How to append or prepend text in specific columns with awk

Assume the following scenario, you have a csv file with the following content and you need to do append / prepend text in specific columns, doing this with awk is very easy as you will see! lets start

How to append text

Assume that we have the following file as example.csv and we want to append text “test” to all values of second column except the headers

name,age,gender 
kostas,43,male 
Nikos,42,male 
Maria,35,female

To do this enter the following one-liner

awk 'BEGIN{OFS=FS=","} NR==1 {print} NR>1 {$2=$2 "_test"; print}' example.csv 
name,age,gender 
kostas,43_test,male 
Nikos,42_test,male 
Maria,35_test,female

Lets break down how it works

• BEGIN{OFS=FS=","} we define comma as delimiter between columns
• NR==1 {print} if number of record is the first one just print it
• NR>1 {$2=$2 "_test"; print} if number of record is the the header, the first one then, for each second value of the row append text “_test”, this translates to set column two to column two + “_test”

How to prepend text

Now lets see another example, how to prepend text to a column, to do this enter the following

awk 'BEGIN{OFS=FS=","} NR==1 {print} NR>1 {$1="test_"$1; print}' example.csv 
name,age,gender 
test_kostas,43,male 
test_Nikos,42,male 
test_Maria,35,female

It isn’t much different than the previous example except that now we do this $1="test_"$1 which translates to set column one as “test_” + column one

How to append and prepend at the same time

Now lets see how we can append and prepend at the same time, enter the following

awk 'BEGIN{OFS=FS=","} NR==1 {print} NR>1 {$1="test_"$1"_test"; print}' example.csv 
name,age,gender 
test_kostas_test,43,male 
test_Nikos_test,42,male 
test_Maria_test,35,female

As you can see is quite easy, $1="test_"$1"_test" which translates to set column1 to “test_” + column one + “_test”.

How to perform operations in multiple columns

Now lets see something more advanced, how we can append / prepend text to multiple columns, lets prepend and append “test” to first and last column

awk 'BEGIN{OFS=FS=","} NR==1 {print} NR>1 {$1="test_"$1;$3=$3"_test";print}' example.csv 
name,age,gender 
test_kostas,43,male_test 
test_Nikos,42,male_test 
test_Maria,35,female_test

$1="test_"$1 prepends text to first column

$2=$2"_test" appends text to last column

How to append / prepend text to last column with a file that has an unknown number of columns

What if the file we work has an unknown number of columns or the number of columns can vary per row and still we want to append / prepend text to the last column? quite easy, assume that we have the following csv

name,age,gender,has_only_fans 
kostas,43,male 
Nikos,42,male 
Maria,35,female 
ilias,44,male,yes

Note that the column named has_only_fans has values only in the last row but we still want to append text to the last column of each row despite the un-equal nunber of columns, we can $NF refers always to last column of the row

awk 'BEGIN{OFS=FS=","} NR==1 {print} NR>1 {$NF=$NF"_test";print}' example.csv 
name,age,gender,has_only_fans 
kostas,43,male_test 
Nikos,42,male_test 
Maria,35,female_test 
ilias,44,male,yes_test

Conclusion

Knowing to work with awk can make you life much easier! of course you can do everything with Python, but still awk exists by default in every system and its one-liners are super easy to learn!