Essential System Administration, 3rd Edition

Getting Input: The read Command

The read command reads one line from standard input and assigns the next word in the line to each successive variable specified as its arguments; extra words are assigned to its final argument. For example, these commands:

cat file.dat | \
while read x y z 
do 
   echo $x $y $z
done

produce output like this:

a b c 
d e f 
...

read can be used either for reading sequentially through a file (as in the earlier example with while) or for getting runtime input from the user. Here is an example using read for command input:

echo "fsck all disks? [y] \c" 
read ans < /dev/console

The bash select command

bash provides the select command for prompting the user to select an item from a menu, as in this example:

$ cat choose.bash
#!/bin/bash
  
PS3="Choose an operating system: "
select os in "aix" "hp-ux" "solaris" "tru64" "linux" "freebsd"
do                                Loop until a valid choice is entered.
  if [ $os ]; then
    echo You chose $os which was choice number $REPLY
    break
  else
    echo -e "\nInvalid choice -- try again.\n"
  fi
done
$ choose.bash
1) aix
2) hp-ux
3) solaris
4) tru64
5) linux
6) freebsd
Choose an operating system: 2
You chose hp-ux which was choice number 2

This code fragment also illustrates the bash echo command’s -e option, which allows you to include backslash escape sequences such as \n.