sed inplace replacement replaces itself

Question

I have a single file shell script (BusyBox, no bash, ksh I believe) that I want to write a path to itself when run. I need #home to be replaced by home=/home. I have this:

var="/home"
#home
sed -i 's/#home/home=$var/g' /this/file.sh

When run it'll replace #home and the string in the sed line and the $var is not expanded:

var="/home"
home=$var
sed -i 's/home=$var/home=$var/g' /this/file.sh

This is my desired outcome:

var="/home"
home=/home
sed -i 's/#home/home=$var/g' /this/file.sh

I'm pulling my hair getting this to work. Any hints?

Answering myself this works as expected:

var="/home"
#home
sed -i "s|^#home|home=${var}/|g" m.sh

see [sed substituting with shell variables](http://stackoverflow.com/documentation/sed/1096/substitution/3519/substitution-using-shell-variables#t=201608300833232824835) for further reading — Sundeep, Aug 30 '16 at 08:34

Harald · Accepted Answer · 2016-08-30T09:33:50.677

2

While there may be several ways to address this question. Try adding the circumflex (^) at the beginning of the pattern to indicate that this pattern should be found at the beginning of the line.

Before running the script (here it is named m.sh)

#!/bin/bash

var="/home"
#home
sed -i 's/^#home/home=$var/g' m.sh

After running the script

#!/bin/bash

var="/home"
home=$var
sed -i 's/^#home/home=$var/g' m.sh

EDIT You may also want to consider removing the g modifier in sed since you only want to match once in the line.

edited Aug 30 '16 at 09:33

answered Aug 30 '16 at 09:10

Harald

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Oops! Looks like user pdg commented on this while I was typing the answer. – Harald Aug 30 '16 at 09:11
Works, but I need home=$var to be expanded to home=/home – stargazer Aug 30 '16 at 09:19
Using pipes as a separator and putting var into braces works. Thanks for the help. – stargazer Aug 30 '16 at 09:27
But since var is already set and home points to var, isn't it unnecessary? – Harald Aug 30 '16 at 09:32
$var is not a set variable, it is only used as an example here. It's the outcome of some querying code. – stargazer Aug 30 '16 at 09:39
Thanks for the g modifier suggestion. – stargazer Aug 30 '16 at 09:55

sed inplace replacement replaces itself

1 Answers1