Find string and replace line in linux

Question

I'd like to do the following, but I can't seem to find an elegant way to do it.

I have a text file that looks like this:

..more values
stuff = 23   
volume = -15
autostart = 12
more values..

Now the "volume" value may change, depending on the circumstance. I need a script that can find that line of text with "volume = xxx" then replace it with the line "volume = 0". What is an elegant solution to this? This line in the text is not guaranteed to be the first line, so I need a way to find it first.

by the time I wrote my answer and tested in a shell, three others were out. I'm getting rusty! — Zlatko
– Zlatko, Commented Sep 30, 2012 at 17:51

Lev Levitsky · Accepted Answer · 2012-09-30 17:49:59Z

13

sed 's/^volume =.*/volume = 0/g' file.txt

answered Sep 30, 2012 at 17:49

Lev Levitsky

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6 Comments

jliu83 Over a year ago

Thanks for the answer, I need to pipe this back into the file correct?

Lev Levitsky Over a year ago

@jliu83 Don't pipe it to the file directly. You can pipe it to a new file, then you can replace the original file with the new one using mv.

jliu83 Over a year ago

Thanks, and why, might I ask, should I not pipe to the same file? I'm guessing some sort of a race condition concerning reading and writing to the same file?

Lev Levitsky Over a year ago

@jliu83 Something like that. Also note that sed has -i option for modifying files "in place", but it actually creates a temporary file, too. For safety, I'd start with the command I show, and if you're happy with the output, you can add the -i option.

jliu83 Over a year ago

Also, can you explain the .* usage? I understand here it means anything that starts with volume, but why not just *, why is there a period (ie, why .* not *)?

|

Thor · Accepted Answer · 2012-09-30 17:49:40Z

2

With sed you can say:

sed '/^volume/s/.*/volume = 0/' infile

answered Sep 30, 2012 at 17:49

Thor

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2 Comments

jliu83 Over a year ago

can you explain this? usually the "s" option is used before delimiter for substitution.

Thor Over a year ago

The first /regex/ decides which line the following commands are applied to, in this case any line starting with volume.

viraptor · Accepted Answer · 2012-09-30 17:50:27Z

1

Pipe the contents into this command:

sed -e 's/^volume\s*=\s*-\?[0-9]\+$/volume = 0/'

answered Sep 30, 2012 at 17:50

viraptor

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Comments

Dan · Accepted Answer · 2012-10-01 05:05:11Z

1

sed 's/^volume=$.*$$/volume=1/g' inputfile

edited Oct 1, 2012 at 5:05

Dan

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answered Oct 1, 2012 at 4:59

vara

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Comments

itsbruce · Accepted Answer · 2012-09-30 18:50:55Z

0

@jliu83, do some reading on regular expressions. It'll make the pattern matching in the sed commands more understandable.

http://en.wikipedia.org/wiki/Regular_expression

answered Sep 30, 2012 at 18:50

itsbruce

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1 Comment

Gustavo Daniel Over a year ago

this is not an answer to the question

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