如何检查grep是否没有输出？ [英] How to check if grep has no output?

问题描述

因此，我需要检查收件人用户名是否位于/ etc / passwd中，该用户名包含我班的所有用户，但我尝试了if语句和grep的几种不同组合，但都没有成功。我能想到的最好的是，但我认为它不能正常工作。
我的逻辑是，如果grep为空，则用户无效。

  send_email（）
 {
 message = 
 address = 
 attachment = 
 validuser = 1 
 until [$ validuser=0] 
 do 
 echo输入电子邮件地址：
读取地址
如果[-z grep $地址/ etc / passwd] 
然后
 validuser = 0 
 else $ b $ b validuser = 1 
 fi 
 echo -n输入消息主题：
读取消息
 echo
 echo输入文件想要附上：
阅读附件
邮件-s$ message$ address<$ attachment
完成
 press_enter 
} 
  

解决方案

使用getent并检查grep的退出代码。避免使用/ etc / passwd。相当于shell：

 > getent passwd | grep -q valid_user 
> echo $？ 
 0 
 
> getent passwd | grep -q invalid_user 
> echo $？ 
 1 
  

So I need to check if the recipient username is in /etc/passwd which contains all the users in my class, but I have tried a few different combinations of if statements and grep with no success. The best I could come up with is below, but I don't think it's working properly. My logic behind it is that if the grep is null, the user is invalid.

send_email()
{
  message=
  address=
  attachment=
  validuser=1
  until [ "$validuser" = "0" ]
    do
    echo "Enter the email address: "
    read address
    if [ -z grep $address /etc/passwd ]
      then
    validuser=0
    else
        validuser=1
    fi
    echo -n "Enter the subject of the message: "
    read message
    echo ""
    echo "Enter the file you want to attach: "
    read attachment
    mail -s "$message" "$address"<"$attachment"
    done
    press_enter 
}

解决方案

Use getent and check for grep's exit code. Avoid using /etc/passwd. Equivalent in the shell:

> getent passwd | grep -q valid_user
> echo $?
0

> getent passwd | grep -q invalid_user
> echo $?
1

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