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How (and why) do these commands with a pipe and a dash work exactly? 

pacman -Qqdt | sudo pacman -Rns -
Jeff Schaller
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Anonymous
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    A dash - is usually used to designate a flag, that tells the program to set a certain option or do something a certain way (if you do pacman --help it'll probably tell you what all the flags do). Meanwhile, a pipe | makes it so that the output of the left side feeds into the standard input on the right side - so, sudo pacman -Rns will behave like taking the output of pacman -Qqdt as input. – Green Cloak Guy Jun 07 '19 at 19:25
  • @GreenCloakGuy that's good enough to be placed as an Answer, if you would! Thank you! – Jeff Schaller Jun 07 '19 at 22:30
  • @GreenCloakGuy Yes, if you could also say something about what the options do and what the purpose of the pipeline might be. – Kusalananda Jun 07 '19 at 22:33
  • Unfortunately, I don't actually know the pacman command, so I wouldn't be able to answer in enough detail for that – Green Cloak Guy Jun 07 '19 at 23:23
  • For many commands that take file arguments, a plain dash indicates "use stdin instead of a file". cat, paste are two examples. – glenn jackman Jun 08 '19 at 12:57
  • @GreenCloakGuy a lone dash isn't an option. It usually means "read from standard input". – terdon Jun 08 '19 at 13:09

1 Answers1

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A lone dash (-), with no option, usually means "read from standard input". This is a very common convention used by many programs. The pipe, |, is a way of connecting the standard output of one program to the standard input of another. Since pacman doesn't read from standard input by default, if you want it to do so, you use the -.

So, the commands you show do (see man pacman):

  • pacman -Qqdt:

    -Q, --query
       Query the package database. This operation allows you to view installed
       packages and their files, as well as meta-information about individual
       packages (dependencies, conflicts, install date, build date, size). This can
       be run against the local package database or can be used on individual
       package files. In the first case, if no package names are provided in the
       command line, all installed packages will be queried. Additionally, various
       filters can be applied on the package list. See Query Options below.

    -q, --quiet
   Show less information for certain query operations. This is useful when
   pacman’s output is processed in a script. Search will only show package
   names and not version, group, and description information; owns will only
   show package names instead of "file is owned by pkg" messages; group will
   only show package names and omit group names; list will only show files and
   omit package names; check will only show pairs of package names and missing
   files; a bare query will only show package names rather than names and
   versions.
    

    -d, --deps
   Restrict or filter output to packages installed as dependencies. This option
   can be combined with -t for listing real orphans - packages that were
   installed as dependencies but are no longer required by any installed
   package.
    

    -t, --unrequired
   Restrict or filter output to print only packages neither required nor
   optionally required by any currently installed package. Specify this option
   twice to include packages which are optionally, but not directly, required
   by another package.

    Combined, these options mean "query the database for packages installed as dependencies of other packages, showing only package names, and restrict the output to those packages not needed by any currently installed package". In other words, show those packages which were installed because they were needed by something else but which are no longer needed because that something else has been removed.

  • sudo pacman -Rns -:

    -R, --remove
   Remove package(s) from the system. Groups can also be specified to be
   removed, in which case every package in that group will be removed. Files
   belonging to the specified package will be deleted, and the database will be
   updated. Most configuration files will be saved with a .pacsave extension
   unless the --nosave option is used. See Remove Options below.

    -n, --nosave
   Instructs pacman to ignore file backup designations. Normally, when a file
   is removed from the system, the database is checked to see if the file
   should be renamed with a .pacsave extension.
    

    -s, --recursive
   Remove each target specified including all of their dependencies, provided
   that (A) they are not required by other packages; and (B) they were not
   explicitly installed by the user. This operation is recursive and analogous
   to a backwards --sync operation, and it helps keep a clean system without
   orphans. If you want to omit condition (B), pass this option twice.

    And the - (emphasis mine):

    Invoking pacman involves specifying an operation with any potential options and targets to operate on. A target is usually a package name, file name, URL, or a search string. Targets can be provided as command line arguments. Additionally, if stdin is not from a terminal and a single hyphen (-) is passed as an argument, targets will be read from stdin.

    So, pacman -Rns - will read package names from standard input and remove any of those, and their dependencies, without keeping backups.

The entire command will therefore find no longer needed packages on your system and remove them. It's a useful way of cleaning your system of unneeded packages.

terdon
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