Provided by: nix-bin_2.16.1+dfsg-3ubuntu1_amd64 bug

Name

       nix-channel - manage Nix channels

Synopsis

       nix-channel  {--add  url  [name] | --remove name | --list | --update [names…] | --rollback
       [generation] }

Description

       A Nix channel is a mechanism that allows you to automatically stay up-to-date with  a  set
       of  pre-built  Nix  expressions.  A  Nix  channel  is  just  a  URL that points to a place
       containing a set of Nix expressions.

       To see the list of official NixOS channels, visit https://nixos.org/channels.

       This command has the following operations:

       • --add url [name]
         Adds a channel named name with URL url to the list of subscribed channels.  If  name  is
         omitted,  it  defaults  to  the  last  component  of  url,  with the suffixes -stable or
         -unstable removed.

         A channel URL must point to a directory containing a file nixexprs.tar.gz.  At  the  top
         level,  that tarball must contain a single directory with a default.nix file that serves
         as the channel’s entry point.

       • --remove name
         Removes the channel named name from the list of subscribed channels.

       • --list
         Prints the names and URLs of all subscribed channels on standard output.

       • --update [names…]
         Downloads the Nix expressions of all subscribed channels  (or  only  those  included  in
         names  if  specified)  and  makes them the default for nix-env operations (by symlinking
         them from the directory ~/.nix-defexpr).

       • --rollback [generation]
         Reverts the previous call  to  nix-channel  --update.  Optionally,  you  can  specify  a
         specific channel generation number to restore.

       Note that --add does not automatically perform an update.

       The list of subscribed channels is stored in ~/.nix-channels.

Common Options

       Most Nix commands accept the following command-line options:

       • --help
         Prints out a summary of the command syntax and exits.

       • --version
         Prints out the Nix version number on standard output and exits.

       • --verbose / -v
         Increases  the  level of verbosity of diagnostic messages printed on standard error. For
         each Nix operation, the information printed on  standard  output  is  well-defined;  any
         diagnostic information is printed on standard error, never on standard output.

         This  option  may  be  specified  repeatedly.  Currently, the following verbosity levels
         exist:

         • 0
           “Errors only”: only print messages explaining why the Nix invocation failed.

         • 1
           “Informational”: print useful messages about what Nix is doing. This is the default.

         • 2
           “Talkative”: print more informational messages.

         • 3
           “Chatty”: print even more informational messages.

         • 4
           “Debug”: print debug information.

         • 5
           “Vomit”: print vast amounts of debug information.

       • --quiet
         Decreases the level of verbosity of diagnostic messages printed on standard error.  This
         is the inverse option to -v / --verbose.

         This option may be specified repeatedly. See the previous verbosity levels list.

       • --log-format format
         This  option  can  be used to change the output of the log format, with format being one
         of:

         • raw
           This is the raw format, as outputted by nix-build.

         • internal-json
           Outputs the logs in a structured manner.

                  Warning

                  While the schema itself is relatively stable, the format of the  error-messages
                  (namely of the msg-field) can change between releases.

         • bar
           Only display a progress bar during the builds.

         • bar-with-logs
           Display the raw logs, with the progress bar at the bottom.

       • --no-build-output / -Q
         By  default,  output written by builders to standard output and standard error is echoed
         to the Nix command’s standard error. This option suppresses this  behaviour.  Note  that
         the  builder’s  standard  output  and  error  are  always  written  to  a  log  file  in
         prefix/nix/var/log/nix.

       • --max-jobs / -j number
         Sets the maximum number of build jobs that Nix will perform in parallel to the specified
         number.  Specify  auto to use the number of CPUs in the system. The default is specified
         by the max-jobs configuration setting, which itself defaults to 1.  A  higher  value  is
         useful on SMP systems or to exploit I/O latency.

         Setting  it  to 0 disallows building on the local machine, which is useful when you want
         builds to happen only on remote builders.

       • --cores
         Sets the value  of  the  NIX_BUILD_CORES  environment  variable  in  the  invocation  of
         builders.  Builders  can  use  this  variable at their discretion to control the maximum
         amount  of  parallelism.  For  instance,  in  Nixpkgs,  if  the   derivation   attribute
         enableParallelBuilding  is  set to true, the builder passes the -jN flag to GNU Make. It
         defaults to the value of the cores configuration setting, if set, or  1  otherwise.  The
         value 0 means that the builder should use all available CPU cores in the system.

       • --max-silent-time
         Sets  the  maximum number of seconds that a builder can go without producing any data on
         standard output or standard error. The  default  is  specified  by  the  max-silent-time
         configuration setting. 0 means no time-out.

       • --timeout
         Sets  the  maximum number of seconds that a builder can run. The default is specified by
         the timeout configuration setting. 0 means no timeout.

       • --keep-going / -k
         Keep going in case of failed builds, to  the  greatest  extent  possible.  That  is,  if
         building  an  input of some derivation fails, Nix will still build the other inputs, but
         not the derivation itself. Without this option, Nix stops if any build fails (except for
         builds  of  substitutes),  possibly  killing  builds in progress (in case of parallel or
         distributed builds).

       • --keep-failed / -K
         Specifies that in case of a build failure, the temporary directory (usually in /tmp)  in
         which  the  build  takes place should not be deleted. The path of the build directory is
         printed as an informational message.

       • --fallback
         Whenever Nix attempts to build a derivation for which substitutes  are  known  for  each
         output  path, but realising the output paths through the substitutes fails, fall back on
         building the derivation.

         The most common scenario in which this is useful is when we have registered  substitutes
         in  order  to  perform  binary  distribution  from,  say,  a  network repository. If the
         repository is down, the realisation of the derivation will fail.  When  this  option  is
         specified, Nix will build the derivation instead. Thus, installation from binaries falls
         back on installation from source. This option is not the default since it  is  generally
         not  desirable  for  a  transient failure in obtaining the substitutes to lead to a full
         build from source (with the related consumption of resources).

       • --readonly-mode
         When this option is used, no attempt  is  made  to  open  the  Nix  database.  Most  Nix
         operations do need database access, so those operations will fail.

       • --arg name value
         This  option  is  accepted  by  nix-env,  nix-instantiate, nix-shell and nix-build. When
         evaluating Nix expressions, the expression evaluator  will  automatically  try  to  call
         functions  that  it  encounters.  It  can  automatically  call functions for which every
         argument has a default value (e.g., { argName ?  defaultValue }: ...). With  --arg,  you
         can  also  call  functions  that  have  arguments without a default value (or override a
         default value). That is, if the evaluator encounters a function with an  argument  named
         name, it will call it with value value.

         For instance, the top-level default.nix in Nixpkgs is actually a function:

       { # The system (e.g., `i686-linux') for which to build the packages.
         system ? builtins.currentSystem
         ...
       }: ...

              So  if  you  call  this  Nix expression (e.g., when you do nix-env --install --attr
              pkgname),  the  function   will   be   called   automatically   using   the   value
              builtins.currentSystem  for the system argument. You can override this using --arg,
              e.g., nix-env --install --attr pkgname --arg system  \"i686-freebsd\".  (Note  that
              since the argument is a Nix string literal, you have to escape the quotes.)

       • --argstr name value
         This  option  is  like  --arg,  only  the value is not a Nix expression but a string. So
         instead of --arg system \"i686-linux\" (the outer quotes are to keep  the  shell  happy)
         you can say --argstr system i686-linux.

       • --attr / -A attrPath
         Select  an  attribute  from the top-level Nix expression being evaluated. (nix-env, nix-
         instantiate, nix-build and nix-shell only.) The attribute path attrPath is a sequence of
         attribute names separated by dots. For instance, given a top-level Nix expression e, the
         attribute path xorg.xorgserver would cause the expression e.xorg.xorgserver to be  used.
         See nix-env --install for some concrete examples.

         In  addition  to attribute names, you can also specify array indices.  For instance, the
         attribute path foo.3.bar selects the bar attribute of the fourth element of the array in
         the foo attribute of the top-level expression.

       • --expr / -E
         Interpret  the  command  line  arguments  as  a list of Nix expressions to be parsed and
         evaluated, rather than as a list of file names  of  Nix  expressions.  (nix-instantiate,
         nix-build and nix-shell only.)

         For  nix-shell,  this option is commonly used to give you a shell in which you can build
         the packages returned by the expression. If you want to get a shell  which  contain  the
         built  packages  ready  for  use,  give  your  expression  to  the  nix-shell --packages
         convenience flag instead.

       • -I path
         Add an entry to the Nix expression search path.   This  option  may  be  given  multiple
         times.  Paths added through -I take precedence over NIX_PATH.

       • --option name value
         Set  the  Nix  configuration  option  name  to value. This overrides settings in the Nix
         configuration file (see nix.conf5).

       • --repair
         Fix corrupted or missing store paths by redownloading or rebuilding them. Note that this
         is slow because it requires computing a cryptographic hash of the contents of every path
         in the closure of the build. Also note the warning under nix-store --repair-path.

Common Environment Variables

       Most Nix commands interpret the following environment variables:

       • IN_NIX_SHELL
         Indicator that tells if the current environment was set up by nix-shell. It can have the
         values pure or impure.

       • NIX_PATH
         A  colon-separated  list  of directories used to look up the location of Nix expressions
         using paths enclosed in angle brackets (i.e., <path>), e.g.  /home/eelco/Dev:/etc/nixos.
         It can be extended using the -I option.

         If  NIX_PATH  is  not set at all, Nix will fall back to the following list in impure and
         unrestricted evaluation mode:

         1.     $HOME/.nix-defexpr/channels
         2.     nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixpkgs
         3.     /nix/var/nix/profiles/per-user/root/channels

         If NIX_PATH is set to an empty string, resolving search paths  will  always  fail.   For
         example, attempting to use <nixpkgs> will produce:

       error: file 'nixpkgs' was not found in the Nix search path

       • NIX_IGNORE_SYMLINK_STORE
         Normally,  the  Nix store directory (typically /nix/store) is not allowed to contain any
         symlink  components.  This  is  to   prevent   “impure”   builds.   Builders   sometimes
         “canonicalise”  paths  by  resolving  all  symlink components. Thus, builds on different
         machines (with /nix/store  resolving  to  different  locations)  could  yield  different
         results.  This  is  generally not a problem, except when builds are deployed to machines
         where /nix/store resolves differently. If you are sure that you’re not going to do that,
         you can set NIX_IGNORE_SYMLINK_STORE to 1.

         Note  that  if  you’re  symlinking  the Nix store so that you can put it on another file
         system than the root file system, on Linux you’re better off using  bind  mount  points,
         e.g.,

       $ mkdir /nix
       $ mount -o bind /mnt/otherdisk/nix /nix

              Consult the mount 8 manual page for details.

       • NIX_STORE_DIR
         Overrides the location of the Nix store (default prefix/store).

       • NIX_DATA_DIR
         Overrides the location of the Nix static data directory (default prefix/share).

       • NIX_LOG_DIR
         Overrides the location of the Nix log directory (default prefix/var/log/nix).

       • NIX_STATE_DIR
         Overrides the location of the Nix state directory (default prefix/var/nix).

       • NIX_CONF_DIR
         Overrides   the   location   of   the   system   Nix  configuration  directory  (default
         prefix/etc/nix).

       • NIX_CONFIG
         Applies settings from Nix configuration from the environment.  The content is treated as
         if  it  was  read  from a Nix configuration file.  Settings are separated by the newline
         character.

       • NIX_USER_CONF_FILES
         Overrides the location of the Nix user configuration files to load from.

         The default are the locations according to the XDG Base  Directory  Specification.   See
         the XDG Base Directories sub-section for details.

         The variable is treated as a list separated by the : token.

       • TMPDIR
         Use  the  specified  directory  to  store  temporary files. In particular, this includes
         temporary build directories; these can take up substantial amounts of  disk  space.  The
         default is /tmp.

       • NIX_REMOTE
         This  variable  should be set to daemon if you want to use the Nix daemon to execute Nix
         operations. This is necessary in multi-user Nix installations. If the Nix daemon’s  Unix
         socket   is   at   some   non-standard   path,   this   variable   should   be   set  to
         unix://path/to/socket. Otherwise, it should be left unset.

       • NIX_SHOW_STATS
         If set to 1, Nix will print some evaluation statistics, such as  the  number  of  values
         allocated.

       • NIX_COUNT_CALLS
         If  set  to  1,  Nix  will  print  how often functions were called during Nix expression
         evaluation. This is useful for profiling your Nix expressions.

       • GC_INITIAL_HEAP_SIZE
         If Nix has been configured to use the Boehm garbage collector, this  variable  sets  the
         initial  size  of  the  heap in bytes. It defaults to 384 MiB. Setting it to a low value
         reduces memory consumption, but will increase runtime due to  the  overhead  of  garbage
         collection.

   XDG Base Directories
       Nix follows the XDG Base Directory Specification.

       For backwards compatibility, Nix commands will follow the standard only when use-xdg-base-
       directories is enabled.  New Nix  commands  (experimental)  conform  to  the  standard  by
       default.

       The  following  environment variables are used to determine locations of various state and
       configuration files:

       • [XDG_CONFIG_HOME]{#env-XDGCONFIGHOME} (default ~/.config)
       • [XDG_STATE_HOME]{#env-XDGSTATEHOME} (default ~/.local/state)
       • [XDG_CACHE_HOME]{#env-XDGCACHEHOME} (default ~/.cache)

Files

       nix-channel operates on the following files.

   Channels
       A directory containing symlinks to Nix channels, managed by nix-channel:

       • $XDG_STATE_HOME/nix/profiles/channels for regular users
       • $NIX_STATE_DIR/profiles/per-user/root/channels for root

       nix-channel uses a profile to store channels.   This  profile  contains  symlinks  to  the
       contents of those channels.

   Subscribed channels
       The list of subscribed channels is stored in

       • ~/.nix-channels
       • $XDG_STATE_HOME/nix/channels if use-xdg-base-directories is set to true

       in the following format:

       <url> <name>
       ...

Examples

       To subscribe to the Nixpkgs channel and install the GNU Hello package:

       $ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
       $ nix-channel --update
       $ nix-env --install --attr nixpkgs.hello

       You can revert channel updates using --rollback:

       $ nix-instantiate --eval --expr '(import <nixpkgs> {}).lib.version'
       "14.04.527.0e935f1"

       $ nix-channel --rollback
       switching from generation 483 to 482

       $ nix-instantiate --eval --expr '(import <nixpkgs> {}).lib.version'
       "14.04.526.dbadfad"

                                                                                   nix-channel(1)