= apt

apt install <package> --no-upgrade  # install package, but don't upgrade package if it already exists
apt install <package> --only-upgrade # upgrade package, but don't install package if it doesn't exist
apt list --upgradeable        # list packages ready to be upgraded
apt list --installed          # list all installed packages
apt list --all-versions       # list all packages available for your system
apt list --all-versions <package>   # list available versions of given package
apt-cache showpkg <package>   # shows reverse dependencies and other metadata of this package
apt-cache stats               # list statistics about packaging cache
apt-cache unmet               # display unmet dependencies
apt-get install apt-listchanges && dpkg-reconfigure apt-listchanges   # show changelog before installation
apt-get check                 # diagnostic tool; it updates the package cache and checks for broken dependencies.
apt-get download <package>    # Downloads the given binary package into the current directory
apt-get changelog <package>   # download a package changelog and display it
apt-get install <package> --print-uris   # don't fetch files to install, but print their URIs and metadata
apt-file search <pattern>     # search for files where some filepath matches pattern among all available packages
apt-file list <pattern>       # list all files contained in packages matching pattern, one file per line
apt-file … --fixed-string|-F  # exact pattern match (e.g. search ⇒ match entire filepath)
apt-file … --regexp|-x        # pattern match by PCRE
apt-get --dry-run …           # short: -s; only simulate, don't run


difference Suggests, Enhances, Depends, Recommends, …
  https://www.debian.org/doc/debian-policy/ch-relationships.html
  Depends:: absolute dependency, i.e. do not configure this package unless all of the packages listed in Depends have been correctly configured (excluding circular dependencies)
  Recommends:: strong but not absolute dependency, i.e. skip configuration of dependencies in unusual installations
  Suggests:: maybe this package is more useful if the given packages are configured too
  Enhances:: reverse Suggests, i.e. maybe given packages are more useful if this package is configured too
  Breaks:: breaks given package, i.e. do not unpack this package if given package is configured
  Conflicts:: (rare) Breaks, do not unpack this and given package at the same time
  Provides:: the given package is virtual and installed if this package (and all others specifying Provides for given package) is configured


readelf -d <shared-object-file.so>       # direct dependencies occur with “(NEEDED)” in second whitespace column