mjhjmtu

$cat contents.txt

cat-1.15

cat-1.15

cat-1.15
cat-1.18

The above output has blank lines

$cat contents.txt | grep cat
results in the word cat being highlighted, but the resultant text is also merged, eliminating blank lines

cat-1.15
cat-1.15
cat-1.15
cat-1.18

How can I grep to highlight without grep affecting the text structure, so that the only difference is the grep term being highlighted ?

With GNU grep this can be accomplished with the -z option.

-z, --null-data

Treat input and output data as sequences of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline. Like the -Z or --null
option, this option can be used with commands like sort -z to process arbitrary file names.

Also this is a UUOC. You can specify an input file with grep.

$ grep --color cat contents.txt
cat-1.15
cat-1.15
cat-1.15
cat-1.18
$ grep --color -z cat contents.txt
cat-1.15

cat-1.15

cat-1.15
cat-1.18

Another possible solution is to provide two patterns to grep: one will be the actual pattern or term to be searched for, and the second one will be an empty string.

$ grep --color -e 'cat' -e '' testfile.txt
cat-1.15

cat-1.15

cat-1.15
cat-1.18

The -e option is used for specify multiple patterns. From the manual:

-e PATTERN, --regexp=PATTERN

Use PATTERN as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a hyphen (-). (-e is specified by POSIX.)

You can also combine those patterns as a single extended regular expression, if required:

$ grep --color -E 'cat|' testfile.txt
cat-1.15

cat-1.15

cat-1.15
cat-1.18

Also, you can simply add another pattern to the list if you need to highlight more than one keyword.

A couple more possibilities with grep:

grep for 0 or more instances of cat:

grep --color '(cat)*' contents.txt

grep for cat or the empty string:

grep -E --color 'cat|' contents.txt

(The -E specifies extended regex syntax. egrep may be used instead of grep -E here.)

Alternatively you can use sed to do the colourization manually using ANSI escape codes:

red='c[[1;31m'
default='c[[0m'
sed "s/cat/$redcat$default/g" contents.txt

Here the red and default shell variables are conveniences only - their values could just as well be placed inline in the sed expression.

I tried with below command

sed -r "/cat/N;s/^$/&=/" filename


awk '' filename