替换操作符

    ${varname:-word} varname存在且非Null，返回其值；否则返回word

${varname:=word} varname存在且非Null，返回其值；否则设置为word

     ${varname:？message} varname存在且非Null，返回其值；否则打印message

     ${varname:+word} varname存在且非Null，返回word；否则返回Null

   ${varname:(-16)} 倒着取变量的字符字数。

     ${varname:offset} 返回字符串的一部分 

     ${varname:offset:offset} 返回字符串的一部分

 

# 匹配前面 因为# 总在数字前。  %  匹配后面， 因为%总在数字后

 

1）path="/home/cam/book/long.file.name"

echo ${path#/*/}                        cam/book/long.file.name

echo ${path##/*/}                             long.file.name

echo ${path%.*}                        /home/cam/book/long.file

echo ${path%%.*}                      /home/cam/book/long

 

2）filename=”alicece”

echo ${filename%ce}                         alice

echo ${filename%%ce}                      alice

echo ${filename%ce*}                       alice

echo ${filename%%ce*}                    ali

 

3）接受一pcx文件，转换成gif文件 ：outfile=${filename%.pcx}.gif

从绝对路径得到文件名 ：bannername=${pathname##*/}

4）  把$PATH变量一个路径一行输出：echo -e ${PATH//:/'\n'}

output:
/home/cam/bin
/usr/local/bin
……

5）${#varname} 返回变量字符串值得长度 如果filename=”alice.c” 则${#vilename} 为7