如何替换文件中的值

Question

I am writing a script to alter the few values in file sysctl.conf in /etc directory

If variable value is less than that then it should alter the value to below given value

For example:

sysctl.conf file contains the following lines (the terms in parentheses are my own comments and are not in the actual file):

kernel.shmall = 5194304 (more than 4194304 so it should leave it as it is)
kernel.shmmax = 2147483648 (it is equal to 2147483648 so it should leave it as it is)
kernel.msgmni = 124 (it is less than 1024 so it should alter the value to 1024)

expecting output is

kernel.shmall = 5194304 
kernel.shmmax = 2147483648
kernel.msgmni = 1024

This is my script I am preparing to replace kernel.shmall if value is less than or equal to 4194303

script:

#!/bin/sh
echo `cat /etc/sysctl.conf|fgrep kernel.shmall` | while read kernel.shmall
if [ "kernel.shmall" -le 4194303 ]; then
    kernel.shmall = 4194304
fi`

Answer 1

You can do something like this

 sed -i "s|\("kernel.shmall" *= *\).*|\14194304|" /etc/sysctl.conf

And same you can use for other properties.

Also if property not exist then you can do like this

if grep -o "kernel.shmall" /etc/sysctl.conf > /dev/null
then
     oldvalue=$(grep kernel.shmall /etc/sysctl.conf | awk '{ print $3 }')

     if [ $oldvalue -lt 4194304 ]
     then
        sed -i "s|\("kernel.shmall" *= *\).*|\14194304|" /etc/sysctl.conf
     fi
else
     echo "kernel.shmall=" >> /etc/sysctl.conf
     sed -i "s|\("kernel.shmall" *= *\).*|\14194304|" /etc/sysctl.conf
fi

Answer 2

Perhaps a clearer way of getting the values from your file is:

value=$(grep kernel.shmall /etc/sysctl.conf | awk '{ print $3 }')

Then, to replace the value in the file:

sed -i "/kernel.shmall =/ s/$value/4194304/" /etc/sysctl.conf