sed property substitution

Question

I've got a property file where I want to do a property substitution, so I wrote a sed patter to change

host = 1234

with another value, but when I execute

echo "host = 1234" | sed 's/\#*\(host[ \t]*\)\=\([ \t]\d*\)/\1\=\1/g'

I got that the substitution is done (host =host) but the \2 atom is also appended to the end of the string (1234). How can I remove it?

 `host =host 1234

It's doing exactly what you told it to do. What are you trying to make it do? Please give sample input line and desired output line. — sorpigal
– sorpigal, Commented Feb 27, 2012 at 12:33
I'd like to change the host = <value> with an host = <newvalue> that for instance can be \1 or a computed one. In the above example the newvalue becomes \1 and the remaining of the line — fluca1978
– fluca1978, Commented Feb 27, 2012 at 12:33

sorpigal · Accepted Answer · 2012-02-27 12:48:15Z

The first problem is that \d doesn't do what you think. Use [0-9] at least.

You still get host =host out, which seems crazy to me.

EDIT:

Okay

echo "host = 1234" | sed 's/#*\host[ \t]*=[ \t]*\([0-9]*\)/host = asdf/g'

Why capture 'host' if it's always the same? Just rewrite it.
Why preserve the exact tab/space information? Just rewrite it.
Why escape things which are not special?

I hope you get the idea.

But here's what you probably want:

sed '/^#/!s/[ \t]*\([^ \t]*\)[ \t]*=[ \t]*\([^ \t]*\)/\1 = newvalue/g' input_file

This will change anything = anything to anything = newvalue in non-commented lines of input_file. To make it a specific key which is replaced by newvalue, use:

sed '/^#/!s/[ \t]*\(host\)[ \t]*=[ \t]*\([^ \t]*\)/\1 = newvalue/g' input_file

to e.g. replace only lines reading host = anything.

@fluca1978: If this answers your question and solves your problem please accept the answer.

synthesizerpatel · Accepted Answer · 2012-02-27 12:37:24Z

Does this suit your needs?

echo "host = 1234" | cut -d"=" -f 1

yields

host

Then,

echo "host = 1234" | cut -d"=" -f 1

yields

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