Bash supports many ways of string manipulation, it includes substring, substring replacement, substring removal, string length.
In
how to rename multiple files in one command, one of the ways I shared to rename *.txt to *.sh is
for f in *.txt
do
mv $f ${f/.txt/.sh}
done
How ${f/.txt/.sh} is doing the magic? Actually this is string replacement in bash.
${f/.txt/.sh} means replacing ".txt" in string $f with ".sh", so for f=abc.txt,
${f/.txt/.sh} will produce abc.sh
[linuxscripter@localhost ~]$ f=abc.txt
[linuxscripter@localhost ~]$ echo $f{f/.txt/.sh}
abc.sh
But the example I gave in
rename multiple files in one command was not so correct.
For f=abc.txt.txt,
${f/.txt/.sh} will produce abc.sh.txt instead of abc.txt.sh
[linuxscripter@localhost ~]$ f=abc.txt.txt
[linuxscripter@localhost ~]$ echo $f{f/.txt/.sh}
abc.txt.sh
To correct this, we need to use
${f/%.txt/.sh}
[linuxscripter@localhost ~]$ f=abc.txt.txt
[linuxscripter@localhost ~]$ echo $f{f/%.txt/.sh}
abc.txt.sh
The syntax of string replacement includes:
1. ${var/Pattern/Replacement} : replacing the first match of "Pattern" with "Replacement"
2. ${var//Pattern/Replacement} : replacing every match of "Pattern" with "Replacement"
3. ${var/#Pattern/Replacement} : if $var starts with "Pattern", replace "Pattern" with "Replacement"
4. ${var/%Pattern/Replacement} : if $var ends with "Pattern", replace "Pattern" with "Replacement"
Some examples will make the syntax clearer.
[linuxscripter@localhost ~]$ f=abc.txt.abc.txt
[linuxscripter@localhost ~]$ ### testing syntax 1
[linuxscripter@localhost ~]$ echo ${f/txt/sh}
abc.sh.abc.txt
[linuxscripter@localhost ~]$ ### testing syntax 2
[linuxscripter@localhost ~]$ echo ${f//txt/sh}
abc.sh.abc.sh
[linuxscripter@localhost ~]$ ### testing syntax 3, [linuxscripter@localhost ~]$ echo ${f/#txt/sh}
abc.txt.abc.txt
[linuxscripter@localhost ~]$ ### testing syntax 3,
[linuxscripter@localhost ~]$ echo ${f/#abc/sh}
sh.txt.abc.txt
[linuxscripter@localhost ~]$ ### testing syntax 4, [linuxscripter@localhost ~]$ echo ${f/%abc/sh}
abc.txt.abc.txt
[linuxscripter@localhost ~]$ ### testing syntax 4
[linuxscripter@localhost ~]$ echo ${f/%txt/sh}
abc.txt.abc.sh
At first these syntax may not appear very intuitive, I found this method help to understand and memorize them.
Variable is referred by add '$' infront of it
1. '/' is used for replacement, same as what we do in awk and sed
2. Once we know '/' is for replacement, '//' should be too difficult for us.
3. On keyboard, '#' is at the left of '$', so '#' is used to replace the left side of the variable.
4. On keyboard, '%' is at the right of '$', so '%' is used to replace the right side of the variable.
Besides string replacement, of course there are a lot more bash can do.
${#var} returns length of $var
[linuxscripter@localhost ~]$ echo {#f}
15
${var:pos:len} returns substring of $var, starting at position "pos", with length upto "len", if len is empty or len is too big, return substring starting at position "pos" until to the end of the string.
[linuxscripter@localhost ~]$ echo ${f:2:5} ${f:2:100} ${f:2}
c.txt c.txt.abc.txt c.txt.abc.txt
${var#substring} delete shortest match of "substring" from front of $var
[linuxscripter@localhost ~]$ echo ${f#abc}
.txt.abc.txt
[linuxscripter@localhost ~]$ echo ${f#*txt}
.abc.txt
${var##substring} delete longest match of "substring" from front of $var
[linuxscripter@localhost ~]$ echo ${f##*abc}
.txt
${var%substring} delete longest match of "substring" from back of $var
[linuxscripter@localhost ~]$ echo ${f%txt}
abc.txt.abc.
[linuxscripter@localhost ~]$ echo ${f%abc*}
abc.txt.
${var%%substring} delete longest match of "substring" from back of $var
[linuxscripter@localhost ~]$ echo ${f%%txt*}
abc.
http://tldp.org/LDP/abs/html/string-manipulation.html
http://bbs.chinaunix.net/forum.php?mod=viewthread&tid=218853&page=7
