14.2. Reading User Input

14.2. Reading User Input

14.2.1 Variables (Review)

In the last chapter we talked about declaring and unsetting variables. Variables are set local to the current shell or as environment variables. Unless your shell script will invoke another script, variables are normally set as local variables within a script. (See "Variables" on page 810.)

To extract the value from a variable, precede the variable with a dollar sign. You can enclose the variable within double quotes and the dollar sign will be interpreted by the shell for variable expansion. Variable expansion is not performed if the variable is enclosed in single quotes.

Example 14.3.

 1   name="John Doe" or declare name="John Doe"  # local variable  2   export NAME="John Doe"  # global variable  3   echo "$name" "$NAME"  # extract the value  

14.2.2 The read Command

The read command is a built-in command used to read input from the terminal or from a file (see Table 14.1). The read command takes a line of input until a newline is reached. The newline at the end of a line will be translated into a null byte when read. If no names are supplied, the line read is assigned to the special built-in variable REPLY . You can also use the read command to cause a program to stop until the user hits Enter. To see how the read command is most effectively used for reading lines of input from a file, see "Looping Commands" on page 903. The “r option to read causes the backslash/newline pair to be ignored; the backslash is treated as part of the line. The read command has four options to control its behavior: “a , “e , “p , and “r . ^[2]

^[2] Options “a , “e , and “p are available only in bash versions 2.x.

Example 14.4.

 (The Script)  #!/bin/bash   # Scriptname: nosy  echo -e "Are you happy? \c"   1  read answer  echo "$answer is the right response."       echo -e "What is your full name? \c"   2  read first middle last  echo "Hello  $first"       echo n "Where do you work? "   3  read  4   echo I guess $REPLY keeps you busy!   -----------------------------------------------------  [a]  5  read -p  "Enter your job title: "   6   echo "I thought you might be an $REPLY."   7   echo -n "Who are your best friends? "   8  read -a friends  9   echo "Say hi to  ${friends[2]}."  -------------------------------------------------------   (The Output)       $  nosy   Are you happy?   Yes  1  Yes is the right response.  2  What is your full name?   Jon Jake Jones   Hello Jon  3  Where do you work?   the Chico Nut Factory  4  I guess the Chico Nut Factory keeps you busy!  5  Enter your job title:   Accountant  6  I thought you might be an Accountant.  7,8  Who are your best friends?   Melvin Tim Ernesto  9  Say hi to Ernesto.  

Table 14.1. The read Command

Format	Meaning
read answer	Reads a line from standard input and assigns it to the variable answer
read first last	Reads a line from standard input to the first whitespace or newline, putting the first word typed into the variable first and the rest of the line into the variable last
read	Reads a line from standard input and assigns it to the built-in variable REPLY (Bash and Korn shell)
read “a arrayname	Reads a list of words into an array called arrayname [a]
read “e	Used in interactive shells with command-line editing in effect; for example, if editor is vi , vi commands can be used on the input line [a]
read “p prompt	Prints a prompt, waits for input, and stores input in REPLY variable [a]
read “r line	Allows the input to contain a backslash [a]

^[a] Not implemented on versions of bash prior to 2.0.

EXPLANATION

1. The read command accepts a line of user input and assigns the input to the variable answer .

2. The read command accepts input from the user and assigns the first word of input to the variable first , the second word of input to the variable middle , and all the rest of the words up to the end of the line to the variable last .

3. A line is read from standard input and stored in the built-in REPLY variable.

4. The value of the REPLY variable is printed.

5. With the “p option, the read command produces a prompt, Enter your job title: and stores the line of input in the special built-in REPLY variable.

6. The value of the REPLY variable is displayed in the string.

7. The user is asked to enter input.

8. With the “a option, the read command takes input as an array of words. The array is called friends . The elements read into the array are Melvin , Tim , and Ernesto .

9. The third element of the friends array, Ernesto , is printed. Array indices start at 0.

Example 14.5.

 (The Script)     #!/bin/bash  # Scriptname: printer_check   # Script to clear a hung-up printer  1   if [ $LOGNAME != root ]     then         echo "Must have root privileges to run this program"         exit 1     fi 2   cat << EOF      Warning: All jobs in the printer queue will be removed.      Please turn off the printer now. Press return when you      are ready to continue. Otherwise press Control C.     EOF 3  read JUNK   # Wait until the user turns off the printer  echo 4  /etc/rc.d/init.d/lpd stop      # Stop the printer  5   echo -e "\nPlease turn the printer on now." 6   echo "Press Enter to continue" 7  read JUNK   # Stall until the user turns the printer back on  echo  # A blank line is printed  8   /etc/rc.d/init.d/lpd start  # Start the printer  

EXPLANATION

1. Checks to see if user is root . If not, sends an error and exits.

2. Creates a here document . Warning message is displayed on the screen.

3. The read command waits for user input. When the user presses Enter, the variable JUNK accepts whatever is typed. The variable is not used for anything. The read in this case is used to wait until the user turns off the printer, comes back, and presses Enter.

4. The lpd program stops the print daemon.

5. Now it's time to turn the printer back on!

6. The user is asked to press Enter when ready.

7. Whatever the user types is read into the variable JUNK , and when Enter is pressed, the program will resume execution.

8. The lpd program starts the print daemon.