pkexec - Execute a command as another user
pkexec [--user username]
pkexec allows an authorized user to execute PROGRAM as another
user. If PROGRAM is not specified, the default shell will be run. If
username is not specified, then the program will be executed as the
administrative super user, root.
Upon successful completion, the return value is the return value of
PROGRAM. If the calling process is not authorized or an authorization
could not be obtained through authentication or an error occured,
pkexec exits with a return value of 127. If the authorization could not
be obtained because the user dismissed the authentication dialog,
pkexec exits with a return value of 126.
pkexec, like any other polkit application, will use the authentication
agent registered for the calling process or session. However, if no
authentication agent is available, then pkexec will register its own
textual authentication agent. This behavior can be turned off by passing the
Executing a program as another user is a privileged operation. By default the
action to check for (see the section called “ACTION AND
AUTHORIZATIONS”) requires administrator authentication. In addition,
the authentication dialog presented to the user will display the full path to
the program to be executed so the user is aware of what will happen.
The environment that PROGRAM will run it, will be set to a
minimal known and safe environment in order to avoid injecting code through
LD_LIBRARY_PATH or similar mechanisms. In addition the PKEXEC_UID
environment variable is set to the user id of the process invoking
pkexec. As a result, pkexec will not by default allow you to
run X11 applications as another user since the $DISPLAY and $XAUTHORITY
environment variables are not set. These two variables will be retained if
the org.freedesktop.policykit.exec.allow_gui annotation on an action
is set to a nonempty value; this is discouraged, though, and should only be
used for legacy programs.
Note that pkexec does no validation of the ARGUMENTS
passed to PROGRAM. In the normal case (where administrator
authentication is required every time pkexec is used), this is not a
problem since if the user is an administrator he might as well just run
pkexec bash to get root.
However, if an action is used for which the user can retain
authorization (or if the user is implicitly authorized) this could be a
security hole. Therefore, as a rule of thumb, programs for which the default
required authorization is changed, should never implicitly trust user
input (e.g. like any other well-written suid program).
By default, the org.freedesktop.policykit.exec action is used. To use
another action, use the org.freedesktop.policykit.exec.path annotation
on an action with the value set to the full path of the program. In addition
to specifying the program, the authentication message, description, icon and
defaults can be specified. If the org.freedesktop.policykit.exec.argv1
annotation is present, the action will only be picked if the first argument to
the program matches the value of the annotation.
Note that authentication messages may reference variables (see the
section called “VARIABLES”), for example $(user) will be
expanded to the value of the user variable.
To avoid modifying existing software to prefix their command-line invocations
with pkexec, it's possible to use pkexec in a she-bang
wrapper like this:
print "Hello, I'm running as uid %d"%(os.getuid())
for n in range(len(sys.argv)):
print "arg[%d]=`%s'"%(n, sys.argv[n])
If this script is installed into /usr/bin/my-pk-test, then the
can be used to select the appropriate polkit action. Be careful to
get the latter annotation right, otherwise it will match any pkexec
invocation of /usr/bin/python scripts.
The following variables are set by pkexec. They can be used in
authorization rules and messages shown in authentication dialogs:
Fully qualified path to the program to be executed.
The requested command-line (do not use this for any
security checks, it is not secure). Example: “cat
The user name of the user to execute the program as.
The full name of the user to execute the program as.
Example: “David Zeuthen”
A representation of the user to execute the program as
that is suitable for display in an authentication dialog. Is typically set to
a combination of the user name and the full name. Example: “David
Written by David Zeuthen <firstname.lastname@example.org> with a lot of help from many