Linux Shell Learning Note two---Custom functions (definition, return value, variable scope) Introduction

The Linux shell can be user-defined functions and can be called casually in shell scripts. Here's how to define it, and what to call attention to. I. Defining shell functions (define function)

Grammar:

[Function] funname [()]

{

Action

[Return int;]

}

Description

• 1, can be with function fun () definition, you can also directly fun () definition, without any parameters.
• 2, the parameter returns, can display add: return returns, if not added, will run the result as the last command, as the return value. Return followed by a value of N (0-255

Example (testfun1.sh):

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#!/bin/sh   fSum 3 2;  function fSum()  {    echo $1,$2;    return $(($1+$2));  }  fSum 5 7;  total=$(fSum 3 2);  echo $total,$?;  sh testfun1.sh testfun1.sh: line 3: fSum: command not found 5,7 3,2 1 5

From the above example we can get a few conclusions:

• 1. The function must be declared before the function is called, and the shell script runs on a line-by-row basis. does not pre-compile like any other language. You must declare the function first before using the function.
• 2, total=$ (fsum 3 2); With this method of invocation, it is clear that in the shell, inside the parentheses, it can be: a command statement. Therefore, we can consider a function in the shell as defining a new command, which is a command, so the individual input parameters are separated directly by a space. Once, the command inside the parameter method can be obtained by: $ ... $n. $ A represents the function itself.
• 3, function return value, only through $? System variables are obtained, directly through =, to obtain a null value. In fact, we follow the above understanding that the function is a command, in the shell to obtain the command return value, all need to get through the $?

Second, function scope, variable action range

Let's look at an example (testfun2.sh):

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#!/bin/sh  echo $(uname); declare num=1000;  uname() {   echo "test!";   ((num++));   return 100; } testvar() {   local num=10;   ((num++));   echo $num;   }  uname; echo $? echo $num; testvar; echo $num;  sh testfun2.sh Linux test! 100 1001 11 1001

Let's analyze the above example together to get the following conclusion:

• 1, the definition function can be the same as the system command, the Shell Search command, the first will be found in the current shell file defined location, find the direct execution.
• 2, need to get the function value: through $? get
• 3. If you need to pass out other types of function values, you can define the variables (this is the global variable) before the function call. It can be modified directly inside the function, and then the modified value can be read out when the function is executed.
• 4, if you need to define your own variables, you can define in the function: local variable = value, then the variable is an internal variable, its modification, will not affect the value of the same variable outside the function.

Linux Shell Learning Note two---Custom functions (definition, return value, variable scope) Introduction