linux - shell中if条件中的函数名称

Question

如果函数名在 shell 脚本 if-else 语句中用作表达式，表达式的值是什么？

在下面的 shell 脚本中，after 函数在失败时返回错误，该错误非零，但控件正在 else 语句中输入。为什么？

#!/bin/sh                                              
test_internet()                                                               
{
        printf "Checking for internet connectivity : \t"                        
        ping -c 4 -q www.google.com                                             
        retval=$?                                                               
        echo $retval                                                            
        if [ $retval != 0 ]                                                     
        then                                                                    
                return "$retval"                                                
        fi                                                                      
}                                                                         
main()                                                                          
{                                                                               
        if test_internet ; then                                                 
                printf "test passed\n"                                          
        else                                                                    
                printf "failed\n"                                               
        fi                                                                                
}                                                                               
main 

Answer 1

if 语句中的函数没什么特别的。调用该函数并考虑其返回值(零 = true，非零 = false)。

来自man 1 bash:

if list; then list; [ elif list; then list; ] ... [ else list; ] fi
    The if list is executed.  If its exit status is zero, the then list is executed.  Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is
    executed and the command completes.  Otherwise, the else list is executed, if present.  The exit status is the exit status of the last command executed, or zero if no condition tested true.

因此，只要简单地执行关键字if之后，其返回值就用于条件检查。

就像您通常检查数字是否相等一样，您可以获得如下所示的可视化报告:

# if [ 1 -eq 2 ]; then do_something; fi
[ 1 -eq 2 ]; echo $?