Quick reference

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at
Execute a command or script at a specified time and at
regular intervals.
at [[-V] [-q
queue
] [-f
file
] [-m] [-l] [-d] [-b] [-v]
time
] |
[-c
job
[
job
...]]
[-V]: Show the version of at.
[-q
queue
]: Specifies the queue to use.
[-f
file
]: Get the job information from a file.
[-m]: Email job status to user after completion.
[-l]: List the jobs currently in the queue.
[-d]: Remove the job from the queue.
[-b]: Allows a job to be run based on the current system
status.
[-v]: Shows completed jobs still in the queue.
time

: The time that the job should run. MM/DD/YY and/or
HH:MM specification is supported.
[-c
job
[
job
...]]: The task that should be performed at the
time specified.
date
Display and set the system date and time
Date [-u | --utc | --universal] [-d | --date=
datestring
]
[-R | --rfc-822] [[
yyyy
]
mmddhhmm
[
yy
][.
ss
]]
[-u | --utc | --universal]: Print/set universal time.
[-d | --date=
datestring
]: Display the time shown by the
string specified here.
[-R | --rfc-822]: Show the time in rfc-822 format.
[[
yyyy

]
mmddhhmm
[
yy
][
.ss
]]: The format for the date
and time.
halt
Power down or restart the system.
halt [-d] [-f] [-h] [-i] [-n] [-p] [-w]
[-d]: Don’t write wtmp record before shutting down.
[-f]: Force a shutdown or restart.
[-h]: Place hard drives into standby before shutting down.
[-i]: Shut down network interfaces before shutting down.
[-n]: Don’t sync before shutting down. Implies a -d.
[-p]: Halt with a poweroff rather than restart.
[-w]: Don’t shutdown or restart, but write wtmp record.
kill
Terminate a system process.
kill [-l [
signal
] | [-s
signal
| -p]] [-a] [--]
pid
...
[-l signal]: Print a list of signal names. Red Hat Linux 9
has 63 signal names.
[-s signal]: The signal to send to the process. Signal 9 for

example, will send a SIGKILL signal to a process
effectively killing it.
[-p]: Just print the process id for the specified process
and do not send it any signals.
[-a]: Don’t restrict commandname-to-pid conversion.
newaliases
Rebuild the mail aliases database.
Newaliases does not take any parameters and is
functionally equivalent to sendmail -bi.
passwd
Manage user’s passwords.
passwd [-d | --delete] [-f | --force] [-i | --inactive=days]
[-k | --keep-tokens] [-l | --lock] [-n | --minimum=
days
] [-S | --status] [--stdin] [-u | --unlock]
[-w | --warning=
days
] [-x | --maximum=
days
]
[username]
[-d | --delete]: Root only: Delete the password for the
user.
[-f | --force]: Force the operation to complete.
[-i | --inactive=
days
]: Root only: The account grace
period. The number of days after a password expires
before an account is disabled.
[-k | --keep-tokens]: Keep authentication tokens that have

not expired.
[-l | --lock]: Root only: Lock the specified account.
[-n | --minimum=
days
]: Root only: The minimum number
of days that must pass before the password can be
changed again.
[-S | --status]: Root only: Display the password status of
the specified account.
[-u | --unlock]: Root only: Unlock the specified account.
[-w | --warning=
days
]: Root only: The number of days
warning that a user receives that his password is to
expire.
[-x | --maximum=
days
]: Root only: The maximum life of
a password.
[username]: The username on which to perform the
operation.
ps
Display process statistics.
ps [a] [-A] [-a] [C] [c] [-C
processname
] [{--cols num |
--columns num | --width num}] [-d] [e] [-e] [{f |
--forest}] [-f] [g] [{-G
RGID
| --Group

RGID
}]
[{-g
groupname
| --group
groupname
}] [{h |
--no-headers}] [-H] [--headers] [--html] [{j | -j}]
[{l | -l}] [L] [{--lines num | --rows num}] [{m | -m}]
[n] [{N | -n}] [-N | --deselect] [{--nul | --null |
--zero}] [O] [{o | -o | --format}] [{p
pid
| -p
pid
| --pid
pid}] [r] [s] [{S | --cumulative}] [{-s session | --sid
session}] [--sort sortkey] [{-t tty | t tty | T tty | --tty
tty}] [{-U
RUID
| --User
RUID
}] [{-u
userid
| --user
userid
}] [v] [{w | -w}] [x] [X] [-y]
[a]: Show information for all processes on this terminal
including those belonging to other users
[-A]: Show information for all processes
[-a]: Show information for all processes with a TTY except

session leaders
[C]: Show the raw CPU time
[c]: Show the true command name
[-C
processname
]: Show information for processes that
match the name specified
[{--cols num | --columns num | --width num}]:
The screen width
[-d]: Show information for all processes except session
leaders
[e]: Show the environment after each command
[-e]: Show information for all processes with a TTY except
session leaders
[{f | --forest}]: Display: process listing with ASCII art
[-f]: Display: full process listing
[g]: Really, we mean it this time. Show ALL process
information.
[{-G
RGID
| --Group
RGID
}]: Show information for
processes with a matching real group ID
[{-g groupname | --group groupname}]: Show
information for processes matching the effective
group name
[{h | --no-headers}]: Display: do not print header lines
[-H]: Display: process hierachy
[--headers]: Repeat the header lines

[--html]: Print html escaped output
[{j | -j}]: Display: jobs format
[{l | -l}]: Display: long listing format
[L]: List all formats
[{--lines
num
| --rows
num
}]: Set the screen height
[{m | -m}]: Display: all threads
[n]: Display: numeric output for WCHAN and USER
[{N | -n}]: Set namelist file
[-N | --deselect]: Negate the selection
[{--nul | --null | --zero}]: Unjustified output with NULs
[O]: Sort order: overloaded
[{o | -o | --format}]: Display: User define format
[{p
pid
| -p
pid
| --pid pid}]: Show information for
processes with a matching process ID
[r]: Show information for running processes
[s]: Display: Signal format
[{S | --cumulative}]: Include dead child process
information
[{-s
session
| --sid
session

}]: Show information for
processes belonging to the matching session
[--sort sortkey]: Sort order
[{-t tty | t tty | T tty | --tty tty}]: Show information for
processes on the terminal specified
[{-U
RUID
| --User
RUID
}]: Show information for
processes with a matching real user ID
[{-u
userid
| --user
userid
}]: Show information for
processes with a matching effective user ID
[v]: Display: virtual memory format
[{w | -w}]: Display: wide output
[x]: Show information for processes that do not have
controlling TTYs
[X]: Display: Linux i386 format
[-y]: Don’t show flags
shutdown
Shut down the system. Must be logged in as root.
shutdown [-a] [-c] [-f] [-F] [-h] [-n] [-k] [-r] [-t
seconds
]
time
[

message
]
[-a]: Use /etc/shutdown.allow.
[-c]: Cancel an in-progress shutdown.
[-f]: Skip the file system check upon reboot.
[-F]: Force a file system check upon reboot.
[-h]: Halt the system after the shutdown is complete.
[-n]: Go down fast and don’t use ‘init’.
[-k]: Only warn about a pending shutdown but don’t
actually do it.
[-r]: Reboot the system after the shutdown.
[-t
seconds
]: The number of seconds to wait before
sending the kill signal.
time
: The time to shutdown (ie - ‘now’)
[
message
]: The warning message to send to attached
users.
sudo
Run a command as another user.
sudo [-k] | [-K] | [-l] | [-v] | [-V] | [-b] [-H] [-P] [-S]
[-p
prompt
] [-u
username
| #
uid

] [-s] | [
command
]
[-k]: “kill”: invalidates a user’s timestamp.
[-K]: “sure kill”: remove a user’s timestamp.
[-l]: Shows the allowed commands for the user.
[-v]: Validate the user’s timestamp.
[-V]: Version
[-b]: Runs the specified command in the background.
[-H]: Sets the HOME variable to match the user specified.
[-P]: Preserve the user’s group vector.
[-S]: Causes sudo to read password from file rather than
terminal.
[-p
prompt
]: Use a custom password prompt.
[-u
username
| #
uid
]: The user or user ID.
[-s]: Runs the shell specified in the SHELL variable.
[
command
]: The command to run.
top
top [b] [c] [C] [d
delay
] [i] [p
pid

] [q] [s] [S]
[n
iterations
]
[b]: Batch mode. Use with n to specify the number of
iterations.
[c]: Display command line rather than command name.
[C]: SMP systems: display total CPU statistics.
[d
delay
]: The delay between screen updates.
[i]: Ignore idle and zombie processes.
[p
pid
]: Monitor processes with the specified process ID.
This parameter can be specified 20 times.
[q]: Refreshes without delay.
[s]: Run in secure mode.
[S]: Show process utilization as a cumulative figure
building on the previous process.
[n
iterations
]: Use with b to run a certain number of
times.
useradd
Add a new user to the system. Must be logged in as root.
useradd [-A {
method
| DEFAULT},...] [-c
comment

]
[-d
homedir
] [-e expiredate] [-f
inactivetime
]
[-g
initialgroup
] [-G
group
[...]] [-m [-k
skeletondir
]]
[-n] [-r] [-s
shell
] [-u
uid
[-o]]
login
[-A {
method
| DEFAULT},...]: The name of the program
that will authenticate the user.
[-c
comment
]: A comment to associate with the user
account.
[-d
homedir
]: The location of the user’s home directory.

SYSTEM
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