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

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