\chapter{System}

\section{To Reload Your \texttt{.bashrc} File}

\lettrine{S}{ometimes} you add something to your \texttt{.bashrc}\index[concepts]{.bashrc}[concepts]\index[concepts]{bashrc} file (such as a new bash alias)\index[concepts]{bash alias} and you need to reload it. Of course, this file is reloaded every time you log in, but you may not wish to log out and then log back in if you are in the middle of important work. There are few different ways to do this:

\begin{Verbatim}[frame=single, breaklines=true]
source ~/.bashrc
\end{Verbatim}
\index{source}

and of course a shorter way:

\begin{Verbatim}[frame=single, breaklines=true]
. ~/.bashrc
\end{Verbatim}

\section{To See if a Package Exists in the \texttt{apt} Repository}\index[concepts]{repositories}

\lettrine{W}{hile} there are many ways to install software on a Linux system, the easiest way is usually through the package manager, which on Ubuntu is \texttt{apt}. \index{apt} To see if a package is available in that repo, we can use 

\begin{Verbatim}[frame=single, breaklines=true]
apt search <package>
\end{Verbatim}
\index{apt!search}

As an example, searching for the Okular document viewer in \texttt{apt} returns the following:

\begin{Verbatim}[]
libokular5core9 - libraries for the Okular document viewer
okular - universal document viewer
okular-backend-odp - Okular backend for ODP documents
okular-backend-odt - Okular backend for ODT documents
okular-dev - development files for the Okular libraries
okular-extra-backends - additional document format support for Okular
okular-mobile - mobile support for Okular
qml-module-org-kde-okular - mobile support for Okular - QML modules
\end{Verbatim}

This does not tell us which version is available however. We have a couple of options here. First, we can use 

\begin{Verbatim}[frame=single, breaklines=true]
apt policy <package>
\end{Verbatim}
\index{apt!policy}

Using that for Okular returns the following:

\begin{Verbatim}[breaklines=true]
okular:
  Installed: (none)
  Candidate: 4:21.12.3-0ubuntu1
  Version table:
     4:21.12.3-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu jammy/universe amd64 Packages
        100 /var/lib/dpkg/status
\end{Verbatim}

We can also use:

\begin{Verbatim}[frame=single, breaklines=true]
apt search <package>
\end{Verbatim}
\index{apt!search}

Which gives a more detailed listing with the available versions of all related packages:

\begin{Verbatim}[]
Sorting... Done
Full Text Search... Done
libokular5core9/jammy 4:21.12.3-0ubuntu1 amd64
  libraries for the Okular document viewer

okular/jammy,now 4:21.12.3-0ubuntu1 amd64 [residual-config]
  universal document viewer

okular-backend-odp/jammy 1:3.2.1+dfsg-5 amd64
  Okular backend for ODP documents

okular-backend-odt/jammy 1:3.2.1+dfsg-5 amd64
  Okular backend for ODT documents

okular-dev/jammy 4:21.12.3-0ubuntu1 amd64
  development files for the Okular libraries

okular-extra-backends/jammy 4:21.12.3-0ubuntu1 amd64
  additional document format support for Okular

okular-mobile/jammy 4:21.12.3-0ubuntu1 amd64
  mobile support for Okular

qml-module-org-kde-okular/jammy 4:21.12.3-0ubuntu1 amd64
  mobile support for Okular - QML modules
\end{Verbatim}

To get even more information, including dependencies, we can use:

\begin{Verbatim}[frame=single]
apt show <package>
\end{Verbatim}

Executing this command for Okular returns:

\begin{Verbatim}[breaklines=true]
Package: okular
Version: 4:21.12.3-0ubuntu1
Priority: extra
Section: universe/graphics
Origin: Ubuntu
Maintainer: Kubuntu Developers <kubuntu-devel@lists.ubuntu.com>
Original-Maintainer: Debian Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 16.4 MB
Provides: pdf-viewer, postscript-viewer
Depends: kinit, kio, libc6 (>= 2.35), libfreetype6 (>= 2.2.1), libjpeg8 (>= 8c), libkf5activities5 (>= 5.68.0~), libkf5archive5 (>= 5.68.0~), libkf5bookmarks5 (>= 5.68.0~), libkf5codecs5 (>= 4.96.0), libkf5completion5 (>= 5.68.0~), libkf5configcore5 (>= 5.68.0~), libkf5configgui5 (>= 5.68.0~), libkf5configwidgets5 (>= 5.81.0), libkf5coreaddons5 (>= 5.68.0~), libkf5crash5 (>= 5.68.0~), libkf5i18n5 (>= 5.68.0~), libkf5itemviews5 (>= 4.96.0), libkf5jobwidgets5 (>= 4.96.0), libkf5kexiv2-15.0.0 (>= 21.12.3~), libkf5kiocore5 (>= 5.68.0~), libkf5kiowidgets5 (>= 5.68.0~), libkf5parts5 (>= 5.68.0~), libkf5pty5 (>= 5.68.0~), libkf5purpose-bin, libkf5purpose5 (>= 5.68.0~), libkf5service-bin, libkf5service5 (>= 4.96.0), libkf5textwidgets5 (>= 5.68.0~), libkf5wallet-bin, libkf5wallet5 (>= 5.68.0~), libkf5widgetsaddons5 (>= 5.77.0), libkf5windowsystem5 (>= 5.68.0~), libkf5xmlgui5 (>= 5.79.0), libokular5core9 (= 4:21.12.3-0ubuntu1), libphonon4qt5-4 (>= 4:4.8.0), libpoppler-qt5-1 (>= 21.11.0), libqmobipocket2 (>= 4:21.12.3~), libqt5core5a (>= 5.15.1), libqt5dbus5 (>= 5.14.1), libqt5gui5 (>= 5.14.1) | libqt5gui5-gles (>= 5.14.1), libqt5printsupport5 (>= 5.13.0~), libqt5texttospeech5 (>= 5.12.0~), libqt5widgets5 (>= 5.15.1), libqt5xml5 (>= 5.13.0~), libspectre1 (>= 0.2.3), libstdc++6 (>= 11), phonon4qt5, zlib1g (>= 1:1.1.4)
Recommends: cups-bsd, okular-extra-backends, qtspeech5-flite-plugin
Suggests: ghostscript, okular-extra-backends (= 4:21.12.3-0ubuntu1), poppler-data, texlive-binaries, unrar
Breaks: kdegraphics-mobipocket (<< 4:4.12), okular-extra-backends (<< 4:16.12.0), okular-mobile (<< 4:17.04.1-0)
Replaces: okular-extra-backends (<< 4:16.12.0), okular-mobile (<< 4:17.04.1-0)
Homepage: https://apps.kde.org/en/okular
Task: kubuntu-desktop, kubuntu-full, ubuntustudio-desktop
Download-Size: 6,206 kB
APT-Sources: http://us.archive.ubuntu.com/ubuntu jammy/universe amd64 Packages
Description: universal document viewer
 Okular is a universal document viewer with support for advanced document
 features, such as annotations, forms, and embedded files.
 .
 This package provides the document viewer application, plus plugins for a wide
 variety of document formats such as:
  * PostScript (PS)
  * Portable Document Format (PDF)
  * TeX Device independent file format (DVI)
  * XML Paper Specification (XPS)
  * G3 fax
  * various electronic book formats: ComicBook, FictionBook, and Plucker
 .
 Additional document formats are in the okular-extra-backends package.
 .
 The Calligra suite provides Okular plugins for OpenDocument formats that can
 be found in the okular-backend-odt and okular-backend-odp packages.
 .
 This package is part of the KDE graphics module.
\end{Verbatim}


\ktcbox{I generally prefer to install software from the \texttt{apt} repository, but sometimes it does not have the most recent version. These tools allow me to quickly check which version the repo does have. If the version is acceptable, I can install from there, or if not, I can explore other options.}

\section{To See Which Packages Have Been Installed}

\lettrine{I}{nstalling} software is easy on a Linux-based system. The problem is that sometimes you forget what you've installed. There are a few ways to do this. The first is

\begin{Verbatim}[frame=single]
apt list --installed
\end{Verbatim}
\index{apt!list}

This will give output like this:

\begin{Verbatim}[breaklines=true]
Listing... Done
abcm2ps/jammy,now 8.14.11-0.1 amd64 [installed]
accountsservice/jammy-security,jammy-updates,now 22.07.5-2ubuntu1.6 amd64 [installed,automatic]
acl/jammy,now 2.3.1-1 amd64 [installed,automatic]
acpi-support/jammy,now 0.144 amd64 [installed,automatic]
acpid/jammy,now 1:2.0.33-1ubuntu1 amd64 [installed,automatic]
adduser/jammy,jammy,now 3.118ubuntu5 all [installed,automatic]
adwaita-icon-theme-full/jammy,jammy,now 41.0-1ubuntu1 all [installed,automatic]
adwaita-icon-theme/jammy,jammy,now 41.0-1ubuntu1 all [installed,automatic]
adwaita-qt/jammy,now 1.4.1-1 amd64 [installed,automatic]
aglfn/jammy,jammy,now 1.7+git20191031.4036a9c-2 all [installed,automatic]
aisleriot/jammy,now 1:3.22.22-1 amd64 [installed,automatic]
alsa-base/jammy,jammy,now 1.0.25+dfsg-0ubuntu7 all [installed,automatic]
[…snip]
\end{Verbatim}

Unfortunately, that will give you a list of \texttt{every} packaged that has been installed on your system, including from when you installed Linux originally. To get a list of manually installed applications, use

\begin{Verbatim}[frame=lines]
apt list --manually-installed
\end{Verbatim}

On my system, using the \texttt{--installed} option produced 3,184 lines of output, whereas using the \texttt{--manual-installed} option produced 237 lines of output—a considerable difference.

If necessary, you can redirect this information to a file using 

\begin{Verbatim}[frame=lines]
apt list --installed > app_list.txt
\end{Verbatim}

Which will produce a warning: ``\texttt{WARNING: apt does not have a stable CLI interface. Use with caution in scripts.}'' What that means is that the \texttt{apt} command is meant to be used in an interactive mode (it includes things like progress bars when downloading and installing software, for example) and its output is not really able to be parsed in a script. (These commands work fine, however, as the output has a simple structure.) For scripting purposes, you really want to use \texttt{apt-get}\index{apt-get} and \texttt{apt-cache}\index{apt-cache}.\index[concepts]{apt stable CLI interface}

\section{Redirecting \texttt{stdout} and \texttt{stderr} Somewhere Else}\index[concepts]{redirection}

\lettrine{E}{very} command in Linux has three components: 

\begin{enumerate}[noitemsep]
\item Something to work on (that is, input) which we call \texttt{stdin}
\item Something it spits out (that is, output) which we call \texttt{stdout}
\item Error messages if something isn't right, which we call \texttt{stderr}
\end{enumerate}\index[concepts]{stdin}\index[concepts]{stdout}\index[concepts]{sterr}

\noindent{}To redirect \texttt{stdout} to a file, you can use 

\begin{Verbatim}[frame=lines]
command > file.txt
\end{Verbatim}

or 

\begin{Verbatim}[frame=lines]
command 1> file.txt
\end{Verbatim}

\noindent{}To redirect \texttt{stderr} to a file, you can use 

\begin{Verbatim}[frame=lines]
command 2> file.txt
\end{Verbatim}