Wednesday, January 9, 2013

Linux tip: Using the read command

http://www.itworld.com/development/333494/linux-tip-using-read-command


This content is excerpted from the new 3rd Ed. of 'A Practical Guide to Linux: Commands, Editors, and Shell Programming', authored by Mark Sobell, ISBN 013308504X, published by Pearson/Prentice Hall Professional, Sept. 2012, Copyright 2013 Mark G. Sobell. For more info please visit www.sobell.com or the publisher site, www.informit.com

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read: Accepts User Input

A common use for user-created variables is storing information that a user enters in response to a prompt. Using read, scripts can accept input from the user and store that input in variables. The read builtin reads one line from standard input and assigns the words on the line to one or more variables:
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$ cat read1
echo -n "Go ahead: "
read firstline
echo "You entered: $firstline"
$ ./read1
Go ahead: This is a line.
You entered: This is a line.

The first line of the read1 script uses echo to prompt for a line of text. The –n option suppresses the following NEWLINE, allowing you to enter a line of text on the same line as the prompt. The second line reads the text into the variable firstline. The third line verifies the action of read by displaying the value of firstline.
The –p (prompt) option causes read to send to standard error the argument that follows it; read does not terminate this prompt with a NEWLINE. This feature allows you to both prompt for and read the input from the user on one line:

$ cat read1a
read -p "Go ahead: " firstline
echo "You entered: $firstline"
$ ./read1a
Go ahead: My line.
You entered: My line.

The variable in the preceding examples is quoted (along with the text string) because you, as the script writer, cannot anticipate which characters the user might enter in response to the prompt. Consider what would happen if the variable were not quoted and the user entered * in response to the prompt:


$ cat read1_no_quote

read -p "Go ahead: " firstline
echo You entered: $firstline
$ ./read1_no_quote
Go ahead: *
You entered: read1 read1_no_quote script.1
$ ls
read1     read1_no_quote     script.1

The ls command lists the same words as the script, demonstrating that the shell expands the asterisk into a list of files in the working directory. When the variable $firstline is surrounded by double quotation marks, the shell does not expand the asterisk. Thus the read1 script behaves correctly:

$ ./read1
Go ahead: *
You entered: *

REPLY
When you do not specify a variable to receive read's input, bash puts the input into the variable named REPLY. The following read1b script performs the same task as read1:

$ cat read1b
read -p "Go ahead: "
echo "You entered: $REPLY"

The read2 script prompts for a command line, reads the user's response, and assigns it to the variable cmd. The script then attempts to execute the command line that results from the expansion of the cmd variable:

$ cat read2
read -p "Enter a command: " cmd
$cmd
echo "Thanks"

In the following example, read2 reads a command line that calls the echo builtin. The shell executes the command and then displays Thanks. Next read2 reads a command line that executes the who utility:

$ ./read2
Enter a command: echo Please display this message.
Please display this message.
Thanks
$ ./read2
Enter a command: who
max     pts/4      2013-06-17 07:50 (:0.0)
sam     pts/12     2013-06-17 11:54 (guava)
Thanks

If cmd does not expand into a valid command line, the shell issues an error message:

$ ./read2
Enter a command: xxx
./read2: line 2: xxx: command not found
Thanks

 The read3 script reads values into three variables. The read builtin assigns one word (a sequence of nonblank characters) to each variable:

$ cat read3
read -p "Enter something: " word1 word2 word3
echo "Word 1 is: $word1"
echo "Word 2 is: $word2"
echo "Word 3 is: $word3"
$ ./read3
Enter something: this is something
Word 1 is: this
Word 2 is: is
Word 3 is: something

When you enter more words than read has variables, read assigns one word to each variable, assigning all leftover words to the last variable. Both read1 and read2 assigned the first word and all leftover words to the one variable the scripts each had to work with. In the following example, read assigns five words to three variables: It assigns the first word to the first variable, the second word to the second variable, and the third through fifth words to the third variable.

$ ./read3
Enter something: this is something else, really.
Word 1 is: this
Word 2 is: is
Word 3 is: something else, really.


Table 10-4 lists some of the options supported by the read builtin.

Table 10-4 read options

Option Function
–a aname (array) Assigns each word of input to an element of array aname.
–d delim (delimiter) Uses delim to terminate the input instead of NEWLINE.
–e (Readline) If input is coming from a keyboard, uses the Readline Library to get input.
–n num (number of characters) Reads num characters and returns. As soon as the user types num characters, read returns; there is no need to press RETURN.
–p prompt (prompt) Displays prompt on standard error without a terminating NEWLINE
before reading input. Displays prompt only when input comes from the
keyboard.
–s (silent) Does not echo characters.
–un (file descriptor) Uses the integer n as the file descriptor that read takes its input from. Thus read –u4 arg1 arg2
is equivalent to
read arg1 arg2 <&4


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