Since you mentioned datatool and glossaries, here are some alternatives.
With datatool, the simplest method requires defining all entries in incremental order (without using a counter). The row number supplies the indexing.
\documentclass{article}
\usepackage{datatool}
\DTLnewdb{data}
\newcommand{\addline}[1]{%
\DTLnewrow{data}%
\DTLnewdbentry{data}{Text}{#1}%
}
\newcommand{\print}[1]{%
\DTLgetvalue{\thisval}{data}{#1}{1}%
\thisval
}
\addline{The first line}
\addline{The second line}
\begin{document}
Line 2: \print{2}.
All lines:
\DTLforeach*{data}{\Text=Text}{\DTLcurrentindex. \Text.\par}
\end{document}
This produces:
Line 2: The second line.
All lines:
1. The first line.
2. The second line.
If you want to define your entries out of order using a counter, this can be done with an additional column:
\documentclass{article}
\usepackage{datatool}
\DTLnewdb{data}
\newcommand{\addline}[2]{%
\DTLnewrow{data}%
\dtlexpandnewvalue
\DTLnewdbentry{data}{Index}{\the\value{#1}}%
\dtlnoexpandnewvalue
\DTLnewdbentry{data}{Text}{#2}%
}
\newcommand{\print}[1]{%
\dtlgetrowindex{\thisrowidx}{data}{1}{#1}%
\ifx\thisrowidx\dtlnovalue
Not found!%
\else
\DTLgetvalue{\thisval}{data}{\thisrowidx}{2}%
\thisval
\fi
}
\newcounter{a}
\setcounter{a}{2}
\addline{a}{The second line}
\setcounter{a}{1}
\addline{a}{The first line}
\begin{document}
Line 2: \print{2}.
All lines:
\DTLforeach*{data}{\theIndex=Index,\Text=Text}{\theIndex. \Text.\par}
\end{document}
This produces:
Line 2: The second line.
All lines:
2. The second line.
1. The first line.
The list is now out of numerical order, but matches the order that the blocks were defined. You can sort them before displaying the list:
\DTLsort{Index}{data}
\DTLforeach*{data}{\theIndex=Index,\Text=Text}{\theIndex. \Text.\par}
Line 2: The second line.
All lines:
1. The first line.
2. The second line.
Here's a glossaries approach:
\documentclass{article}
\usepackage{glossaries-extra}
\glssetexpandfield{name}
\newcommand{\addline}[2]{%
\edef\thisidx{\the\value{#1}}%
\newglossaryentry{\thisidx}{name={\thisidx},description={#2}}%
}
\newcommand{\print}[1]{%
\glsentrydesc{#1}%
}
\newcounter{a}
\setcounter{a}{2}
\addline{a}{The second line}
\setcounter{a}{1}
\addline{a}{The first line}
\begin{document}
Line 2: \print{2}.
All lines:
\renewcommand{\glstreenamefmt}[1]{#1}
\renewcommand{\glossarysection}[2][]{}
\printunsrtglossary[style=index]
\end{document}
This produces:
Again this lists in order of definition. If you want the list sorted, you can use the following instead:
\documentclass{article}
\usepackage[automake,nopostdot]{glossaries}
\makeglossaries
\glssetexpandfield{name}
\newcommand{\addline}[2]{%
\edef\thisidx{\the\value{#1}}%
\newglossaryentry{\thisidx}{name={\thisidx},description={#2}}%
}
\newcommand{\print}[1]{%
\glsentrydesc{#1}\glsadd{#1}%
}
\newcounter{a}
\setcounter{a}{2}
\addline{a}{The second line}
\setcounter{a}{1}
\addline{a}{The first line}
\begin{document}
Line 2: \print{2}.
All lines:
\renewcommand{\glstreenamefmt}[1]{#1}
\renewcommand{\glossarysection}[2][]{}
\printglossary[style=index,nonumberlist]
\end{document}
This produces:
Line 2: The second line.
All lines:
2 The second line
This only lists the entry that has been indexed (with \glsadd). If you want all the entries to be listed, use \glsaddall (after all entries have been defined).
\documentclass{article}
\usepackage[automake,nopostdot]{glossaries}
\makeglossaries
\glssetexpandfield{name}
\newcommand{\addline}[2]{%
\edef\thisidx{\the\value{#1}}%
\newglossaryentry{\thisidx}{name={\thisidx},description={#2}}%
}
\newcommand{\print}[1]{%
\glsentrydesc{#1}%
}
\newcounter{a}
\setcounter{a}{2}
\addline{a}{The second line}
\setcounter{a}{1}
\addline{a}{The first line}
\glsaddall
\begin{document}
Line 2: \print{2}.
All lines:
\renewcommand{\glstreenamefmt}[1]{#1}
\renewcommand{\glossarysection}[2][]{}
\printglossary[style=index,nonumberlist]
\end{document}
This produces:
Line 2: The second line.
All lines:
1 The first line
2 The
second line
Expansion
It's important that the indexing value \the\value{#1} is fully expanded before being stored, otherwise it will keep changing as the counter changes value. Both datatool and glossaries have a way to switch expansion on or off when adding/defining a new entry.
In the case of datatool, expansion is switched on using \dtlexpandnewvalue. In the case of glossaries, expansion is switched on for a particular field using \glssetexpandfield{field-label}.
The code you want to add (in the final argument of \addline) may well contain fragile commands, in which case it's important not to expand the value. With datatool, the expansion is switched back off again using \dtlnoexpandnewvalue. With glossaries, the value is being stored in the description key and expansion is off by default for that field.
