Short listing of directory contents

ls - Short listing of directory contents

-a        list hidden files

-d        list the name of the current directory

-F        show directories with a trailing '/'

            executable files with a trailing '*'

-g        show group ownership of file in long listing

-i        print the inode number of each file

-l        long listing giving details about files  and directories

-R        list all subdirectories encountered

-t        sort by time modified instead of name

cp - Copy files

cp  myfile yourfile

Copy the files "myfile" to the file "yourfile" in the current working directory. This command will create the file "yourfile" if it doesn't exist. It will normally overwrite it without warning if it exists.

cp -i myfile yourfile

With the "-i" option, if the file "yourfile" exists, you will be prompted before it is overwritten.

cp -i /data/myfile

Copy the file "/data/myfile" to the current working directory and name it "myfile". Prompt before overwriting the  file.

cp -dpr srcdir destdir

Copy all files from the directory "srcdir" to the directory "destdir" preserving links (-poption), file attributes (-p option), and copy recursively (-r option). With these options, a directory and all it contents can be copied to another dir

ln - Creates a symbolic link to a file.

ln -s test symlink

Creates a symbolic link named symlink that points to the file test Typing "ls -i test symlink" will show the two files are different with different inodes. Typing "ls -l test symlink" will show that symlink points to the file test.

locate - A fast database driven file locator.

slocate -u

This command builds the slocate database. It will take several minutes to complete this command.This command must be used before searching for files, however cron runs this command periodically  on most systems.locate whereis Lists all files whose names contain the string "whereis". directory.

more - Allows file contents or piped output to be sent to the screen one page at a time

less - Opposite of the more command

cat - Sends file contents to standard output. This is a way to list the contents of short files to the screen. It works well with piping.

whereis - Report all known instances of a command

wc - Print byte, word, and line counts

bg

bg jobs Places the current job (or, by using the alternative form, the specified jobs) in the background, suspending its execution so that a new user prompt appears immediately. Use the jobs command to discover the identities of background jobs.

cal month year - Prints a calendar for the specified month of the specified year.

cat files - Prints the contents of the specified files.

clear - Clears the terminal screen.

cmp file1 file2 - Compares two files, reporting all discrepancies. Similar to the diff command, though the output format differs.

diff file1 file2 - Compares two files, reporting all discrepancies. Similar to the cmp command, though the output format differs.

dmesg - Prints the messages resulting from the most recent system boot.

fg

fg jobs - Brings the current job (or the specified jobs) to the foreground.

file files - Determines and prints a description of the type of each specified file.

find path -name pattern -print

Searches the specified path for files with names matching the specified pattern (usually enclosed in single quotes) and prints their names. The find command has many other arguments and functions; see the online documentation.

finger users - Prints descriptions of the specified users.

free  - Displays the amount of used and free system memory.

ftp hostname

Opens an FTP connection to the specified host, allowing files to be transferred. The FTP program provides subcommands for accomplishing file transfers; see the online documentation.

head files - Prints the first several lines of each specified file.

ispell files - Checks the spelling of the contents of the specified files.

kill process_ids

kill - signal process_ids

kill -l

Kills the specified processes, sends the specified processes the specified signal (given as a number or name), or prints a list of available signals.

killall program

killall - signal program

Kills all processes that are instances of the specified program or sends the specified signal to all processes that are instances of the specified program.

mail - Launches a simple mail client that permits sending and receiving email messages.

man title

man section title - Prints the specified man page.

ping host - Sends an echo request via TCP/IP to the specified host. A response confirms that the host is operational.

reboot - Reboots the system (requires root privileges).

shutdown minutes

shutdown -r minutes

Shuts down the system after the specified number of minutes elapses (requires root privileges). The -r option causes the system to be rebooted once it has shut down.

sleep time - Causes the command interpreter to pause for the specified number of seconds.

sort files - Sorts the specified files. The command has many useful arguments; see the online documentation.

split file - Splits a file into several smaller files. The command has many arguments; see the online documentation

sync - Completes all pending input/output operations (requires root privileges).

telnet host - Opens a login session on the specified host.

top - Prints a display of system processes that's continually updated until the user presses the q key.

traceroute host - Uses echo requests to determine and print a network path to the host.

uptime - Prints the system uptime.

w - Prints the current system users.

wall - Prints a message to each user except those who've disabled message reception. Type Ctrl-D to end the message.

change file owner and group

chown - change file owner and group

Usage

chown [OPTION] OWNER[:[GROUP]] FILE

chown [OPTION] :GROUP FILE

chown [OPTION] --reference=RFILE FILE

Options

Change the owner and/or group of each FILE to OWNER and/or GROUP. With --reference, change the owner and group of each FILE to those of RFILE.

 -c, changes like verbose but report only when a change is made

 -dereference affect the referent of each symbolic link, rather than the symbolic link itself

 -h, no-dereference affect each symbolic link instead of any referenced file (useful only on systems that can         change the ownership of a symlink)

 -from=CURRENT_OWNER:CURRENT_GROUP

  change the owner and/or group of each file only if its current owner and/or group match those specified here.  Either  may  be  omitted,  in which case a match is not required for the omitted attribute.

-no-preserve-root do not treat `/' specially (the default)

-preserve-root fail to operate recursively on `/'

-f, -silent, -quiet  suppress most error messages

-reference=RFILE use RFILE's owner and group rather than the specifying OWNER:GROUP values

-R, -recursive operate on files and directories recursively

-v, -verbose output a diagnostic for every file processed

The  following options modify how a hierarchy is traversed when the -R option is also specified. If more than one is specified, only the final one  takes effect.

-H     if a command line argument is a symbolic link to a directory, traverse it

-L     traverse every symbolic link to a directory encountered

-P     do not traverse any symbolic links (default)

chmod - change file access permissions

Usage

chmod [-r] permissions filenames

 r  Change the permission on files that are in the subdirectories of the directory that you are currently in.        permission  Specifies the rights that are being granted. Below is the different rights that you can grant in an alpha  numeric format.filenames  File or directory that you are associating the rights with Permissions

u - User who owns the file.

g - Group that owns the file.

o - Other.

a - All.

r - Read the file.

w - Write or edit the file.

x - Execute or run the file as a program.

Numeric Permissions:

CHMOD can also to attributed by using Numeric Permissions:

400 read by owner

040 read by group

004 read by anybody (other)

200 write by owner

020 write by group

002 write by anybody

100 execute by owner

010 execute by group

001 execute by anybody

Basic Linux Commands

mkdir - make directories

Usage

mkdir [OPTION] DIRECTORY

Options

Create the DIRECTORY(ies), if they do not already exist.

 Mandatory arguments to long options are mandatory for short options too.

 -m, mode=MODE  set permission mode (as in chmod), not rwxrwxrwx - umask

 -p, parents  no error if existing, make parent directories as needed

 -v, verbose  print a message for each created directory

 -help display this help and exit

 -version output version information and exit

cd - change directories

Use cd to change directories. Type cd followed by the name of a directory to access that directory.Keep in mind that you are always in a directory and can navigate to directories hierarchically above or below.

mv- change the name of a directory

Type mv followed by the current name of a directory and the new name of the directory.

 Ex: mv testdir newnamedir

pwd - print working directory

will show you the full path to the directory you are currently in. This is very handy to use, especially when performing some of the other commands on this page

 rmdir - Remove an existing directory

 rm -r - Removes directories and files within the directories recursively.

Important Bash Shell Variables

These variables are most often inherited or declared when a shell is started. A great reference for bash shell variable, bash builtin commands, and bash in general is SSC (2000).

• HOME - home directory, abbreviated as ~
• MAIL - name of file that mail is stored in (mailbox)
• MAILCHECK - sets the frequency at which bash checks for mail
• PATH - directory paths to search for executable files. According to A Practical Guide to Linux, p.329, the PATH is set in /etc/profile. On my Linux box, /etc/profile adds /usr/X11R6/bin to the path, which means that the path must be set before by another configuration file. The .bash_profile file in my home directory adds /home/ambler/bin to the path. On my Linux box, the first command to set the path would seem to be in /etc/rc.d/rc.sysinit, which is one of the shell scripts invoked by the init process (inittab). Then, /etc/profile adds /usr/X11R6/bin. Finally, .bash_profile adds /home/ambler/bin.
• PS1 - prompt string. Things that can be put in the prompt string include \h (hostname), \u (username), \w (absolute pathname of working directory), \W (name of working directory w/o path), \d (date), \t (time). See p.331 of A Practical Guide to Linux for more details. On my Red Hat boxes, the primary prompt string is set in the /etc/bashrc file. The prompt is also set in /etc/profile, but the setting in bashrc seems to take precedence. I also have a .bashrc file, which in turn runs /etc/bashrc, which sets the prompt. This means that the same prompt is used by xterm and rxvt in X sessions. On my Slackware box, the command line prompt is set in /etc/profile. The xterm and rxvt prompts are different, since I don't have a .bash_profile file which is run when an xterm is started. In fact, I don't know where the prompt is set in X sessions. It is not set by /usr/X11R6/lib/X11/app-defaults/XTerm.
• PS2 - secondary prompt string.

Important Directories

Different distributions have different directory structures, despite attempts at standardization such as the the Linux Filesystem Hierarchy Standard (FHS) organization.

• /bin - essential UNIX commands such as ls, etc. Should contain all binaries needed to boot the system or run it in single-user mode
• /boot - files used during booting and possibly the kernel itself are stored here
• /dev - contains device files for various devices on system
• /etc - files used by subsystems such as networking, NFS, and mail. Includes tables of disks to mount, processes to run on startup, etc.
• /etc/profile.d - contains scripts that are run by /etc/profile upon login.
• /etc/rc.d - contains a number of shell scripts that are run on bootup at different run levels. There is also typically an rc.inet1 script to set up networking (in Slackwar), an rc.modules script to load modular device drivers, and an rc.local script that can be edited to run commands desired by the administrator, along the lines of autoexec.bat in DOS.
• /etc/rc.d/init.d - contains most of the initialization scripts themselves on an rpm-based system.
• /etc/rc.d/rc*.d - where ``*'' is a number corresponding to the default run level. Contains files for services to be started and stopped at that run level. On rpm-based systems, these files are symbolic links to the initialization scripts themselves, which are in /etc/rc.d/init.d.
• /etc/skel - directory containing several example or skeleton initialization shells. Often contains subdirectories and files used to populate a new user's home directory.
• /etc/X11 - configuration files for the X Window system
• /home - home directories of individual users
• /lib - standard shared library files
• /lib/modules - modular device driver files, most with .o extensions
• /mnt - typical mount point for many user-mountable devices such as floppy drives, cd-rom readers, etc. Each device is mounted on a subdirectory of /mnt.
• /proc - virtual file system that provides a number of system statistics
• /root - home directory for root
• /sbin - location of binaries used for system administration, configuration, and monitoring
• /tmp - directory specifically designed for programs and users to store temporary files.
• /usr - directory containing a number of subdirectory with programs, libraries, documentation, etc.
• /usr/bin - contains most user commands. Should not contain binaries necessary for booting the system, which go in /bin. The /bin directory is generally located on the same disk partition as /, which is mounted in read-only mode during the boot process. Other filesystems are only mounted at a later stage during startup, so putting binaries essential for boot here is not a good idea.
• /usr/bin/X11 - most often a symbolic link to /usr/X11R6/bin, which contains executable binaries related to the X Window system
• /usr/doc - location of miscellaneous documentation, and the main location of program documentation files under Slackware
• /usr/include - standard location of include files used in C programs such as stdio.h
• /usr/info - primary location of the GNU info system files
• /usr/lib - standard library files such as libc.a. Searched by the linker when programs are compiled.
• /usr/lib/X11 - X Window system distribution
• /usr/local/bin - yet another place to look for comon executables
• /usr/man - location of manual page files
• /usr/sbin - other commands used by superuser for system administration
• /usr/share - contains subdirectories where many installed programs have configuration, setup and auxiliary files
• /usr/share/doc - location of program documentation files under Mandrake and Red Hat
• /usr/src - location of source programs used to build system. Source code for programs of all types are often unpacked in this directory.
• /usr/src/linux - often a symbolic link to a subdirectory whose name corresponds to the exact version of the Linux kernel that is running. Contains the kernel sources.
• /var - administrative files such as log files, used by various utilities
• /var/log/packages - contains files, each of which has detailed information on an installed package in Slackware. The same file can also be found at /var/adm/packages, since the adm subdirectory is a symbolic link to log. Each package file contains a short description plus a list of all installed files.
• /var/log/scripts - package installation scripts in Slackware are stored here. You can inspect these scripts to see what special features are included in individual packages.
• /var/spool - temporary storage for files being printed, mail that has not yet been picked up, etc.

Useful Files

• /boot/vmlinuz - the typical location and name of the Linux kernel. In the Slackware distribution, the kernel is located at /vmlinuz.
• /dev/fd0 - first floppy disk drive
• /dev/fd0H1440 - driver for the first floppy drive in high density mode. Generally, this is invoked when formatting a floppy drive for a particular density. Slackware comes with drivers that allow for formatting a 3.5" diskette with up to 1.7MB of space. Red Hat and Mandrake do not contain these device driver files by default.
• /dev/fd1 - second floppy disk drive
• /dev/hda - first IDE hard drive
• /dev/hdc - on many machines, the IDE cdrom drive. Most often, there is a symbolic link called /dev/cdrom which is just a link to the true cdrom driver file.
• /dev/null - used when you want to send output into oblivion
• /etc/aliases - file containing aliases used by sendmail and other MTAs (mail transport agents). After updating this file, it is necessary to run the newaliases utility for the changes to be passed to sendmail.
• /etc/bashrc - system-wide default functions and aliases for the bash shell
• /etc/conf.modules - aliases and options for configurable modules
• /etc/crontab - shell script to run different commands periodically (hourly, daily, weekly, monthly, etc.)
• /etc/DIR_COLORS - used to store colors for different file types when using ls command. The dircolors command uses this file when there is not a .dir_colors file in the user's home directory. Used in conjunction with the eval command (see below).
• /etc/exports - specifies hosts to which file systems can be exported using NFS. Man exports contains information on how to set up this file for remote users.
• /etc/fstab - contains information on partitions and filesystems used by system to mount different partitions and devices on the directory tree
• /etc/HOSTNAME - stores the name of the host computer
• /etc/hosts - contains a list of host names and absolute IP addresses.
• /etc/hosts.allow - hosts allowed (by the tcpd daemon) to access Internet services
• /etc/hosts.deny - hosts forbidden (by the tcpd daemon) to access Internet services
• /etc/group - similar to /etc/passwd but for groups
• /etc/inetd.conf - configures the inetd daemon to tell it what TCP/IP services to provide (which daemons to load at boot time). A good start to securing a Linux box is to turn off these services unless they are necessary.
• /etc/inittab - runs different programs and processes on startup. This is typically the program which is responsible for, among other things, setting the default runlevel, running the rc.sysinit script contained in /etc/rc.d, setting up virtual login terminals, bringing down the system in an orderly fashion in response to [Ctrl][Alt][Del], running the rc script in /etc/rc.d, and running xdm for a graphical login prompt (only if the default runlevel is set for a graphical login).
• /etc/issue - pre-login message. This is often overwitten by the /etc/rc.d/rc.S script (in Slackware) or by the /etc/rc.d/rc.local script (in Mandrake and Red Hat, and perhaps other rpm-based distributions). The relevant lines should be commented out (or changed) in these scripts if a custom pre-login message is desired.
• /etc/lilo.conf - configuration file for lilo boot loader
• /etc/motd - message of the day file, printed immediately after login. This is often overwritten by /etc/rc.d/rc.S (Slackware) or /etc/rc.d/rc.local (Mandrake/Red Hat) on startup. See the remarks in connection with /etc/issue.
• /etc/mtab - shows currently mounted devices and partitions and their status
• /etc/passwd - contains passwords and other information concerning users who are registered to use the system. For obvious security reasons, this is readable only by root. It can be modified by root directly, but it is preferable to use a configuration utility such as passwd to make the changes. A corrupt /etc/passwd file can easily render a Linux box unusable.
• /etc/printcap - shows the setup of printers
• /etc/profile - sets system-wide defaults for bash shell. It is this file in Slackware that sets up the DIR_COLORS environment variable for the color ls command. Also sets up other system-wide environment variables.
• /etc/resolv.conf - contains a list of domain name servers used by the local machine
• /etc/securetty - contains a list of terminals on which root can login. For security reasons, this should not include dialup terminals.
• /etc/termcap - ASCII database defining the capabilities and characteristics of different consoles, terminals, and printers
• /etc/X11/XF86Config - X configuration file. The location in Slackware is /etc/XF86Config.
• /proc/cpuinfo - cpu information
• /proc/filesystems - prints filesystems currently in use
• /proc/interrupts - prints interrupts currently in use
• /proc/ioports - contains a list of the i/o addresses used by various devices connected to the computer
• /proc/kcore - The command ls -l /proc/kcore will give the amount of RAM on the computer. It's also possible to use the free command to get the same information (and more).
• /proc/version - prints Linux version and other info
• /var/log/messages - used by syslog daemon to store kernel boot-time messages
• /var/log/lastlog - used by system to store information about last boot
• /var/log/wtmp - contains binary data indicating login times and duration for each user on system