12.2. Reading User Input
The read command is used to take input from the terminal or from a file until the newline is reached. The Korn shell provides some additional options for the read command. See Table 12.1 for different read formats. See Table 12.2 for read options.
Example 12.5.
(The Script)
#!/bin/ksh
# Scriptname: nosy
print –n "Are you happy? "
1 read answer
print "$answer is the right response."
print –n "What is your full name? "
2 read first middle last
print "Hello $first"
print –n "Where do you work? "
3 read
4 print I guess $REPLY keeps you busy!
5 read place?"Where do you live? "
# New ksh read and print combined
print Welcome to $place, $first $last
(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? Tandem
4 I guess Tandem keeps you busy!
5 Where do you live? Timbuktu
Welcome to Timbuktu, Jon Jones
Table 12.1. read FormatsFormat | 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 response?"Do you feel okay?" | Displays the string Do you feel okay? to standard error and waits for the user to type a reply, then puts the reply in the variable response. This form of read requires and accepts only one variable. Whatever the user types, until the newline, will be stored in response. | read –u3 line | Reads a line from file descriptor 3 into variable line. | read | Reads input into a built-in variable, REPLY. |
Table 12.2. read OptionsOptions | Meaning |
|---|
–p | Reads a line of input from a coprocess. | –r | Treats newline character, the \n, as a literal. | –s | Copies a line into the history file. | –un | Reads from file descriptor n; the default is fd 0, or standard input. | On Versions of ksh Newer Than 1988 | –A | Stores the fields as an array, index starting at zero. | –d char | Used as an alternate delimiter for terminating input; newline is the default. | –t sec | Puts a limit of seconds on the user's response time. |
EXPLANATION
The read command accepts a line of user input and assigns the input to the variable answer. The read command accepts input from the user and assigns the first word of input to the variable first, assigns the second word of input to the variable middle, and all the rest of the words to the end of the line to the variable last. The read command, without an argument, accepts a line of input from the user and assigns the input to the built-in variable REPLY. After the shell has performed variable substitution, the print function prints the string, showing the value of the built-in REPLY variable. If the variable following the read command is appended with a question mark (?), the string after the question mark is displayed as a prompt. The user input is stored in the variable place.
12.2.1 read and File Descriptors
When the system boots up, three files called streams (stdin, stdout, and stderr) are opened and assigned to an array of file descriptors. The first three file descriptors, 0, 1, and 2, are for standard input, standard output, and standard error, respectively. The next file descriptor available is file descriptor 3. The –u option allows the read command to read directly from the file descriptor.
Example 12.6.
(The Command Line)
1 $ cat filex
Captain Kidd
Scarlett O'Hara
2 $ exec 3< filex # filex is assigned to file descriptor 3 for reading
3 $ read –u3 name1 # read from filex and store input in variable, name1
4 $ print $name1
Captain Kidd
5 $ read –u3 name2
$ print $name2
Scarlett O'Hara
6 $ exec 3<&- # close file descriptor 3
7 $ read -u3 line
ksh: read: bad file unit number
EXPLANATION
The contents of filex are displayed. The exec command is used to open file descriptor 3 for reading from filex. The read command reads one line directly from unit 3 (file descriptor 3, filex) and assigns that line to the variable name1. The line stored in name1 is printed. The file filex is still open, and this read command reads the next line from the file and stores that line in the variable name2. File descriptor 3 (unit 3) is closed. filex is no longer open. Because file descriptor 3 (filex) has been closed, the read command fails when attempting to read input from that descriptor into variable line.
12.2.2 Reading Through Files
Example 12.7 uses the read command with a while loop. The loop will iterate through the file one line at a time. When end of file is reached, the loop terminates. The files are opened with descriptors (units) for reading.
Example 12.7.
(The Files)
1 $ cat names
Merry Melody
Nancy Drew
Rex Allen
$ cat addresses
150 Piano Place
5 Mystery Lane
130 Cowboy Terrace
------------------------------------------------------------------
(The Script)
#!/bin/ksh
# Scriptname: readit
2 while read -u3 line1 && read -u4 line2
do
3 print "$line1:$line2"
4 done 3<$1 4<$2
------------------------------------------------------------------
(The Command Line)
5 $ readit names addresses
Merry Melody:150 Piano Place
Nancy Drew:5 Mystery Lane
Rex Allen:130 Cowboy Terrace
---------------------------------------------------------------
EXPLANATION
The contents of two files, names and addresses, are displayed. The while loop is started. The read command reads a line of input from file descriptor 3 (unit 3) and, if successful, reads another line from file descriptor 4. The file descriptors (units) are assigned filenames on line 4. The filenames are being passed as arguments, or positional parameters 1 and 2. The value of the first variable, a colon, and the value of the second variable are displayed. The input assigned to file descriptor 3 is the first command-line argument, names. The input assigned to file descriptor 4 is the second command-line argument, addresses. The script is executed with command-line arguments (the names of two files).
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