sed command to replace last occurrence of a word in a file with the content of another file

Question

1. macOS sed command to replace last occurrence of a word in a file
2. Replace with the content from another file
3. Replace only the last occurrence, means only once.
4. The word can be a substring abchello or helloabc
5. There can be trailing whitespace or newline after the word

sample_file_path = "/Users/saurav/sample.text" sample_file_path_1 = "/Users/saurav/sample1.text"

sample.txt:

hello
hi
hello
ok

sample1.txt:

I
am 
doing
great

Expected output (sample.txt):

hello
hi
I
am 
doing
great
ok

Need to use file path variable

Answer 1

In three steps, using sed syntax compatible with /usr/bin/sed on macOS, and either bash or zsh (the two main shells on current macOS systems):

sed -n '/hello/=' sample.txt |
sed -e '$!d' -e $'s/.*/&r sample1.txt\\\n&d/' |
sed -f /dev/stdin sample.txt

This uses sed in three steps:

1. Finds all lines in sample.txt that matches hello and output the line numbers corresponding to those lines.

2. Deletes all but the last line number outputted by the first step (using $!d, "if this is not the last line, delete it"), and creates a two-line sed script that would modify the last matching line by first reading sample1.txt and then deleting the original line. Given that the last match of hello is on line 3 in the original file, this script would look like

   3r sample1.txt
   3d
   

3. Applies the constructed sed script on the file sample.txt.

Would you want to make the edit "in-place", so that the original sample.txt is modified, then use

sed -n '/hello/=' sample.txt |
sed -e '$!d' -e $'s/.*/&r sample1.txt\\\n&d/' |
sed -i '' -f /dev/stdin sample.txt

---

The same set of commands, but using your variables $sample_file_path and $sample_file_path_1 for the two file paths:

sed -n '/hello/=' "$sample_file_path" |
sed -e '$!d' -e 's,.*,&r '"$sample_file_path_1"$'\\\n&d,' |
sed -i '' -f /dev/stdin "$sample_file_path"

Note that I have changed the delimiters in the second command from / to , as the file path contains slashes. You may use any character as a delimiter in the s/// command that is not otherwise part of the regular expression or replacement text.

Answer 2

Try tac and awk combination:

tac sample.txt \
| awk -v replacement="$(tac sample1.txt)" '/hello/ && !n++ {gsub(/hello/,replacement,$0)}1' \
| tac

Note, this will also replace the hello part from nothello. If you want to replace only independent hello, you need to add word boundaries to the pattern: /\<hello\>/ instead of /hello/

Answer 3

With a single invocation + using the Posix-ly constructs of sed we can do as shown. The USP of this approach is that the sed code itself generates the file which it shall read later into the printing stream.

sed -n '
  /\n/{
    s/.$//w file.tmp
    b a
  }
  /hello/!{
    H;1h;$!d
    s/.*//;x
    //!{p;q;}
    G;D
  }
  x;1!p;$!d;:a
  r file2
  r file.tmp
' file1

Brief explanation of how it works:

• Accumulate hello line and the subsequent non hello lines in hold space.
• Now when we read the next line and it happens to be hello, then we know that the hello line in space cannot be the last, so we flush out the hold area.
• This is the next cycle of events wherein we store the hello line at the top and subsequent non hellos underneath it.
• The cycle (flush hold;store /hello/+!/hello/+!/hello/...) keeps going on until we hit the EOF.
• Now at eof, one of the 3 possible scenarios can arise:
  • There never was a /hello/ line in the whole file, so the hold ballooned to contain the total lines and we flush it via the //!{p;q;} construct. At this stage, we already are at eof and we tested for the regex /hello/ , so we can reuse it as //.
  • Second scenario is hold had /hello/+atleast one non hello. What we need to do here is clip the top of the hold , the /hello/ line, and replace it with the contents of file2 since this is the LAST /hello/ line. But the fly in the ointment is we want the other portion (the non hello lines). If we just did r file2 this would place the contents at the end. So we have to come up with something to overcome it. What we do is , store these non hello lines in a file and then do a r tmpfile . Pbm solved.
  • Third possibility is the last line was a hello line. Here the tmpfile would not exist and we exploit the property of read command to not complain about non existent files.

Answer 4

We can use the sed stream editor utility to get the desired output.

sed -n '
  /hello/!{
    H;1h;$!d;g
    s///w file.tmp
    t a
    p;q
  }
  x;1!p;$!d
  :a
  r file2
' file1
sed -e '1d' file.tmp

• accumulates the lines in hold until it sees the hello line
• on a hello line, it prints what was saved in hold and starts the cycle again.
• only at EOF or the last line we print the hold and pattern spaces.

---

Or, another way is the tac-perl-tac pipeline:

tac file1 |
perl -0777 -pe 's?hello\n?qx/tac file2/?e' - | 
tac