awk matching incorrect hostname in /etc/hosts

问题

I am using this awk code to pick out an IP address in the /etc/hosts file and although I can force it to match the correct hostname, awk will select an "almost like" hostname if it see's it first. I have researched this and I cannot figure it out.

sUSER=machine_005
ip=$(awk '/^[[:space:]]*($|#)/{next}/'$sUSER'/{print $1; exit}' /etc/hosts)

I know the first part looks for $ or # and if found calls the next line. This feature is needed in the final formula. The other feature needed is to stop after the first match. I know that the next /xxx/ searches for the pattern and if found prints $1 to $ip.

Here is a sample of my hosts file

555.555.555.555 machine.005
222.222.222.222 machine_005

if I $sUSER=machine_005 $ip is 222.222.222.222 just like you'd expect.

if I $sUSER=machine.005 $ip is 555.555.555.555 just like you'd expect

But if my /etc/hosts is:

222.222.222.222 machine_005
555.555.555.555 machine.005

Then if I $sUSER=machine_005 $ip is 222.222.222.222 just like you'd expect.

However, if I $sUSER=machine.005 $ip is 222.222.222.222 It takes machine_005 ip address.

Could this be a bug? or is this on purpose?

Thanks.

Hello. I wanted to post my final line of code that provides the correct output. I appreciate all of the help and guidance from those who responded. I wanted to post the final code in case someone in the future can find it useful.

 ip=$(awk -v sUSER=$sUSER 'BEGIN{gsub(/\./,"\\.",sUSER)}match($0,/[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/) && $0 ~ ("[^[:alpha:]]"sUSER"$") && $0 !~ /^$/ && $0 !~ /^#/{print $1}' /etc/hosts)

回答1:

In a regular expression, . matches any character. You need to escape it to make it match literally.

sUser='machine\.005'

If you're getting the value from user input, you can use the shell's substitution operator:

sUser=${sUser//./\\.}

Also see How do I use shell variables in an awk script? for a better way to incorporate the shell variable into the awk script.

回答2:

EDIT2: as per OP need to remove lines start from # and empty lines I believe then try following.

sUser="machine.005"
ip=$(awk -v sUSER="$sUSER" 'BEGIN{sub(/\./,"\\.",sUser)} match($0,/[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/) && $0 ~ sUSER  && $0 !~ /^$/ && $0 !~ /^#/{print $1}' /etc/hosts)

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EDIT: Or to escape DOT inside awk code itself try following.

sUser="machine.005"
awk -v sUser="$sUser" 'BEGIN{sub(/\./,"\\.",sUser)} match($0,/[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/) && $0 ~ sUser{print $2}'  Input_file

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Since DOT is considered as any character so that is why it is catching everything between machine and 005|6, so we need to escape it to tell program that it is an actual DOT and shouldn't be considered as special meaning.

Could you please try following.

sUser="machine\\\.005"
awk -v u="$sUser" 'match($0,/[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/) && $0 ~ u{print $2}' Input_file

Output will be as follows.

machine.005

回答3:

getent is a tool meant to retrieve entries from /etc/hosts (and other config databases):

getent hosts "${USER}" | cut -d' ' -f1

来源：https://stackoverflow.com/questions/53353494/awk-matching-incorrect-hostname-in-etc-hosts