使用SED命令添加文件之前，请检查文件中是否存在换行

Question

I have one small doubt regarding sed command. For example, I have a file like below.

iptable.save

-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

So next time before adding same line to file , here need a check that whether the new line which we are going to enter is present or not in the file. If present, don't do anything, else enter the new line to the file. Thanks In Advance.

Answer 1

I would rather use grep for this purpose:

if [ $(grep -cx "$line" iptables.save) -eq 0 ]; then
    echo "$line" >> iptables.save  # Append to the file
fi

-xc means that we want to count the number of lines that are exactly the same as $line .

But if you really prefer to use sed :

if [ $(sed -n '/^'"$line"'$/p' iptables.save | wc -l) -eq 0 ]; then
    echo "$line" >> iptables.save  # Append to the file
fi

Answer 2

如果我理解正确，这是awk的工作：

awk -v line="$line" '$0==line{f=1} END{if (!f) print line}' file >> file

Answer 3

This may also help. Example, checking the file /etc/ssh/sshd_config, if it does not end in an empty line, add one, if it does end with an empty line, do nothing

if [[ $(tail -n 1 /etc/ssh/sshd_config) != "" ]]; then echo "" >> /etc/ssh/sshd_config; fi