I want to replace the 5th line of multiple text files (file1.txt,file2.txt,file3.txt,file4.txt) with the string " Good Morning " using a single terminal command.
All the text files are located on my ~/Desktop.
Note: My desktop consists of 6 .txt files.I want to apply the change to above mentioned 4 text files only.
Here are a few approaches. I am using brace expansion (file{1..4}.txt) which means file1.txt file2.txt file3.txt file4.txt
Perl
perl -i -pe 's/.*/ Good Morning / if $.==5' file{1..4}.txt
-i: causes perl to edit the files in place, changing the original file.
If -i is followed with a file extension suffix, then a backup is created for every file that is modified. Ex: -i.bak creates a file1.txt.bak if file1.txt is modified during the execution.
-p: means read the input file line by line, apply the script and print it.
-e: allows you to pass a script from the command line.
s/.*/ Good Morning /: That will replace the text in the current line (.*) with Good Morning.
$. is a special Perl variable that holds the current line number of the input file. So, s/foo/bar/ if $.==5, means replace foo with bar only on the 5th line.
sed
sed -i '5s/.*/ Good Morning /' file{1..4}.txt
-i: Like for perl, edit file in place.By default, sed prints each line of the input file. The 5s/pattern/replacement/ means substitute pattern with replacement on the 5th line.
Awk
for f in file{1..4}.txt; do
awk 'NR==5{$0=" Good Morning "}1;' "$f" > foobar && mv foobar "$f";
done
awk has no equivalent to the -i option¹ which means that we need to create a temporary file (foobar) which is then renamed to overwrite the original. The bash loop for f in file{1..4}.txt; do ... ; done simply goes through each of file{1..4}.txt, saving the current file name as $f. In awk, NR is the current line number and $0 is the content of the current line. So, the script will replace the line ($0) with " Good Morning " only on the 5th line. 1; is awk for "print the line".
¹Newer versions do as devnull showed in his answer.
coreutils
for f in file{1..4}.txt; do
(head -4 "$f"; echo " Good Morning "; tail -n +6 "$f") > foobar &&
mv foobar "$f";
done
The loop is explained in the previous section.
head -4: print the first 4 lines
echo " Good Morning ": print " Good Morning "
tail -n +6: print everything from the 6th line to the end of the file
The parentheses ( ) around those three commands allow you to capture the output of all three (so, 1st 4 lines, then " Good morning ", then the rest of the lines) and redirect them to a file.
