Could someone look at my problem in shell-exec?

Hello, everybody, I am a new user in PHP. When I try to execute shell command in php, I met some problem. Either I running shell_exec or backstick(``), when I asked server to run tar command, it did not respond. For example, if I type ls -al, cd $dir, `more $file`, the server worked properly. However, when I type `tar cvf a.out $filename', it doesn't work. When I type tar tvf b.out(b.out is the result when I run tar on server instead from php`), it works OK. Does someone else meet the same problem like me? If the server is in safemode, it should not run any shell command in php, right? Since my server can run other shell command, why cannot it run tar cvf? I had read a lot about it from manul but did not get the answer. could you help me? Thank you very much. LJ __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com