Linux shell script, search by an input string - bash

Question

So, there is a large file where I have to conduct several search using bash shell scripting.
The file is like this:
TITLE and AUTHOR ETEXT NO.
Aspects of plant life; with special reference to the British flora,      56900
by Robert Lloyd Praeger
The Vicar of Morwenstow, by Sabine Baring-Gould 56899
[Subtitle: Being a Life of Robert Stephen Hawker, M.A.]
Raamatun tutkisteluja IV, mennessä Charles T. Russell 56898
[Subtitle: Harmagedonin taistelu]
[Language: Finnish]
Raamatun tutkisteluja III, mennessä Charles T. Russell 56897
[Subtitle: Tulkoon valtakuntasi]
[Language: Finnish]
Tom Thatcher's Fortune, by Horatio Alger, Jr. 56896
A Yankee Flier in the Far East, by Al Avery 56895
and George Rutherford Montgomery
[Illustrator: Paul Laune]
Nancy Brandon's Mystery, by Lillian Garis 56894
Nervous Ills, by Boris Sidis 56893
[Subtitle: Their Cause and Cure]
Pensées sans langage, par Francis Picabia 56892
[Language: French]
Helon's Pilgrimage to Jerusalem, Volume 2 of 2, by Frederick Strauss 56891
[Subtitle: A picture of Judaism, in the century
which preceded the advent of our Savior]
Fra Tommaso Campanella, Vol. 1, di Luigi Amabile 56890
[Subtitle: la sua congiura, i suoi processi e la sua pazzia]
[Language: Italian]
The Blue Star, by Fletcher Pratt 56889
Importanza e risultati degli incrociamenti in avicoltura, 56888
di Teodoro Pascal
[Language: Italian]
The Junior Classics, Volume 3: Tales from Greece and Rome, by Various 56887
~ ~ ~ ~ Posting Dates for the below eBooks: 1 Mar 2018 to 31 Mar 2018 ~ ~ ~ ~
TITLE and AUTHOR ETEXT NO.
The American Missionary, Volume 41, No. 1, January, 1887, by Various 56886
Morganin miljoonat, mennessä Sven Elvestad 56885
[Author a.k.a. Stein Riverton]
[Subtitle: Salapoliisiromaani]
[Language: Finnish]
"Trip to the Sunny South" in March, 1885, by L. S. D 56884
Balaam and His Master, by Joel Chandler Harris 56883
[Subtitle: and Other Sketches and Stories]
Susien saaliina, mennessä Jack London 56882
[Language: Finnish]
Forged Egyptian Antiquities, by T. G. Wakeling 56881
The Secret Doctrine, Vol. 3 of 4, by Helena Petrovna Blavatsky 56880
[Subtitle: Third Edition]
No Posting 56879
First love and other stories, by Iván Turgénieff                         56878
Now I have to search it with etext no, author name and title..
Like If I search by an etext no: like etext 56900:
It should return
Aspects of plant life; with special reference to the British flora, 56900
Well I am new to shell scripting. And I can only read the file.
With this:
#!/bin/sh
read -p 'string to search ' searchstring
grep --color searchstring GUTINDEX.ALL | #condition
I don't know what kind of condition I should use to search by author name or etext no....

Answer 1

As others have already pointed out, the use of grep alone is not how you would really approach this. A rather substantial improvement could be accomplished by using Awk instead of grep, but for a real production system, you would parse out the fields into a relational database, and use SQL to search instead. With database indexing, searching will then be much much quicker than sequentially scanning the entire index file for each search.
But if you are confined to just grep, here is a quick and dirty attempt.
author () { grep -E "(by|par|di|mennessä) $#" GUTINDEX.ALL; }
index () { grep " $#\$" GUTINDEX.ALL; }
title () { grep "^$#" GUTINDEX.ALL; }
This declares three shell functions which search different parts of the file, by way of supplying an anchor expression (^ matches beginning of line, $ matches end of line) or a suitable context.
Having the search expression as a command-line argument instead of requiring interactive input is generally a huge usability improvement. Now, you can use your shell's history mechanism to recall and possibly edit earlier searches, and build new scripts on top of these simple building blocks.
(By the way, "mennessä" is not at all a correct Finnish localization here. I have reported a bug to Project Gutenberg.)

Answer 2

You could start with something like this, but as #tom-fenech points out, it's rather unreliable in the absence of structured input.
For instance, the author names are not consistently prefixed, they appear sometimes under "Subtitle", and rarely under "Author" tag.
#!/bin/bash
CATALOG=/tmp/s
function usage()
{
echo "Usage:"
echo "$0 [etext <key>] [author <id>]"
exit 1;
}
function process_etext()
{
local searchKey=$1
egrep "${searchKey}" ${CATALOG} | awk -F"${searchKey}" '{print $1}'
}
function process_author()
{
local searchKey=$1
egrep -b1 "${searchKey}" ${CATALOG} | egrep "[[:digit:]]{5}"
}
for key in "$#"
do
key="$1"
case $key in
etext|author)
process_${key} $2
shift; shift;
;;
*)
[ -z ${key} ] || usage
;;
esac
done