[SOLVED] sed: how to replace only the first instance of a line

RandomTroll · 11-15-2015, 11:49 AM

I want to use sed to replace the first instance of a line but not subsequent instances. My Linux books tell me that:

Code:

sed 's/apple/banana/1'

should do it. It replaces all instances instead. 'info sed' tells me that the number flag for the s command isn't specified in Posix, that GNU matches every instance beginning with the numberth. So I tried:

Code:

sed 's/apple/banana/1' file | sed 's/banana/apple/2'

but the latter instruction didn't change anything sed 's/apple/banana/2' changed no lines.

What's the right way?

The file contains 123 instances of apple.

grail · 11-15-2015, 12:25 PM

The number where you have it currently will at the line it is on and only replace the n'th item, so if you have a line:

Code:

apple pear apple

and use your first example you would get:

Code:

$ sed 's/apple/banana/1' fruit_file
banana pear apple
$ sed 's/apple/banana/2' fruit_file
apple pear banana

So the numbers relate to position on the line.

You want your script to stop after it makes the necessary change, so I would use:

Code:

sed '/apple/{s/apple/banana/1;q}' file

One of the gurus may have a way to shorten that, but it should work till then

RandomTroll · 11-15-2015, 01:07 PM

Quote:

Originally Posted by grail

the numbers relate to position on the line.

Used sed for 20 years and didn't know this - never needed to. Thanks.

MadeInGermany · 11-15-2015, 01:32 PM

The q skips the remainder of the file, doesn't it.
The following prints the remainder

Code:

sed '1,/apple/ s/apple/banana/' file

syg00 · 11-15-2015, 07:40 PM

The quit is (very) useful on large files - especially if the target is near the beginning of the file. Update in-place (-i) avoids having to read the entire file.
Sometimes applicable, sometimes not.

MadeInGermany · 11-16-2015, 03:22 AM

? The q is efficient because it skips the remainder. But the OP says there are 123 instances, so I assume the following 122 should be kept.
? The -i option leads to short shell code. But still the entire file is to be read (unless a q is met).

syg00 · 11-16-2015, 04:07 AM

Sorry - I meant -i as well as quit. Very handy.