${var1%$pattern2}"
echo '${var1%%pattern2} = ' "${var1%%$pattern2}"
echo

exit 0

 

5.修改文件扩展名

#! /bin/sh
E_BADARGS=65
case $# in
0|1)
 echo "Usage: `basename $0` old file suffix new file suffix"
 exit $E_BADARGS
 ;;
esac

for filename in *.$1
do
  mv $filename ${filename%$1}$2
done

exit 0

 

6.用模式匹配来解析任意字符串

#! /bin/sh
var1=abcd-1234-defg
echo "var1 = $var1"
t=${var1#*-*}
echo "var1 (with everything,up to and including first - strippedout) = $t"

t=${var1##*-*}
echo "If var1 contains a /"-/",returns empty string... var1=$t"

t=${var1%*-*}
echo "var1 (with everything from the last - on stripped out) = $t"
echo

path_name=/home/bozo/ideas/thoughts.for.today
echo "path_name=$path_name"
t=${path_name##/*/}  ------截去最大匹配/*/
echo "path_name,stripped of prefixes = $t"

t=${path_name%/*.*} ------从尾部截去/*.*
echo "path_name,stripped of suffixes = $t"
echo

t=${path_name:11}
echo "$path_name,with first 11 chars stripped of = $t"
t=${path_name:11:5}
echo "$path_name,with first 11 chars stripped off,length 5 = $t"

echo
t=${path_name/bozo/clown}
echo "$path_name with /"bozo/" replaced by /"clown/" = $t"

t=${path_name/today/}
echo "$path_name with /"today/" delete = $t"

t=${path_name//o/O}
echo "$path_name with all o's capitalized = $t"

t=${path_name//o/}
echo "$path_name with all o's deleted = $t"

exit 0

 

7.对字符串的前缀和后缀使用匹配模式

${var/#pattern/replacement}----如果变量var的前缀匹配pattern，那么就是用replacement来替换匹配到的pattern字符串

${var/%pattern/replacement}----如果变量var的后缀匹配pattern，那么就是用replacement来替换匹配到的pattern字符串

 

#! /bin/bash
v0=abc1234zip1234abc
echo "v0 = $v0"
echo

v1=${v0/#abc/ABCDEF}

echo "v1 = $v1"

v2=${v0/%abc/ABCDEF}
echo "v2 = $v2"
echo

v3=${v0/#123/100}
echo "v3 = $v3"

v4=${v0/%123/100}
echo "v4 = $v4"

exit 0