[Introduction to Basic/linux basics of Linux]shell programming

declaring the execution program

#!/bin/bash

Used to tell the system to use the/bin/bash program to execute the script. For example, a Python script can be written like this:

#!/usr/bin/python

assignments and references

Assignment formula:

Variable name = value

When you take the value of a variable, you need to precede the variable name with the $ symbol, for example:

var="Hello bash"echo $var

If there is a concatenation, and there is no space for the delimiter, this time you need to use braces to enclose the variable, such as:

num=3echo"This is the${num}th"

The result is: the 3th

The shell defaults to string manipulation. So if you're going to calculate it, you can't use it that way.

var=1var= $var +1echo $var

The result is:

For numerical calculations, there are the following methods

Var=1 Let"var+=1"# Let to represent the mathematical operation var="$[$var +1]"# $[] means that the expression inside the parentheses is mathematically calculated as the (()) sense concept (Var++)) var=$ (($var +1)) var="$ (Expr"$var"+ 1)"# expr denotes integer operation var="' Expr"$var"+ 1 '"

If it is too messy, it is advisable to remember (($var + 1)). Just keep in mind that the two-parenthesis content will be counted. Process Control

If statement

if  Then   ... elif  Then   ... Else    ... fi

Some of the common judgments

 [-F  file_path   " " # to determine if a file exists [-x  file_path   ] # to determine whether a file has executable permissions [-N  "  $var   "  # to determine if the $var variable exists [  "  $var 1   =   $var 2  ] # determine if $var1 and $var2 are equal 

[] is used to represent a condition test. Note that spaces are required before and after parentheses.

&& | |
As with most other languages,&& and | | The meaning is almost the same.
&&: If the first condition is ture, check the second condition/execute the second statement;
|| : Checks the second condition/executes the second statement if the first condition is false;
So you can use it like this:

" ${file_path} " Echo " cannot read ${file_path} " 1; }

This means that if the file is unreadable, the error message is printed and exited.

Case statement
Case is an expression used to match a string, not a number.
Case expression

 Case inch ...)  ...;; Esac

Example

#!/bin/Bash Case " $" inch"1")  Echo 1;;"2")  Echo " Both";;*)  Echo "your input is $";;Esac

Cycle
While expression

 while  Do   ...  Done

for-expression

 for inch  Do   ...  Done

Both of these expressions are relatively simple, and there is nothing special to say. I found some differences between the different systems. I found that red Hat can use the following expressions, but Debian is not. I don't know whether the system version or the system itself is different.

 for (i=0;i++;i<  ...  Done

Shell Functions

function_name () {    function}

Functions need to be mastered in any language. The Shell's function only needs to be defined before it is called. As in C, but no need to declare, and the C language is still different.
You do not need to write formal parameters when defining a shell function. Call the time according to a space separated by a number of parameters passed on it. Then use the $, $, $ ... these parameters to invoke.
Here is an example of a simple recursive invocation:

#!/bin/bashrecursion () {Echo$1; if[" $"-eq"0"]; Then    Echo " Done"  Elserecursion $ (($1-1))  fi}Echo "Start"; recursion5;

Although the above is a very simple example, but explains how the function is called, how to pass the parameter.

[Getting Started with Linux]shell programming Basics/linux