Sorting the file leaving the first line and writing in a file

Now we can see how to sort a file by leaving the first line as it is. For example, we take a file sample.txt which contains the below data.

chrom:index:forward:reverse

chr01:13:1:2

chr13:23432:4:7

chr01:3445:1:6

chr02:2311:3:1

chr01:212:5:2

chr03:12:1:4

chr02:345:12:6

chr01:45:45:0

If we sort the above data by leaving the first line, the output would be

chrom:index:forward:reverse

chr01:13:1:2

chr01:45:45:0

chr01:212:5:2

chr01:3445:1:6

chr02:345:12:6

chr02:2311:3:1

chr03:12:1:4

chr13:23432:4:7

Now we are going to see the command which sorts the content of the file by ignoring the header line (i.e. the first line). This command sorts the first field in ascending order(String) and the second field in the ascending order(numeric)

head -1 sample.txt  

tail -n +2 sample.txt | sort -t : -k1,1 -k2,2n

1) The first command prints the first line of the file on to the screen.

2) The second command will print all the lines except the first line and then sorts the data using the first field (string sort) and then with the second field(numeric sort).

If you want the output to be written in a file, redirect the output using the re-directional operators.

head -1 sample.txt > output.txt

tail -n +2 sample.txt | sort -t : -k1,1 -k2,2n >> output.txt

The symbol > is for writing into a file. This will clear the output.txt if exists and then writes the data. If the file does not exist, it will create the file and then write the output to it.

The symbol >> will append the data to the existing data in the output.txt file.