可移植性真的很重要：on shell and tr

道理很简单，但是不小心造成的麻烦会很大！

关键词：shell, tr, regexp, [A-Z] vs. '[A-Z]'

 

1. Make sure where we are.
bash-3.2$ pwd
/home/zedware/test

 

2. What we have
bash-3.2$ ls -al
total 8
drwxr-xr-x  2 zhangw8 zedware   80 Apr  8 22:11 .
drwxr-xr-x 21 zedware zedware 3072 Apr  8 22:11 ..

 

3. Which shell
bash-3.2$ ps -p $$
  PID TTY          TIME CMD
27643 pts/0    00:00:00 bash

 

4. Convert uppercase to lowercase. It works?!
bash-3.2$ echo Bill | tr [A-Z] [a-z]
bill

 

5. Create a file x
bash-3.2$ touch x
bash-3.2$ ls -al
total 8
drwxr-xr-x  2 zedware zedware   80 Apr  8 22:12 .
drwxr-xr-x 21 zedware zedware 3072 Apr  8 22:11 ..
-rw-r--r--  1 zedware zedware    0 Apr  8 22:12 x

 

6. Convert again. It works? NO!
bash-3.2$ echo Bill | tr [A-Z] [a-z]
Bill

 

7. Why?
[A-Z] and [a-z] are interpreted by current shell - bash as regular expression.
Where [a-z] matches x now.

sh-3.2$ echo Bill | tr [A-Z] x
Bill

 

8. Supress the regexp by ''. And it works.
bash-3.2$ echo Bill | tr '[A-Z]' '[a-z]'
bill

 

9. Or use the simpler syntax. It also works.
bash-3.2$ echo Bill | tr A-Z a-z
bill

 

10. Delete file x and try the tests.
bash-3.2$ rm -rf x
bash-3.2$ ls -al
total 8
drwxr-xr-x  2 zedware zedware   80 Apr  8 22:13 .
drwxr-xr-x 21 zedware zedware 3072 Apr  8 22:11 ..

 

11. '' still works.
bash-3.2$ echo Bill | tr '[A-Z]' '[a-z]'
bill

 

12. Also works.
bash-3.2$ echo Bill | tr A-Z a-z
bill

 

13. Now, try it in another shell - csh.
[zedware@fedora ~/test]$ ps -p $$
  PID TTY          TIME CMD
26144 pts/0    00:00:00 csh

 

14. It shows error message. That's better than slient.
[zedware@fedora ~/test]$ echo Bill | tr [A-Z] [a-z]
tr: No match.

 

15. '' still works.
[zedware@fedora ~/test]$ echo Bill | tr '[A-Z]' '[a-z]'
bill

 

16. Also works.
[zedware@fedora ~/test]$ echo Bill | tr A-Z a-z
bill

 

17. Create file x.
[zedware@fedora ~/test]$ touch x

 

18. Error out as expected.
[zedware@fedora ~/test]$ echo Bill | tr [A-Z] [a-z]
tr: missing operand after `x'
Two strings must be given when translating.
Try `tr --help' for more information.

 

19. '' still works.
[zedware@fedora ~/test]$ echo Bill | tr '[A-Z]' '[a-z]'
bill

 

20. Also works.
[zedware@fedora ~/test]$ echo Bill | tr A-Z a-z
bill

 

Conclusion:
1. Be careful of the differences among shells.
2. Always use the standard, portable syntax of shell commands.