*Bash：字符串替换。
我们在编写bash脚本的时候，经常需要替换掉字符窗中特殊的字符，我们看看有几种方法可以实现。
- 最常用的方法是使用sed命令。例如
[nhuang@localhost test]$ a="This is a / and you will know /" ; echo "$a" | sed "s/\//\\\\\//g" This is a \/ and you will know \/
但是相当复杂，应为在替换部分要使用双反斜杠，"\\\\\/"，而不是"\\\/",为什么呢？应为在escape"\“的时候，必须要使用"\\"，而不是对待"/"的"\/"。例如
[nhuang@localhost test]$ a="This is a / and you will know /" ; echo "$a" | sed -s "s/\//\//g" This is a / and you will know / [nhuang@localhost test]$ a="This is a / and you will know /" ; echo "$a" | sed -s "s/\//\\\\/g" This is a \ and you will know \ [nhuang@localhost test]$ a="This is a / and you will know /" ; echo "$a" | sed -s "s/\//\\/g" sed: -e expression #1, char 8: unterminated `s' command
- 那么，第二种方法是什么呢？使用${}这个符号，例如：
[nhuang@localhost test]$ a="This is a / and you will know /" ; echo ${a//\//\\} This is a \ and you will know \ [nhuang@localhost test]$ a="This is a / and you will know /" ; echo ${a//\//\\\/} This is a \/ and you will know \/
这里，我们不再需要使用"\\"了，而是正常的使用正则表达式。所以，第二种方法，最为方便。如果使用expr substr，则会把问题复杂化。dan
- 但是，如果要同时替换多个不同位置的字符串，例如"/", "\", "?",等，要在他们前面都加上反斜杠，使用sed，用-e，或者"{;}"来操作。
[nhuang@localhost test]$ str="asdasdf / asdfasdf / asdf,adf" ; echo $str | sed "{s/\//\\\\\//g;s/,/:/g}" asdasdf \/ asdfasdf \/ asdf:adf
- 我们也可以使用多次的${}来做替换，例如：
[nhuang@localhost test]$ str="This is / and This is & " ; str=`echo ${str//\//\\\/}` ; str=`echo ${str//&/\\\&}` ; echo $str This is \/ and This is \& [nhuang@localhost test]$ str="This is / and This is \\" ; str=`echo ${str//\//\\\/}` ; str=`echo ${str//\\\/\\\\\\\}` ; echo $str This is \\/ and This is \\ [nhuang@localhost test]$ str="This is / and This is \ " ; str=`echo ${str//\//\\\/}` ; str=`echo ${str//\\\/\\\\\\\}` ; echo $str This is \\/ and This is \\
第二个和第三个有点复杂：（，
资料：
http://www.cnblogs.com/frydsh/p/3261012.html
