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apply_unix_commands_to_all_but_the_first_line

## 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

% echo -e "value\n8\n2\n6\n3"
value
8
2
6
3

we want to output

value
2
3
6
8

We can't directly use 'sort' since it will sort the header as well.

% echo -e "value\n8\n2\n6\n3" | sort
2
3
6
8
value

### Bare bones solution

Create a script called 'body' with the following contents

#!/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"
"$@" Make it executable % chmod +x body place it somewhere in your PATH (say ~/bin) % mv body ~/bin This script will apply any unix command to all but the first line. For example, using it on our example % echo -e "value\n8\n2\n6\n3" | body sort value 2 3 6 8 ### Practical solution I got the above script from https://github.com/jeroenjanssens/dsutils/blob/master/body . The underlying repository (https://github.com/jeroenjanssens/dsutils) contains many such useful scripts (ex:- header - to add, replace, and delete header lines). A more practical approach is to clone that entire repository and add it to the shell's PATH. I did it as follows. Remove the bare bones script added in the previous step % rm ~/bin/body Clone the repository % mkdir -p ~/github/jeroenjanssens % cd ~/github/jeroenjanssens % git clone git@github.com:jeroenjanssens/dsutils.git Cloning into 'dsutils'... remote: Enumerating objects: 62, done. remote: Counting objects: 100% (62/62), done. remote: Compressing objects: 100% (52/52), done. remote: Total 62 (delta 20), reused 48 (delta 10), pack-reused 0 Receiving objects: 100% (62/62), 18.59 KiB | 9.29 MiB/s, done. Resolving deltas: 100% (20/20), done. Update the PATH in ~/.bashrc by adding the following lines #------------------------------------------------------------------------------ # Add data science utils such as body, header export PATH=~/github/jeroenjanssens/dsutils:$PATH
#------------------------------------------------------------------------------

Open a new bash session and verify that the scripts are picked up from the correct location.

% which body
/home/rajulocal/github/jeroenjanssens/dsutils/body

/home/rajulocal/github/jeroenjanssens/dsutils/header

Verify that the scripts are working as expected.

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

### 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.

% echo -e "value\n8\n2\n6\n3" | body grep 8
value
8

### 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:

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