如何检查“grep"是否没有任何输出? [英] How can I check if 'grep' doesn't have any output?

问题描述

我需要检查接收者用户名是否在包含我班级所有用户的文件 /etc/passwd 中，但我尝试了几种不同的 if 组合> 语句和 grep 没有成功.我能想到的最好的方法如下，但我认为它不能正常工作.

I need to check if the recipient username is in file /etc/passwd which contains all the users in my class, but I have tried a few different combinations of if statements and grep without success. The best I could come up with is below, but I don't think it's working properly.

我的逻辑是，如果 grep 为空，则用户无效.

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
}

推荐答案

如果像这样做一个简单的:

Just do a simple if like this:

if grep -q $address  /etc/passwd
then 
   echo "OK";
else
   echo "NOT OK";
fi

这里使用 -q 选项只是为了让 grep 安静(不输出...)

The -q option is used here just to make grep quiet (don't output...)

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