Linux read command

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The Linux read command is used to read numerical values from standard input.

The read internal command is used to read a single line of data from standard input. This command can be used to read keyboard input, and when used with redirection, it can read a line of data from a file.

Syntax

read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Parameter Description:

• -a Followed by a variable, which is considered as an array, and then assign a value to it, with a space as the default delimiter.
• -d Followed by a flag, actually only the first character after it is useful, serving as the end mark.
• -p Followed by prompt information, which prints the prompt information before entering.
• -e Can use command completion feature when entering.
• -n Followed by a number, defining the length of the input text, which is very practical.
• -r The escape character, if this option is not available, it is treated as a normal character if it is present.
• -s Quiet mode, does not display input characters on the screen, for example, when entering a password during login.
• -t followed by seconds, defines the waiting time for input characters.
• -u followed by fd, reads from the file descriptor, which can be a new one opened by exec.

Online examples

1, simple reading

#!/bin/bash
#Here, a new line is默认会换行  
echo "Enter the website name: "  
#Read input from the keyboard  
read website  
echo "The website name you entered is $website"  
exit 0  #exit

Test results are:

Enter the website name: 
www.oldtoolbag.com
The website name you entered is www.oldtoolbag.com

2,-p parameter, allowing a prompt to be specified directly in the read command line.

#!/bin/bash
read -p "Enter the website name: " website
echo "The website name you entered is $website" 
exit 0

Test results are:

Enter the website name:www.oldtoolbag.com
The website name you entered is www.oldtoolbag.com

3,-t parameter specifies the number of seconds the read command waits for input. When the timer is up, the read command returns a non-zero exit status.

#!/bin/bash
if read -t 5 -p "Enter the website name: " website
then
    echo "The website name you entered is $website"
else
    echo "\nSorry, you have exceeded the time limit for input."
fi
exit 0

The executing program does not input, waiting 5 seconds later:

Enter the website name:
Sorry, you have exceeded the time limit for input

4, in addition to inputting time counting, you can also use -n Parameter settings read The command counts the input characters. When the number of input characters reaches the predetermined number, it will automatically exit and assign the input data to the variable.

#!/bin/bash
read -n1 -p "Do you want to continue [Y/N]?" answer
case $answer in
Y | y)
      echo "fine ,continue";;
N | n)
      echo "ok,good bye";;
*)
     echo "error choice";;
esac
exit 0

This example uses-n option, followed by a number 1, indicating that the read command will exit as soon as it receives a character. Just press a character to answer, and the read command will immediately accept the input and pass it to the variable without pressing the Enter key.

Only accepts 2 Input and exit:

#!/bin/bash
read -n2 -p "Please enter two characters at will: " any
echo "\nYou entered the two characters are:$any"
exit 0

Execute program and enter two characters:

Please enter two characters at will: 12
The two characters you entered are:12

5,-s The option can make read The data entered in the command does not appear on the command terminal (in fact, the data is displayed, but read The command sets the text color to the same as the background color). This option is commonly used when entering a password.

#!/bin/bash
read  -s  -p "Please enter your password: " pass
echo "\nYou entered the password is $pass"
exit 0

When executing the program, the password entered is not displayed:

Please enter your password:
The password you entered is w3codebox

6.Read file

Each call to the read command reads a "line" of text from the file. When there are no more readable lines in the file, the read command exits with a non-zero status.

How to pass the data in the file to read? Use the cat command and pass the result directly to the while command containing the read command through a pipe.

The content of the test file test.txt is as follows:

123
456
w3codebox

Test Code:

#!/bin/bash
count=1    # Assignment statement, no space
cat test.txt | while read line      # The output of the cat command is input to the read command, the value read to > is placed in line
do
   echo "Line $count:$line"
   count=$[ $count + 1 ]          # Note the space in the brackets.
done
echo "finish"
exit 0

Execution result is:

Line 1:123
Line 2:456
Line 3:w3codebox
finish

Use -e arguments, the following example input characters a Press Tab Press the key and it will output the relevant filenames (the directory exists):

$ read -e -p "Enter filename:" str 
Enter filename: a
a.out    a.py     a.pyc    abc.txt  
Enter filename: a

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