$ ls
file1  file2  file3  file4
$ cat mv1.sh
#!/bin/bash
for file in `ls file*`
do
   mv $file `echo "${file}.txt" `
done
$ bash mv1.sh 
$ ls
file1.txt  file2.txt  file3.txt  file4.txt

$ ls
file.1  file.2  file.3  file.4
$ cat mv1.sh
#!/bin/bash
for file in `ls file*`
do 
   #mv $file  `echo ${file}.txt|sed 's/\.//1' `
   mv $file $(echo ${file}.txt|sed 's/\.//1')
done
$ bash mv1.sh 
$ ls
file1.txt  file2.txt  file3.txt  file4.txt

$ ls
file1.txt  file2.txt  file3.txt  file4.txt 
$ cat mv3.sh
#!/bin/bash
for file in `ls file*`
do
   mv $file ${file%.txt}.sh    #第一种方法
done
$ bash mv3.sh 
$ ls
file1.sh  file2.sh  file3.sh  file4.sh
$ cat mv4.sh
#!/bin/bash
for file in `ls file*`
do
   mv $file `echo $file |sed 's/\.sh/\.pdf/'`   #第二种方法
done
$ bash mv4.sh
$  ls
file1.pdf  file3.pdf  file2.pdf  file4.pdf 

$  ls
file1.pdf  file3.pdf  file2.pdf  file4.pdf 
$ cat mv5.sh
#!/bin/bash
for file in `ls file*`
do
   mv $file `echo $file |sed 's/\.pdf//'`
done
bash mv5.sh
$ ls
file1  file2  file3  file4