Using variables in sed command

Question

I am trying to replace a particular string in a file using the sed command. However, it is not working. Structure of data in my file:

"SomeServerName": {
    "serverIP": "192.168.0.132",
    "serverPort": "9999",
    "active": false
}

In this, I want to change active status to true from false on the basis of IP which is a variable input I get from the terminal.

The sed command I have used is :

sudo sed -i "s/$someIP/!b;n;n;c\ \ \ \ \ \ \ \ "active": true/" filename

Here, $someIP=192.168.0.132. In sed, if I replace $someIP with '192.168.0.132' it works fine. However, if I pass the variable, it does not work.

EDIT: Below is my whole script for the above sed:

#! /bin/sh

someIP=$(hostname -I)
sudo sed -i "s/$someIP/!b;n;n;c\ \ \ \ \ \ \ \ "active": true/" filename

are you executing the command from script or commandline? where is someIP set? i am having trouble reproducing this behavior. — Sam
– Sam, Commented Sep 7, 2018 at 17:06
I am executing this from a script. I get the someIP by running the command hostname -I and then assigning it to the variable someIP — SKTripathy
– SKTripathy, Commented Sep 7, 2018 at 17:08
I am sorry. I have edited the sed command to exactly my requirement. — SKTripathy
– SKTripathy, Commented Sep 7, 2018 at 17:13
might want to try putting your $someIP into ${someIP} but im not postive on that — moore1emu
– moore1emu, Commented Sep 7, 2018 at 17:14

Sam · Accepted Answer · 2018-09-07 20:11:50Z

1

how about

sed -zE "s/($someIP[^\n]*\n[^\n]*\n[^\n]*\"active\": )false/\1true/g" filename

you can not literally "pass" a variable to sed. bash replaces $someIP in the argument with the string it refers to before it starts sed, i.e. it's a pass by value.

beware that $someIP does not just match 192.168.0.132 though, but also 192:168:0:132 or 1924168903132 because to sed '.' means "any character", if you don't escape it.

this command will work as long as the matched IP and the "active" setting have exactly two lines and otherwise unspecified content between them.

edited Sep 7, 2018 at 20:11

answered Sep 7, 2018 at 17:42

Sam

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4 Comments

Benjamin W. Over a year ago

This treats the whole input as a single line and will change false to true everywhere. I assume the real input has more than one JSON object in it.

Sam Over a year ago

with the g flag it should, as long as every entry has this same basic structure. if things are more complex, i will agree with you that sed is simply the wrong tool.

Benjamin W. Over a year ago

The point is that since the whole file is treated as a single line, your command will turn it to true everywhere, but you'd want it just for the entry right after the IP address match.

Sam Over a year ago

it makes one replacement for each match found. but you are right, there is one major concern. if active was already set for one IP block the next block with active set to false would be affected. will fix.

iamauser · Accepted Answer · 2018-09-07 21:21:47Z

1

Here is a sed solution. Note, you need to use double-quotes "" for sed and escape any other double quotes in the search or replace strings.

 $ input_ip="192.168.0.132"
 $ sed -i "/$input_ip/{n;n;s/\"active\": false/\"active\": true/}" inputFile

This assumes, active: false string appears two lines after the $input_ip. n;n; skips two lines at the end of $input_ip.

answered Sep 7, 2018 at 21:21

iamauser

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