how to replace value in a file

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`

Jayesh Bhoi · Accepted Answer · 2014-04-22 12:37:41Z

2

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

edited Apr 22, 2014 at 12:37

answered Apr 16, 2014 at 12:55

Jayesh Bhoi

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8 Comments

Jayesh Bhoi Over a year ago

@user3541225 i give you only example here forkernel.shmall but you can do use same command for other properties with respective values.

user3541225 Over a year ago

out put of grep kernel.shmall /emblocal/sysctl.conf2| awk '{ print $1}' kernel.shmall=4194304 I am trying to delimit =

Jayesh Bhoi Over a year ago

@user3541225 It seems correct...have you getting expected result?

user3541225 Over a year ago

achived it by following grep -v '^#' /emblocal/sysctl.conf|grep kernel.shmall|sed 's/=/ /g'| awk '{ print $2}'

user3541225 Over a year ago

if grep -o "kernel.shmall" /etc/sysctl.conf > /dev/null then oldvalue=$(grep -v '^#' /etc/sysctl.conf|grep kernel.shmall|sed 's/=/ /g'| awk '{ print $2}') 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 is throwing error [: -lt: unary operator expected because as there is no variable "kernel.shmall" in file. How to add it as above script is not adding when there is no variable

|

tkocmathla · Accepted Answer · 2014-04-16 13:13:25Z

1

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

answered Apr 16, 2014 at 13:13

tkocmathla

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