Shift usage in shell programming

Source: Internet
Author: User
Location parameters can be used shiftCommand left shift. For example shift 3Indicates the original $4Change $1, The original $5Change $2And so on. $1, $2, $3Discard, $0Do not move. Without Parameters shiftThe command is equivalent shift 1. Very useful Unix Command: shift. We know that the number of location variables or command line parameters must be fixed, or when the shell program does not know the number, all parameters can be assigned to the variable $ * together *. If you want shell to process parameters one by one without knowing the number of location variables, that is, after $1, after $2, it is $3. The value of $1 before the shift command is executed is not available after the shift command is executed.


# Test shift command (
Until [$ #-EQ 0]
Echo "the first parameter is $1. The number of parameters is $ #"
Run the preceding program
$./ 1 2 3 4

The result is as follows:

The first parameter is: 1. The number of parameters is: 4.
The first parameter is: 2. The number of parameters is: 3.
The first parameter is 3. The number of parameters is 2.
The first parameter is: 4. The number of parameters is: 1.

We can see that the number of variables ($ #) is reduced by one every time the shift command is executed, and the variable value is one bit in advance. The following code uses the until and shift commands to calculate the sum of all command line parameters.

# Shift application of the previous command (
If [$ #-EQ 0]
Echo "Usage: Parameters"
Exit 1
Sum = 0
Until [$ #-EQ 0]
Sum = 'expr $ sum + $1'
Echo "sum is: $ sum"

Execute the above program:

$ 10 20 15

The result is as follows:


The shift command has another important purpose. bsh defines nine location variables from $1 to $9, which does not mean that you can only use nine parameters in the command line, the shift command can be used to access more than 9 parameters.

The number of moving parameters at a time is specified by the parameter included in the shift command. For example, after the shell program processes the first nine command line parameters, you can use the shift 9 command to move $10 to $1.

Related Article

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.