Reverse file using tac and sed - linux

Question

I have a usecase where I need to search and replace the last occurrence of a string in a file and write the changes back to the file. The case below is a simplified version of that usecase:
I'm attempting to reverse the file, make some changes reverse it back again and write to the file. I've tried the following snippet for this:
tac test | sed s/a/b/ | sed -i '1!G;h;$!d' test
test is a text file with contents:
a
1
2
3
4
5
I was expecting this command to make no changes to the order of the file, but it has actually reversed the contents to:
5
4
3
2
1
b
How can i make the substitution as well as retain the order of the file?

Answer 1

You can tac your file, apply substitution on first occurrence of desired pattern, tac again and tee result to a temporary file before you rename it with the original name:
tac file | sed '0,/a/{s//b/}' | tac > tmp && mv tmp file

Answer 2

Another way is to user grep to get the number of the last line that contains the text you want to change, then use sed to change that line:
$ linno=$( grep -n 'abc' <file> | tail -1 | cut -d: -f1 )
$ sed -i "${linno}s/abc/def/" <file>

Answer 3

Try to cat test | rev | sed -i '1!G;h;$!d' | rev
Or you can use only sed coomand:
For example you want to replace ABC on DEF:
You need to add 'g' to the end of your sed:
sed -e 's/\(.*\)ABC/\1DEF/g'
This tells sed to replace every occurrence of your regex ("globally") instead of only the first occurrence.
You should also add a $, if you want to ensure that it is replacing the last occurrence of ABC on the line:
sed -e 's/\(.*\)ABC$/\1DEF/g'
EDIT
Or simply add another | tac to your command:
tac test | sed s/a/b/ | sed -i '1!G;h;$!d' | tac

Answer 4

Here is a way to do this in a single command using awk.
First input file:
cat file
a
1
2
3
4
a
5
Now this awk command:
awk '{a[i++]=$0} END{p=i; while(i--) if (sub(/a/, "b", a[i])) break;
for(i=0; i<p; i++) print a[i]}' file
a
1
2
3
4
b
5
To save output back into original file use:
awk '{a[i++]=$0} END{p=i; while(i--) if (sub(/a/, "b", a[i])) break;
for(i=0; i<p; i++) print a[i]}' file >> $$.tmp && mv $$.tmp f

Answer 5

Another in awk. First a test file:
$ cat file
a
1
a
2
a
and solution:
$ awk '
$0=="a" && NR>1 { # when we meet "a"
print b; b="" # output and clear buffer b
}
{
b=b (b==""?"":ORS) $0 # gether the buffer
}
END { # in the end
sub(/^a/,"b",b) # replace the leading "a" in buffer b with "b"
print b # output buffer
}' file
a
1
a
2
b
Writing back the happens by redirecting the output to a temp file which replaces the original file (awk ... file > tmp && mv tmp file) or if you are using GNU awk v. 4.1.0+ you can use inplace edit (awk -i inplace ...).