In Linux, there are two special files /dev/null and /dev/tty. /dev/null will drop all the data written to it, i.e, when program writes data to this file, it means the program has completed the data write operation. But in fact it does nothing, if you just want the status of a command but not the output of a command, this feature will be very useful. See below shell codes:
/> vi test_dev_null.sh
#!/bin/bash
if grep hello TestFile > /dev/null
then
echo "Found"
else
echo "NOT Found"
fi
After exiting vi, type below command:
/> chmod +x test_dev_null.sh
/> cat > TestFile
hello my friend
CTRL + D #Save and exit
/> ./test_dev_null.sh
Found #Output
Modify the above script:
/> vi test_dev_null.sh
#!/bin/bash
if grep hello TestFile
then
echo "Found"
else
echo "NOT Found"
fi
/> ./test_dev_null.sh
hello my friend
Found
Let's see /dev/tty command now, /dev/tty stands for the controlling terminal (if any) for the current process. To find out which tty's are attached to which processes use the "ps -a" command at the shell prompt (command line). Look at the "tty" column. For the shell process you're in, /dev/tty is the terminal you are now using. Type "tty" at the shell prompt to see what it is (see manual pg. tty(1)). /dev/tty is something like a link to the actually terminal device name with some additional features for C-programmers: see the manual page tty(4).
/> vi test_dev_tty.sh
#!/bin/bash
printf "Enter new password: " #Prompt input
stty -echo
read password < /dev/tty
printf "\nEnter again: "
read password2 < /dev/tty
printf "\n"
stty echo
echo "Password = " $password
echo "Password2 = " $password2
echo "All Done"
/> chmod +x test_dev_tty.sh
/> ./test_dev_tty
Enter new password:
Enter again:
Password = hello
Password2 = hello
All Done
Reference : http://www.cnblogs.com/stephen-liu74/archive/2011/11/10/2240461.html
