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My CompTIA Linux+ LX-103 Study Guide

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These are my notes as I prepare to take the CompTIA Linux+ LX-103 Exam.

[Source: http://linuxacademy.com]

System Architecture

SECTION 1

Determine and Configure Hardware Settings

  • modprobe -- auto load/unload dependent kernel modules. preferred to insmod and rmmod.
  • lsusb -- usb bus info; reads /dev/bus/usb dir for config and contents.
  • /proc -- virtual system info.
  • /proc/interrupts -- show IRQ's in use.
  • /proc/dma -- show DMA addresses in use.
  • DMA's -- alternative method to communicate with I/O ports.
  • lsmod -- list modules loaded in the running kernel.
  • insmod -- old method to insert kernel modules.
  • rmmod -- old method to remove kernel modules.

SECTION 2

Boot the System

  • dmesg -- extract info about boot process.
  • kernel -- main part of OS. runs threads, devices, filesystems, video, etc.
  • BIOS -- basic input output system. prepare system for boot. find CPU, mem, disk.
  • EFI/UEFI -- new systems use instead of BIOS. (? abbr.)
  • boot process 1 -- CPU starts then runs BIOS.
    • boot process 2 - BIOS checks for and configures hardware, looks for boot sector on disk.
    • boot process 3 -- BIOS hands off to GRUB boot loader.
    • boot process 4 -- GRUB finds kernel, loads into memory, executes it.
    • boot process 5 -- GRUB hands off to kernel. kernel performs system tasks, loads init from /sbin/init.
    • boot process 6 -- init given PID of 1.
  • /var/log/messages -- global system messages. contains post-boot messages. also: messages, errors, and info from apps that communicate with kernel, modules, drivers.
  • dmesg -- /var/log/messages generally. /var/log/dmesg on some distros (?). contains kernel ring buffer info. shows messages about devices that kernel detected during bootup.

SECTION 3

Change Runlevels and Shutdown or Reboot the System

  • etc/inittab -- sysvinit systems file for setting default run level. systemd redirects this to lib/systemd/system/<target-name>.target.
  • shutdown -k -- warn of impending shutdown but doesn't shutdown.
  • shutdown -r -- reboot after shutdown.
  • shutdown -h -- halt and power-off.
  • `shutdown -P -- (?) half-action to turn off power.
  • shutdown -f -- skip fsck on next boot.
  • shutdown -F -- force fsck on next boot.
  • shutdown -c -- cancel a pending shutdown.
  • shutdown -t -- tell init to wait X # of seconds before shutting down.
  • init -- change run level or shutdown. last process to run during boot.
  • run level 0 -- shut down.
  • run level 1, s, S -- single-user mode for low-level system maintenence: e.g. resizing partitions.
  • run level 2 -- debian multi-user mode with X running and graphical login. undefined on other distros.
  • run level 3 -- RHEL/fedora/mandriva/etc. multi-user mode with console non-graphical login.
  • run level 4 -- undefined; customizable.
  • run level 5 -- RHEL/fedora/mandriva multi-user mode with X and graphical login.
  • run level 6 -- reboot/transitional.
  • init 1 -- change to run level 1.
  • id:2:initdefault -- set to default run level 2 in /etc/inittab.
  • N 2 - previous:current run level (?).
  • id:runlevels:action:process -- (?).
  • id -- 1-4 character sequence that idenfifies a function (?).
  • runlevels -- query run levels for a given entry (?).
  • action(?)wait(?) -- tells init to start process when entering specified run level and wait for process to terminate.
  • respawn -- tell init to restart the process whenever it terminates.

Linux Installation and Package Management

SECTION 4

Design a Hard Disk Layout or Partitioning Scheme

  • /var -- contains files that change frequently.
  • /home -- user directories.
  • partitions -- /var and /home could be on separate partitions.

SECTION 5

Install a Boot Manager

  • Type codes:
    • 0x0c - FAT.
    • 0x05 - deprecated extended partition type.
    • 0x07 - NTFS.
    • 0x0f - new extended partition type.
    • 0x82 - Linux swap.
    • 0x83 - Linux file system.
  • GRUB1 config: /boot/grub/menu.list.
  • GRUB2 config: /boot/grub/grub.cfg.

SECTION 6

Manage Shared Libraries

  • ldd -- print shared library dependencies for a given program/file.
  • LD_LIBRARY_PATH -- bash env variable for additional library locations to search when looking for shared libraries.
  • ldconfig -- clear library cache after adding library locations to /etc/ld.so.conf.

SECTION 7

Use Debian Package Management

  • /etc/apt/sources.list -- list of repositories to search for system-compatible software for install.
  • apt-get -- package handling and installation for debian-based distros. install by name; include dependent packages.
  • dpkg -- install .deb package files in debian-based systems.
    • -i -- install and configure.
    • -r -- remove package.
    • -configure -- configure package.
    • -c -- list contents of package.
    • -s -- list status of packages (installed or not).
  • apt-cache -- search packages named or installed.
  • apt-cache [pkgnames] -- show installed.
  • apt-cache search [pkgnames] -- search for named package (not-installed).
  • dpkg-reconfigure -- reconfigure already installed packages
  • aptitude -- high-level package management interface for debian-based distros.

SECTION 8

Use RPM and YUM Package Management

  • prm -- package installation utility for red hat distros.
    • -nodeps -- install regardless of installed dependencies.
    • -i -- install.
    • -K -- check package signature.
    • -V -- verify package.
    • -a -- all packages.
      • -Va -- verify all packages.
    • -U -- upgrade or install new package
    • -F -- upgrade installed packaged (only if it exists)
    • -q -- query a package. determine if installed or not.
    • -e -- erase/uninstall.
    • -f -- query package owning file (?).
    • -p -- package (named).
    • -l -- list files in package.
    • -rebuild -- rebuild source package.
    • -rebuilddb -- rebuild rpm database.
    • -qa -- print all installed packages.
    • ql -- list files in an installed package.
    • qf -- determine which installed package a file belongs to.
    • -qpl -- list all files in an RPM package.
    • -checksig -- same as -K; check package signature.
  • /etc/yum.repos.d -- directory containing yum repository files.
  • cpio -- create CPIO archive (?).
    • -d -- create leading directories where needed.
    • -i -- extract.
    • -u -- replace all files without asking.
    • -m -- retain previous modification times when creating files.
  • rpm2cpio -- convert RPM packages to CPIO compressed files. extract files from RPM package without installing it.
    • rpm2cpio file.rpm | cpio -dium

GNU and Unix Commands

SECTION 9

Working on the Command Line

  • make -- compile source code, install binaries.
  • env -- show current session environment variables.
  • bash -- bourne again shell. command language shell interpreter. executes commands from standard input or from files.
  • uname -- prints system information.
    • -n -- node name
    • -s -- kernel name
    • -v -- kernel version
    • -r -- kernel release
    • -m -- machine/cpu info
    • -p -- processor info
    • -i -- hardware info
    • -o -- operating system name
    • -a -- print all info
  • export -- set an environment variable.
    • export HOME=/home/user -- set $HOME to specified directory in env.
  • unset $HOME -- unset/remove the $HOME variable from env.
  • man -- manual pages:
    • 1 -- executable programs and shell commands
    • 2 -- system calls (kernel functions)
    • 3 -- library calls (library program functions)
    • 4 -- special files (usually found in /dev)
    • 5 -- file formats and conventions, e.g. /etc/passwd
    • 6 -- games
    • 7 -- miscellaneous, including macro packages and conventions
    • 8 -- system administration commands (root)
    • 9 -- kernel routines [non-standard]

SECTION 10

Process Text Streams Using Filters

  • cat -- concatentate/combine files, display a single file.
  • fmt -- reformat streams or files for display, e.g. width of columns.
  • join -- join file lines based on delimiters:
    • -t -- delimiter as specified
    • -1 [field] -- join on given field of file #1
    • -2 [field] -- join on given field of file #2
  • pr -- convert file for printing. adds header, footer, and page breaks.
  • split -- split a file into pieces:
    • split -l 2 file.txt newname -- split every two lines and name newnamea, newnameb, etc.
    • (?) -- default -b split 512 bytes (?).
  • unexpand -- convert spaces to tabs.
  • expand -- convert tabs to spaces.
  • cut -- remove sections from file lines using delimiters and fields.
  • head -- display only the first part of a file.
    • -c [X] -- display only the first [X] bytes.
    • -n [X] -- display only the first [X] lines.
  • nl -- add line numbers to file for display or redirect purposes.
  • sed -- stream editor for filtering and formatting text.
    • sed 's/ugly/beautiful/g' myfile.txt -- replace all instances of 'ugly' with 'beautiful' in myfile.txt.
  • regex -- regular expressions.
    • GNU tool under POSIX standard
    • includes sed, awk, grep, egrep, ed, etc.
    • [ \t] -- match 'white space tab`
    • [ \t\r\n] -- match 'line break'
    • ([A-Z]) -- match alpha character range in caps
  • tail -- display only the last part of a file.
    • -c [X] -- display only the last [X] bytes.
    • -n [X] -- display only the last [X] lines.
  • uniq -- report or omit repeated lines.
    • -d -- only print duplicate lines
    • -u -- only print unique lines
    • -r -- recursive (?)
  • od -- dump files in octal format (?).
  • paste -- merge lines of given files.
  • sort -- sort file contents forward or reverse by alphanumeric characters.
  • tr -- translate/squeeze/delete characters from standard input writing to output (?).
    • tr test [texttotrim] < test -- send contents of test file to standard input and trim out 'texttotrim'.
  • wc -- perform a word or character count on given file.

SECTION 11

Perform Basic File Management

  • cp -- copy files and directories.
    • -a -- archive
    • -backup -- back up each destination file
    • -d -- preserve links (?)
  • mv -- move files and directories
    • -f -- do not prompt for overwrite
    • -i -- prompt for overwrite; interactive mode
    • -n -- no clobber; prevent overwrite of existing files
  • touch -- change file date/time attributes
    • -a -- update access time only
    • -c -- do not create if file doesn't exist
    • -d -- parse string and use string instead of current time (?)
    • -m -- update modification time only
    • -r -- use given file's times instead of current time (?)
    • -t -- use specified timestamp
  • dd -- copy file, converting and formatting according to operands.
  • gzip/gunzip -- compression utility. used with tar (for archiving).
    • -d -- decompress
    • -f -- force
    • -h -- help
    • -l -- list contents
    • -q -- quiet mode
    • -t -- test but don't perform operation
    • -v -- verbose mode
  • mkdir -- make directory
    • -p -- create all directories in the given parent chain, e.g. /dir/subdir/subsubdir
  • tar -- tape archive utility for backups
    • -d -- show differences between archive and source file system
    • --delete -- delete specified contents/files from archive
    • -c -- create new archive
    • -A -- append tar files to existing archive
    • -r -- append files to existing archive
    • -u -- append only newer versions of files to existing archive
    • -t -- list archive contents
    • -x -- extract files from archive
    • -z -- filter archive through gzip
    • -j -- filter archive through bzip2
    • -J -- filter archive through xz compression tool
  • file -- determine file type
  • bzip2 -- compression utility. used with tar (for archiving).

SECTION 12

Use Streams, Pipes, and Redirects

  • && -- run [command1] and then [command2]. e.g. cd .. && ls moves up one level then lists directory contents.
  • tee -- save standard input to file and also send to standard output.
  • xargs -- build and execute command with multiple arguments
    • -a -- reads items from file instead of standard input
    • -0 -- input items are terminated by null character instead of by white spaces (?)
    • -d -- input items are terminated by specified character

SECTION 13

Create, Monitor, and Kill Processes

  • jobs -- display basic info about running processes associated with the current session
  • ps -- display processes run (by default) from owner's terminal.
    • -Ae -- display all users' processes
    • -u -- display processes of specified user
    • (?) H -F -- group processes together and show hierarchy of relationships
    • ps -w > ps.txt -- does not truncate results; sends output to given file
  • uptime -- find uptime and display load average
  • bg -- restores a job to the running status, in the background.
  • fg -- use Ctrl+Z to pause a program; fg to send to foreground
  • kill -- stop running processes based on process ID (PID).
  • nohup -- run a command immune to hangups, with output to console or non-TTY.
  • killall -- kill all processes sharing a given name
  • free -- show free memory and swap space
  • common kill signals:
    • SIGHUP 1 HANGUP -- (?)
    • SIGINT 2 INTERRUPT FROM KEYBOARD -- (?)
    • SIGKILL 9 KILL SIGNAL -- abrubtly terminate the program
    • SIGTERM 15 TERMINATION SIGNAL -- asks program to finish and then exist. preferred method over sigkill
    • SIGSTOP 17,19,23 STOP THE PROCESS -- when child process exits from parent process is sends a SIGHUP 1
    • man -k signal -- signal's man page

SECTION 14

Modify Process Execution Priorities

  • nice -- run a program with a modified scheduling priority.
  • renice -- alter the priority of a running process; -20 highest, 19 lowest.
  • top -- display running processes dynamically.
    • k -- kill process
    • q -- quit top
    • r -- renice a process
    • s [X]/d [X] -- change update delay to every [X] seconds
    • P -- sort by CPU usage
    • M -- sort by memory usage
    • -p [PID],[PID],[PID] -- display up to 20 specific processes (command line only)
    • -n [X] -- update [X] times and then quit
    • -b -- run in batch mode. for output to other files. non-interactive. command line only.

SECTION 15

Search Text Files Using Regular Expressions

  • grep -- search using regular expressions
    • -v -- find lines NOT matching given string/parameters
    • -i -- ignore case
    • [^abc] -- match any character not contained in brackets
    • ^example -- match lines starting with 'example'
    • example$ -- match lines ending with 'example'
    • * -- match the preceding element 0 or more times. e.g. ab*c matches ac, abc, and abbbbc.
    • + -- match the preceding element 0 or more times
  • egrep -- extended regular expressions. same as grep -e.
  • fgrep -- search using fixed strings (more literal). find any number of lines given as search strings/parameters. same as grep -f.

SECTION 16

Perform Basic File Editing Operations Using vi

  • vi/vim -- command line text editor
    • i -- insert mode. insert text left of the cursor.
    • I -- insert text at beginning of line
    • a -- append text to the right of the cursor
    • o -- insert text on new line below last line of file
    • O -- insert text on new line above first line of file
    • /word -- find next occurrence of 'word'
    • n -- repeat last search; find next occurrence
    • ?word -- find previous occurrence of 'word'
    • c -- change text under cursor to new word typed (?)
    • d -- delete text that {motion} move over
    • [X]dd -- delete [X] number of lines
    • y -- yank (copy) text that {motion} passes over (to be pasted elsewhere later)
    • [X]yy -- yank [X] number of lines
    • p -- put/paste text copied previously (after the cursor)
    • P -- put/paste text copied previously (before the cursor)
    • h -- move LEFT
    • j -- move DOWN
    • k -- move UP
    • l -- move RIGHT
    • 2h -- move LEFT 2 spaces
    • ZZ -- save and quit
    • wq! -- save and force quit
    • q! -- quit without saving
    • e! -- discard changes and edit original again

Devices, Linux Filesystems, and the Filesystem Hierarchy Standard

SECTION 17

Create Partitions and Filesystems

  • fdisk -- partition a disk
    • -n -- interactive mode
  • mkfs -- build a filesystem
    • -t [type] -- specify filesystem type: ext2/3/4, FAT, btrfs, etc.
    • -c -- check device for bad blocks before building
  • mkswap -- create swap partition on existing filesystem.
    • swapon/swapoff -- turn swap on or off

SECTION 18

Maintain the Integrity of Filesystems

  • du -- estimate file space usage on a disk
    • -c -- show grand total
    • -h -- human-readable format
    • --max-depth [X] -- print the total for given directory only down to [X] number of levels
    • -b -- display size in bytes
  • dumpe2fs -- obtain filesystem information
  • xfs_info -- obtain filesystem information for xfs filesystems
  • df -- display filesystem usage, across filesystems.
    • df . -- show mount location of current working directory
    • shows only total usage of each filesystem
  • mke2fs -- create an ext2/3/4 filesystem.
  • tune2fs -- change filesystem parameters, fs cannot be mounted.
    • maximum mount count
    • time between checks
    • add a journal
    • set reserved blocks
    • obtain filesystem level (?)
  • fsck -- check and repair a filesystem
    • -A -- try to check all filesystems listed in /etc/fstab
    • -a -- attempt to automatically repair all errors (?)
    • -C -- display completion/progress
    • -N -- don't execute; simulate what would be done instead
  • debugfs -- modify filesystem interactively
    • undelete a file from an unmounted file system with inode number

SECTION 19

Control Mounting and Unmounting of Filesystems

  • mount -- attempt to mount filesystems to specified directories
    • -a -- mount all filesystems listed in etc/fstab as indicated
    • -o -- override mount options listed in etc/fstab during manual mount
  • unmount -- unmount given filesystem(s)
    • -f -- force unmount even if busy
  • etc/fstab -- persistent filesystem mounting configuration file (for mounting on boot).

SECTION 20

Manage Disk Quotas

  • quotaon -- turn quotas on
  • edquota -- edit a given quota
    • -p -- duplicate/copy the quotas of an existing user quota to new users
  • repquota -- summarize quotas for a given filesystem
    • -a -- print quotas of all filesystems configured with a quota mount
    • -g -- print only group quotas
    • -v -- print a header line before printing each filesystem quota (?)

SECTION 21

Manage File Permissions and Ownership

  • setuid/setgid -- set user and group ID. allow to run executables as if they were the owners of those files/programs.
    • chmod 2711 -- sets group id
    • chmod 4711 -- sets user id
    • chmod 6711 -- sets group and user id
    • u+s -- set user ID, octal bit '4'
    • g+s -- set group ID, octal bit '2'
    • +t -- set sticky bit, octal bit '1'
  • chmod -- change file/directory access attributes
    • 1 -- execute
    • 2 -- write
    • 4 -- read
  • chgrp -- change group ownership of files/directories
  • umask - change default user level permissions for files/directories when created
  • chown -- change ownership of files and directories
  • -R -- recursion switch for chown, chgrp, chmod, etc.

SECTION 22

  • ln -- create link between files and directories. hard link by default.
    • -s -- soft link
    • if you remove the source file on a hard link the link file will still exist

SECTION 23

Find System Files and Place Files in the Correct Location

  • find -- find files on local or any mounted filesystem
  • which -- display the path of the indicated command. must be in $PATH variable.
  • whereis -- locate sources, binaries, and manual-sections for specific files.
  • /etc/updatedb.conf -- location database configuration file
  • locate -- find files by name. uses location database /etc/updatedb.conf.
  • FHS -- filesystem hierarchy standard.
    • unified directory trees -- all files, devices, and disks accessible from single directory tree
    • system files include: startup scripts, daemons, program files, binaries, support files, config files, databases, log files, /etc/shadow for encrypted passwords
    • system files and programs use system accounts for access
    • /home/user -- default user file storage location
    • shareable files -- shared between computers, includes data and binaries, may be placed on Network File System (NFS)
    • unshareable files -- contain system-specific info and configurations
    • static files -- only change through sysadmin edits typically, e.g. program executables
    • variable files -- modified by users, scripts, servers, etc.
  • FHS common linux directories:
    • / -- root
    • /etc -- system config files
      • /etc/fstab -- define partition mount parameters
      • /etc/passwd -- primary account definition file
      • /etc/X11 -- 'X' windows system
      • /etc/samba -- samba file server configuration directory
    • /boot -- boot files, linux kernel, initramfs/initial RAM disk
    • /bin -- program files that users may run
    • /sbin -- program files seldom run by users
    • /lib -- libraries (code used by other programs)
    • /usr -- non-critical programs and data
    • /home -- user home directories. isolate on separate partition to protect user data.
    • /root -- root user home directory
    • /var -- transient, frequently modified files
      • /var/tmp -- temporary files; not to be deleted on reboot
    • /tmp -- temporary files to be deleted on reboot (ok to delete)
    • /mnt -- removable media mount point with subdirectories for each mounted filesystem
    • /media -- new(er) mount point for removable media, with subdirectories for mounted filesystems
    • /dev -- device files and system hardware directories
  • FHS access standards -- users can access /media and /mnt, but not most system directories.
  • executable directory program files -- stored in /sbin, /bin, /usr/bin, /usr/sbin, /usr/local/sbin, and /usr/local/bin.
  • library directories -- stored in /lib, /usr/lib, and /usr/local/lib.
  • example distribution of a single program's files -- executables in /usr/bin; libraries in /usr/lib; configuration files in /etc or /home/user.

2015 Objective Updates

SECTION 24

Upstart

  • Upstart -- replaces old SysVInit daemon init program.
    • can start jobs/services asynchronously.
    • can listen for and execute jobs based on system events.
    • config in /etc/init.
    • backwards compatible with run levels and SysVInit.
    • service -- command to manage jobs.
  • Upstart new commands:
    • initctl list -- list jobs from /etc/init.
    • init-checkconfig /etc/file.conf -- check job config syntax.
    • start [jobname] -- start a job.
    • stop [jobname] -- stop a job.
    • restart [jobname] -- restart a job.
    • job management backwards compatible with service command.

SECTION 25

Systemd

  • Systemd -- system-wide daemon-management daemon.
  • first daemon to start at boot; last daemon to stop at shutdown.
  • allows for the running of proc/daemon/services in parallel during bootup, etc.
  • start daemons on-demand; SysVInit was sequencial (one-by-one) and limited.
  • controls shutdown, cron, journald, and other low-level components.
  • more utilities to be added to systemd over time.
  • journald -- new systemd daemon for event logging; by default non-persistent; persistence can be enabled.
  • systemctl -- new command which replaces service.
    • also replaces chkconfig.
  • targets -- replaces run levels.
  • systemd PID = 1.
  • journalctl -- query systemd journal

SECTION 26

Systemd Commands and Directories

  • systemctl restart httpd.service -- restart httpd.
  • systemctl enable httpd.service -- enable to start at boot.
  • systemctl stop httpd.service -- stop httpd.
  • systemctl isolate graphical.target -- move into graphical target.
  • systemctl set-default graphical.target -- make graphical the default target to boot into.
  • systemctl get-default -- get current default target setting.
  • /usr/lib/systemd/system -- core system unit config file directory.
  • /etc/systemd/system -- custom config directory. overridden by like-configurations in system unit config library.
  • systemctl -t help -- display available unit types on system.
  • systemctl list-units --type=target | grep target -- show all active (enabled) targets.
  • systemctl list-unit-files --type=target -all -- show all available targets, enabled or disabled.
  • default target symlink -- goes from /etc/systemd/system/default.target to target config file (?).
  • wall -- broadcast message to all logged in users.
    • -n -- suppress 'from user' notification.
    • type message, end with ctrl+d to send message.

SECTION 27

Managing MBR and GPT Partitions

  • GPT -- 128 partitions max.
  • GPT -- 9.4ZB or 8ZiB max size.
  • MBR -- older partition format; 4 max.
  • MBR -- 2TiB max size.
  • fdisk -- manage MBR partition tables.
  • gdisk -- manage GPT partition tables.
  • parted -- manage GPT partition tables.

SECTION 28

Commands and Compression

  • screen -- full-screen window manager. multiplexes physical terminal between several processes.
    • -d -- start in detached mode. forks a new process.
    • -D -- start in detached mode. same process. command exists if session terminates (?).
    • -S -- session name.
    • -x -- attach to a session that is not detached.
    • -X -- send specified command to an already running screen session.
    • screen -list -- list all current screens.
    • screen -r [name] -- reattach named screen.
  • XZ compression:
    • better compression ratio than gzip.
    • more memory intensive.
    • xz -flag [filename] syntax.
      • -z -- compress
      • -d -- decompress/uncompress.
      • -l -- list compressed file info and compression ratio.
    • must first tar (archive) the directory.
  • pkill and pgrep
    • identify process based on regex/pattern
    • pgrep -u user -- list all processes from 'user'.
    • pgrep -l -u user -- list and display all processes from 'user' (?).
    • pkill -t ttyid -- kill all processes from specified terminal.
    • pgrep -v -u user -- show all processes not belonging to 'user'.
    • pgrep -n user -- show PID for most recent process from 'user'.
    • pgrep -l -u user sshd -- show all sshd processes from 'user'.
    • pkill httpd -- kill all httpd processes.
    • pkill -u user sshd -- kill all sshd processes from 'user'.
  • dmesg -- view messages in kernel ring buffer.
    • troubleshoot kernel, memory, boot issues.
    • dmesg > boot_messages -- output contents to 'boot_messages' file.
    • dmesg | grep -l tty -- output first 'tty' message in kernel ring buffer.
    • dmesg | grep memory -- output messages about memory.

SECTION 29

GRUB2

  • grub2-mkconfig > /boot/grub2/grub.cfg -- make or remake grub2 config file.
  • /boot/grub2/grub.cfg -- comprised of configuration files from /etc/default/grub and /etc/grub.d.
  • grub2-install /dev/device -- install or reinstall grub2 on specified device.

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