Up to now, our scripts have not been interactive. That is, they did not require any input from the user. In this lesson, we will see how your scripts can ask questions, and get and use responses.
read
To get input from the keyboard, you use the read command. The read command takes input from the keyboard and assigns it to a variable. Here is an example:
#!/bin/bash
echo -n "Enter some text > "
read text
echo "You entered: $text"
As you can see, we displayed a prompt on line 3. Note that “-n” given to the echo command causes it to keep the cursor on the same line; i.e., it does not output a carriage return at the end of the prompt.
Next, we invoke the read command with “text” as its argument. What this does is wait for the user to type something followed by a carriage return (the Enter key) and then assign whatever was typed to the variable text.
Here is the script in action:
[me@linuxbox me]$ read_demo.bash
Enter some text > this is some text
You entered: this is some text
If you don’t give the read command the name of a variable to assign its input, it will use the environment variable REPLY.
The read command also takes some command line options. The two most interesting ones are -t and -s. The -t option followed by a number of seconds provides an automatic timeout for the read command. This means that the read command will give up after the specified number of seconds if no response has been received from the user. This option could be used in the case of a script that must continue (perhaps resorting to a default response) even if the user does not answer the prompts. Here is the -t option in action:
#!/bin/bash
echo -n "Hurry up and type something! > "
if read -t 3 response; then
echo "Great, you made it in time!"
else
echo "Sorry, you are too slow!"
fi
The -s option causes the user’s typing not to be displayed. This is useful when you are asking the user to type in a password or other security related information.
Arithmetic
Since we are working on a computer, it is natural to expect that it can perform some simple arithmetic. The shell provides features for integer arithmetic.
What’s an integer? That means whole numbers like 1, 2, 458, -2859. It does not mean fractional numbers like 0.5, .333, or 3.1415. If you must deal with fractional numbers, there is a separate program called bc which provides an arbitrary precision calculator language. It can be used in shell scripts, but is beyond the scope of this tutorial.
Let’s say you want to use the command line as a primitive calculator. You can do it like this:
[me@linuxbox me]$ echo $((2+2))
As you can see, when you surround an arithmetic expression with the double parentheses, the shell will perform arithmetic evaluation.
Notice that whitespace is not very important:
[me@linuxbox me]$ echo $((2+2))
4
[me@linuxbox me]$ echo $(( 2+2 ))
4
[me@linuxbox me]$ echo $(( 2 + 2 ))
4
balajiommudali 