How to change the first occurrence of a line containing a pattern? - sed

Question

I need to find the line with first occurrence of a pattern, then I need to replace the whole line with a completely new one.
I found this command that replaces the first occurrence of a pattern, but not the whole line:
sed -e "0,/something/ s//other-thing/" <in.txt >out.txt
If in.txt is
one two three
four something
five six
something seven
As a result I get in out.txt:
one two three
four other-thing
five six
something seven
However, when I try to modify this code to replace the whole line, as follows:
sed -e "0,/something/ c\COMPLETE NEW LINE" <in.txt >out.txt
This is what I get in out.txt:
COMPLETE NEW LINE
five six
something seven
Do you have any idea why the first line is lost?

Answer 1

The c\ command deletes all lines between and inclusive the first matching address through the second matching address, when used with 2 addresses, and prints out the text specified following the c\ upon matching the second address. If there is no line matching the second address in the input, it just deletes all lines (inclusively) between the first matching address through the last line. Since you want to replace one line only, you shouldn't use the c\ command on an address range. The c\ is immediately followed by a new-line character in normal usage.
The 0,/regexp/ address range is a GNU sed extension, which will try to match regexp in the first input line too, which is different from 1,/regexp/ in that aspect. So, the correct command in GNU sed could be
sed '0,/something/{/something/c\
COMPLETE NEW LINE
}' < in.txt
or simplified as pointed out by Sundeep
sed '0,/something/{//c\
COMPLETE NEW LINE
}' < in.txt
or a one-liner,
sed -e '0,/something/{//cCOMPLETE NEW LINE' -e '}' < in.txt
if a literal new-line character is not desirable.
This one-liner also works as pointed out by potong:
sed '0,/something/!b;//cCOMPLETE NEW LINE' in.txt

Answer 2

This might work for you (GNU sed):
sed '1!b;:a;/something/!{n;ba};cCOMPLETE NEW LINE' file
Set up a loop that will only operate from the first line.
Within in the loop, if the key word is not found in the current line, print the current line, fetch the next and repeat until the end of the file or a match is found.
When a match is found, change the contents of the current line to the required result.
N.B. The c command terminates any further processing of sed commands in the same way the d command does.
If there are lines in the input following the key word match, the negation of address at the start of the sed cycle will capture these lines and result in their printing and no further processing.
An alternative:
sed 'x;/./{x;b};x;/something/h;//cCOMPLETE NEW LINE' file
Or (specific to GNU and bash):
sed $'0,/something/{//cCOMPLETE NEW LINE\n}' file

Answer 3

Just use awk:
$ awk '!done && sub(/something/,"other-thing"){done=1} {print}' file
one two three
four other-thing
five six
something seven
$ awk '!done && sub(/.*something.*/,"other-thing"){done=1} {print}' file
one two three
other-thing
five six
something seven
$ awk '!done && /something/{$0="other-thing"; done=1} {print}' file
one two three
other-thing
five six
something seven
and look what you can trivially do if you want to replace the Nth occurrence of something:
$ awk -v n=1 '/something/ && (++cnt == n){$0="other-thing"} {print}' file
one two three
other-thing
five six
something seven
$ awk -v n=2 '/something/ && (++cnt == n){$0="other-thing"} {print}' file
one two three
four something
five six
other-thing