Replace with multiple spaces starting a string with sed

Question

I have the input in /etc/hosts:

127.0.0.1 fake.hostname.net   fake

I want to replace the second fake with real. The following sed statement attempts to filter the two apart by matching two spaces before, but it does nothing:

sed -i -e 's/  fake/  new/g' /etc/hosts

Result is:

127.0.0.1 fake.hostname.net   fake

Expected result is:

127.0.0.1 fake.hostname.net   new

I have also tried the following with the same results:

sed -i -e 's/\s\sfake/\s\snew/g' /etc/hosts

Why does this happen, and what can I do to fix it? I am running Ubuntu 14.04.1 Server. I do not wish to simply replace the second match, as I am expecting things like:

127.0.0.1 asdf.hostname.net   fake

as well, so the space matching is the only acceptable method.

echo "127.0.0.1 fake.hostname.net   fake" | sed -e 's/  fake/  new/g'

Returns the expected result, but the same statement does not write to the file. Simply not putting the extra spaces in the statement writes to the file, so it's not a filesystem permissions issue.

can you confirm that the expression works as expected if you use cat: cat /etc/hosts | sed -e 's/ fake/ new/g'? also, confirm that the whitespace is, in fact, spaces, not tabs? — Willi Ballenthin
– Willi Ballenthin, Commented Mar 29, 2015 at 19:07
@WilliBallenthin It does: echo "127.0.0.1 fake.hostname.net fake" | sed -e 's/ fake/ new/g' and yes, it's spaces, not tabs. If I don't put the spaces in the expression, the file updates, so it's not a permissions issue. Stack Overflow is nuking the double space even in code formatting. — Yunyu L.
– Yunyu L., Commented Mar 29, 2015 at 19:10

hek2mgl · Accepted Answer · 2015-03-29 19:20:44Z

1

It should be

sed -i 's/fake$/real$/' /etc/hosts

$ matches the end of the line. You don't need the g option since there is only one match per line.

answered Mar 29, 2015 at 19:20

hek2mgl

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4 Comments

Yunyu L. Over a year ago

That solved my problem, thanks. I'm still wondering about the space behavior, however.

hek2mgl Over a year ago

probably it was a tab?

Yunyu L. Over a year ago

I'm certain it was a space, even ran it through a hex dump.

Walter A Over a year ago

This will replace fake in 127.0.0.1 fake but that might not cause any problems.

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