Basic shell programming conditions, iterative process control, and arithmetic operations

Basic shell programming

1. Start Script Programming

<1> #! The/bin/sh line is the shebang line, which tells the system that the following command is executed by shell.

#!/bin/shPERSON="Yuesichiu"echo $PERSONecho "what is your name?"read PERSONecho "Hello,$PERSON"

Generally, the variable name must be in uppercase. "=" no blank characters are allowed on both sides!

<2> special Variables

$ ----- Indicates the ID of the Current Shell Process

? ---- Output status of the previous command

----- Options called when the current shell is started

! ---- PID of the previous command running in the background

0 ---- File Name of the current script

1-9 ---- 1st to 9th command line parameters provided when the current script is called: $1 is the value of 1st command line parameters, and so on

 

Echo $?

2. Process Control

<1> conditional Process Control

If Conditon

Then

Something happenings

Fi

#!/bin/shecho "Guess the secret color:"read COLORif [ $COLOR="pink" ]thenecho "You are correct."#elif [ $COLOR="red" ] #  echo "you are close."else   echo "You are stupid!"fi

The test command is used to calculate the value of a condition.

The test can contain the following parameters:

-D: the specified file exists and is a directory.

-E: the specified file exists.

-F: the specified file exists and is a common file.

-G all file group ids match with file IDS

-The first file specified by NT is newer than the last file (file1-nt file2)

-The previous file specified by ot is older than the previous one (file1-nt file2)

-Rxw: the user who executes the command has the read and write permissions.

-S file exists and is not empty

Case statement format

#! /Bin/sh

# Caseselect
Echo = N "enter a number from 1 to 5 :"
Read ans
Case $ ans in
1) echo "select 1"
;;
2) echo "select 2"
;;
3) echo "select 3"
;;
4) echo "select 4"
;;
*) Echo "'basename $ 0': This is not between 1 and 4"> & 2
Exit 1
;;
Esac <2> Iterative Process Control

 

Supplement: shell arithmetic operations

BASH Shell has four arithmetic operations:
1: Use expr external program

Add r = 'expr 4 + 5' // reverse quotation marks
Echo $ R
Note! There must be a blank space between '4' and ''5 '.
R = 'expr 4 * 5' # error
Multiplication r = 'expr 4 \ * 5'

2: use $ (())
R = $(4 + 5 ))
Echo $ R

3: use $ []

R = $[4 + 5]
Echo $ R

Multiplication
R = 'expr 4 \ * 5'
R = $(4*5 ))
R = $ [4*5]
Echo $ R

Division
R = 'expr 40/5'
R = $(40/5 ))
R = $ [40/5]
Echo $ R

Subtraction
R = 'expr 40-5'
R = $(40-5 ))
R = $ [40-5]
Echo $ R

Returns the remainder.
R = $ [100% 43]
Echo $ R

Power (such as power 3 of 2)
R = $(2 ** 3 ))
R = $[2 ** 3]
Echo $ R
Note: expr has no power.

4: Use the let Command
Addition:
N = 10
Let n = n + 1
Echo $ N # n = 11

Multiplication:
Let M = N * 10
Echo $ m

Division:
Let R = m/10
Echo $ R

Calculate the remainder:
Let R = m % 7
Echo $ R

Multiplication limit:
Let R = m ** 2
Echo $ R

Although bash shell has four arithmetic operations, not each of them is cross-platform. We recommend that you use expr.
In addition, we often haveAdd 1 Operation, The following four methods can be used:
M = $ [M + 1]
M = 'expr $ m + 1'
M = $ ($ m + 1 ))
Let M = m + 1

Shell programming is prone to errors:

-Bash:./configure:/bin/sh ^ m: Bad Interpreter: no such file or directory

This is because the file is in DOC format. In VI Mode: Set ff, you can view the format: Set FF = UNIX to convert the DOC format to Unix format.