linux shell read command-Getting User Input Via Keyboard--ref

ref：http://bash.cyberciti.biz/guide/Getting_User_Input_Via_Keyboard

You can accept input from the keyboard and assign an input value to a user defined shell variable using read command.

Contents

• 1 read Command Syntax
  • 1.1 Handling Input
  • 1.2 Examples
    • 1.2.1 Multiple Input (number.sh)
    • 1.2.2 Display Domain Owner Information
    • 1.2.3 Timeout Input
    • 1.2.4 Handling Passwords
• 2 Handling multiple values
  • 2.1 $IFS
    • 2.1.1 But how do I use $IFS and read command together?
    • 2.1.2 How do I change the IFS separator value?
• 3 See also

read Command Syntax

read -p "Prompt" variable1 variable2 variableN

Where,

• -p "Prompt" : Display prompt to user without a newline.
• variable1 : The first input (word) is assigned to the variable1.
• variable2 : The second input (word) is assigned to the variable2.

Handling Input

Create a script called greet.sh as follows:

#!/bin/bash
read -p "Enter your name : " name
echo "Hi, $name. Let us be friends!"

Save and close the file. Run it as follows:

chmod +x greet.sh
./greet.sh

Sample Outputs:

Enter your name : Vivek Gite
Hi, Vivek Gite. Let us be friends!

Examples

Try the following examples.

Multiple Input (number.sh)

#!/bin/bash
# read three numbers and assigned them to 3 vars
read -p "Enter number one : " n1
read -p "Enter number two : " n2
read -p "Enter number three : " n3
 
# display back 3 numbers - punched by user.
echo "Number1 - $n1"
echo "Number2 - $n2"
echo "Number3 - $n3"

Display Domain Owner Information

A shell script to display the Internet domain name owner information (domain.sh):

#!/bin/bash
read -p "Enter the Internet domain name (e.g. nixcraft.com) : " domain_name
whois $domain_name

Timeout Input

You can time out read command using the -t option. It causes read to time out and return failure if a complete line of input is not read within TIMEOUT seconds. For example, if no input provided within 10 second, program will be aborted (domain2.sh):

#!/bin/bash
read -t 10 -p "Enter the Internet domain name (e.g. nixcraft.com) : " domain_name
whois $domain_name

Handling Passwords

The -s option causes input coming from a terminal do not be displayed on the screen. This is useful for password handling (readpass.sh):

#!/bin/bash
read -s -p "Enter Password  : " my_password
echo
echo "Your password - $my_password"

Handling multiple values

Consider the following example:

read -p "Enter directory to delete : " dirname
echo "$dirname"

Sample outputs:

Enter directory to delete : foo bar /tmp/data
foo bar /tmp/data

The user supplied three values instead of one. The string is now made of three different fields. All three words are assigned to dirname using $IFS internal field separator. The $IFS determines how shell recognizes fields.

$IFS

To display default value of $IFS, enter:

echo "$IFS"

You will see a whitespace which is nothing but a space, a tab, and a newline (default). You can print actual values of IFS using the following command (see Here strings):

cat -etv <<<"$IFS"

Sample outputs:

 ^I$
$

Where,

• $ - end of line i.e. newline
• ^I$ - tab and newline

But how do I use $IFS and read command together?

Create a variable called nameservers and give it total 3 values as follows (note all values are separated by a whitespace):

nameservers="ns1.nixcraft.net ns2.nixcraft.net ns3.nixcraft.net"

Display the value of a variable nameservers with echo command or printf command:

echo "$nameservers"

OR

printf "%s" $nameservers

Now, you can simply split $nameservers using the read command as follows (see Here strings):

read -r ns1 ns2 ns3 <<< "$nameservers"

Where,

• The read command reads input from $nameservers variable.
• The default value of $IFS is used to assign values to three separate variables. Your input is broken into tokens using $IFS and assigned to three variables.
• In other words, the IFS variable worked as token delimiter or separator.
• The first token (ns1.nixcraft.net) is saved as the value of the first variable ($ns1)
• The second token (ns2.nixcraft.net) is saved as the value of the second variable ($ns2).
• The third token (ns3.nixcraft.net) is saved as the value of the third variable ($ns3).
• To display the value of each variable use echo command or printf command as follows:

echo "DNS Server #1 $ns1"
echo " #2 $ns2"
echo " #3 $ns2"

OR use the printf command

printf "DNS Server #1 %s\n #2 %s\n #3 %s\n" $ns1 $ns2 $ns3

Sample outputs:

DNS Server #1 ns1.nixcraft.net
 #2 ns2.nixcraft.net
 #3 ns3.nixcraft.net

How do I change the IFS separator value?

Consider the following /etc/passwd line:

gitevivek:x:1002:1002::/home/gitevivek:/bin/sh

Assign the above line to a variable called pwd:

pwd="gitevivek:x:1002:1002::/home/gitevivek:/bin/sh"

Save the Internal Field Separator to a variable called old:

old="$IFS"

Set the Internal Field Separator to a colon (i.e. change the Internal Field Separator):

IFS=:

Read $pwd and generate tokens using $IFS and store them into respective fields:

read -r login password uid gid info home shell <<< "$pwd"
printf "Your login name is %s, uid %d, gid %d, home dir set to %s with %s as login shell\n" $login $uid $gid $home $shell

Sample outputs:

Your login name is gitevivek, uid 1002, gid 1002, home dir set to /home/gitevivek with /bin/sh as login shell

Finally, restore the Internal Field Separator value using $old:

IFS="$old"

Where,

• : - act as token separator on $pwd i.e. the contents of the IFS variable are used as token delimiters.

• login - Field # 1 is generated using the first token and is saved as the value of the first variable ($login)
• password - Field # 2 is generated using the second token and is saved as the value of the second variable ($password)
• uid - Field # 3 and so on...
• gid - Field # 4
• info - Field # 5
• home - Field # 6
• shell - Field # 7