实验案例
该脚本会检查基于 Red Hat 的标准软件包管理器（rpm、dnf 和 flatpak）。它会对每个
软件包管理器使用 which 命令，并在 if 条件语句中使用单括号。
如果找到了软件包管理器，那么针对该管理器的一个特殊布尔变量就会被设置为 TRUE（1），如果没有找到，则会被设置为FALSE（0），如下所示：

$ cat PackageMgrCheck.sh 
#!/bin/bash 
# Checks system for popular package managers 
# 
#################### User Introduction ###################### 
echo "########################################################" 
echo 
echo " This script checks your Linux system for popular" 
echo "package managers and application containers, lists" 
echo "what's available, and makes an educated guess on your" 
echo "distribution's base distro (Red Hat or Debian)." 
echo 
echo "#######################################################" 
# 
##################### Red Hat Checks ####################### 
# 
echo 
echo "Checking for Red Hat-based package managers &"

echo "application containers..." 
##### 
if (which rpm &> /dev/null) 
then 
    item_rpm=1 
    echo "You have the basic rpm utility." 
else 
    item_rpm=0  
fi 
#### 
if (which dnf &> /dev/null) 
then 
    item_dnfyum=1 
    echo "You have the dnf package manager." 
# 
elif (which yum &> /dev/null) 
then 
    item_dnfyum=1 
    echo "You have the yum package manager." 
else 
    item_dnfyum=0  
fi 
#### 
if (which flatpak &> /dev/null) 
then 
    item_flatpak=1 
    echo "You have the flatpak application container."  
else 
    item_flatpak=0 
fi 
#### 
redhatscore=$[$item_rpm + $item_dnfyum + $item_flatpak] 
# 
##################### Debian Checks ####################### 
# 
echo 
echo "Checking for Debian-based package managers &" 
echo "application containers..." 
##### 
if (which dpkg &> /dev/null) 
then 
    item_dpkg=1 
    echo "You have the basic dpkg utility." 
else 
    item_dpkg=0  
fi 
####
if (which apt &> /dev/null) 
then 
    item_aptaptget=1 
    echo "You have the apt package manager." 
# 
elif (which apt-get &> /dev/null) 
then 
    item_aptaptget=1 
    echo "You have the apt-get/apt-cache package manager."  
else 
 item_aptaptget=0 
fi 
#### 
if (which snap &> /dev/null) 
then 
    item_snap=1 
    echo "You have the snap application container."  
else 
    item_snap=0  
fi      
debianscore=$[$item_dpkg + $item_aptaptget + $item_snap] 
# 
# 
##################### Determine Distro ####################### 
# 
echo 
if [ $debianscore -gt $redhatscore ] 
then 
    echo "Most likely your Linux distribution is Debian-based." 
elif [ $redhatscore -gt $debianscore ] 
then 
    echo "Most likely your Linux distribution is Red Hat-based." 
else 
    echo "Unable to determine Linux distribution base." 
fi 
# 
echo 
# 
#############################################################  
exit 
$