===== Apply unix commands to all but the first line =====
==== Situation ====
Let's say we want to sort a series of numbers in ascending order but keep the header at the top. For example, given
<code>
% echo -e "value\n8\n2\n6\n3"
value
8
2
6
3
</code>
we want to output
<code>
value
2
3
6
8
</code>

We can't directly use 'sort' since it will sort the header as well.
<code>
% echo -e "value\n8\n2\n6\n3" | sort
2
3
6
8
value
</code>

==== Bare bones solution ====
Create a script called 'body' with the following contents
<code>
#!/usr/bin/env bash
#
# body: apply expression to all but the first line.
# Use multiple times in case the header spans more than one line.
#
# Example usage:
# $ echo -e "value\n8\n2\n6\n3" | body sort
#
IFS= read -r header
printf '%s\n' "$header"
"$@"
</code>

Make it executable
<code>
% chmod +x body
</code>

place it somewhere in your PATH (say ~/bin)
<code>
% mv body ~/bin
</code>

This script will apply any unix command to all but the first line. For example, using it on our example

<code>
% echo -e "value\n8\n2\n6\n3" | body sort
value
2
3
6
8
</code>

==== Practical solution ====
I got the above script from https://github.com/jeroenjanssens/dsutils/blob/master/body . The underlying repository (https://github.com/jeroenjanssens/dsutils) contains many such useful scripts (ex:- header - to add, replace, and delete header lines).

A more practical approach is to clone that entire repository and add it to the shell's PATH.

I did it as follows.

Remove the bare bones script added in the previous step
<code>
% rm ~/bin/body
</code>

Clone the repository
<code>
% mkdir -p ~/github/jeroenjanssens
% cd ~/github/jeroenjanssens

% git clone git@github.com:jeroenjanssens/dsutils.git
Cloning into 'dsutils'...
remote: Enumerating objects: 62, done.
remote: Counting objects: 100% (62/62), done.
remote: Compressing objects: 100% (52/52), done.
remote: Total 62 (delta 20), reused 48 (delta 10), pack-reused 0
Receiving objects: 100% (62/62), 18.59 KiB | 9.29 MiB/s, done.
Resolving deltas: 100% (20/20), done.
</code>

Update the PATH in ~/.bashrc by adding the following lines
<code>
#------------------------------------------------------------------------------
# Add data science utils such as body, header
export PATH=~/github/jeroenjanssens/dsutils:$PATH
#------------------------------------------------------------------------------
</code>

Open a new bash session and verify that the scripts are picked up from the correct location.
<code>
% which body
/home/rajulocal/github/jeroenjanssens/dsutils/body

% which header
/home/rajulocal/github/jeroenjanssens/dsutils/header
</code>

Verify that the scripts are working as expected.
<code>
% echo -e "value\n8\n2\n6\n3" | body sort
value
2
3
6
8
</code>

==== Works with any command ====
The beauty of this approach is that you can use it with any unix command (and not just sort). For example, you can grep a value and it will show the header along with the value.
<code>
% echo -e "value\n8\n2\n6\n3" | body grep 8
value
8
</code>

==== How I came across it ====
The 'body' and 'header' commands are discussed in the book "Data Science at the Command Line" (2nd Edition) by Jeroen Janssens (https://smile.amazon.com/Data-Science-Command-Line-Explore/dp/1492087912). It is available for free at https://datascienceatthecommandline.com/2e/ . See for example https://datascienceatthecommandline.com/2e/chapter-5-scrubbing-data.html#bodies-and-headers-and-columns-oh-my .

==== Close but no cigar ====
Here I will try to list some alternative approaches that are close but not perfect.

Solution 1:
<code>
$ cat input.txt
value
8
2
6
3

$ (head -n 1 input.txt; tail -n +2 input.txt | sort)
value
2
3
6
8
</code>

Disadvantages:
  * Requires two passes on the input (once to extract the header and another to process the body)
  * Does not work if the input is coming from stdin