How to insert text into a conf file?

In nginx.conf I have:

server {
  listen 81;
  ...snip...
}  

I want the file to look like this:

server {
  listen 81;
  if (-f $document_root/system/maintenance.html) {
    return 503;
  }
  ...snip...
}  

So the text (if..) should only be inserted if it does not already exist.

Is this possible to do using some in-built program at the command line?

Something like:

• Open a file
• See if it has certain text in it
• If it does not then locate certain text (listen 81;) and on the next line a开发者_C百科fter it insert a few lines of text

This is a job for sed, wrapped up in a shell script. Completely untested; invoke as "./script /path/to/nginx.conf"; note that it does not look for the complete text you wanted to add, only a hopefully-sufficiently-long token from its first line.

#! /bin/sh

if grep -q "document_root/system/maintenance" "$1"
then :
else
     set -e
     sed < "$1" > "$1.tmp" \
     '/listen 81;/a\
  if (-f $document_root/system/maintenance.html) {\
    return 503;\
  }
  '
     mv -f "$1.tmp" "$1"
fi