Basic Commands in Linux

From WikiGuruja

Contents 1 Getting Help 1.1 man 1.2 info 1.3 apropos 1.4 whatis 1.5 makewhatis 2 File System Utilities 2.1 ls 2.2 mkdir 2.3 fdisk 2.4 cd 2.5 pwd 2.6 chroot 2.7 cp 2.8 mv 2.9 rm 2.10 rmdir 2.11 shred 2.12 touch 2.13 df 2.14 ln 2.15 chown 2.16 chmod 3 Finding Files 3.1 find 3.2 whereis 3.3 which 3.4 locate 3.5 grep 4 Network 4.1 Setting up the network configuration 4.2 ifconfig 5 Others 5.1 loadkeys 5.2 TAR 5.3 RPM 5.4 shutdown

Getting Help

man

man displays the manual page for the specified command

A useful option is:

-k TEXT  search manual page titles and synopsis lines for TEXT

Examples:

To display the manual page for the chown command:

$ man chown

man has different sections.

• section 1 is user commands
• section 2 is system calls (used by programs to communicate with the kernel)
• section 3 is library reference (for programming in C)
• section 4 is device drivers
• section 5 is configuration files and other file formats
• section 6 is games
• section 7 is miscellaneous (for example, "ascii" map and C "operator" precedence)
• section 8 is system commands (like user commands, but mostly for root)

A section number can be specified before the page name. For example, man chmod normally shows the user command "chmod". To see the system call "chmod":

$ man 2 chmod

To search the man pages for "newsgroups",

$ man -k newsgroups
actsync              (8)  - synchronize newsgroupsoups
newsgroups           (1)  - a program to list unsubscribed newsgroups

• If this does not work you may have to run the makewhatis command.

info

info is an advanced man command that is sometimes available. It displays the improved manual pages in Info format for specified command.

Examples:

To display the manual page for the grep command:

$ info grep

To find occurrence of 'grep' in all info manual pages:

$ info --apropos grep
"(autoconf-2.13)Examining Declarations" -- EGREP_CPP
"(autoconf-2.13)Examining Declarations" -- EGREP_HEADER
"(autoconf-2.13)Old Macro Names" -- HEADER_EGREP
...

To see the physical location of 'grep' info manual page:

$ info -w grep
/usr/share/info/grep.info.gz

To view a file a info page:

$ info -f ./some_cmd.info.gz

apropos

apropos searches the manual page short descriptions for a specified keyword

On many systems this is exactly the same as the -k option of the man command.

Examples:

$ apropos newsgroups
active               (5)  - list of active Usenet newsgroups
newsgroups           (1)  - a program to list unsubscribed newsgroups

whatis

whatis displays short man page descriptions. Unlike

Examples:

$ whatis info
info (1)             - read Info documents
info (5)             - readable online documentation

$ whatis chmod
chmod (1) - change file modes
chmod, fchmod (2) - change mode of file

makewhatis

makewhatis creates the database for the whatis, appropos, and man -k commands. This is commonly run automatically by your system however sometimes you need to run this manually.

Examples:

# makewhatis

$ sudo makewhatis

File System Utilities

ls

ls is a utility for listing the files in a directory.

Most used options are:

-a 

all files (include files with . prefix)

-l 

long detail (provide file statistics)

-t 

order by creation time

-r 

reverse order

-F 

mark directories with /, executables with *, symbolic links with @, local sockets with =, named pipes (FIFOs) with |

Other options include:

-Fx --color 

color-coded listing

-s 

show filesizes

-s -h

show filesizes in kilobytes and megabytes

ls *st* : list files that contain st in name
ls > list : output list to file named "list"
ls | more : fit listing to screen

Examples

$ ls
fish  hello.txt

$ ls -l
-rw-r--r--    1 username    groupname        0 Apr 11 00:09 fish
-rw-r--r--    1 username    groupname       11 Apr 11 00:10 hello.txt

Several systems have an alias ll which does the same as ls -l:

$ ll
-rw-r--r--    1 username    groupname        0 Apr 11 00:09 fish
-rw-r--r--    1 username    groupname       11 Apr 11 00:10 hello.txt

Be careful with the -F option. Here is one example:

$ ls -F /usr/X11R6/bin/X*
/usr/X11R6/bin/X@          /usr/X11R6/bin/Xnest*      /usr/X11R6/bin/Xprt*
/usr/X11R6/bin/Xmark*      /usr/X11R6/bin/Xorg*       /usr/X11R6/bin/Xvfb*

We do not know yet if there is a symbolic link "X" and an executable "Xmark" or if "X@" and "Xmark*" are just the names of normal files. (Though "@" and "*" are not much found in filenames, they are possible.) So we check by dropping the -F:

$ ls /usr/X11R6/bin/X*
/usr/X11R6/bin/X           /usr/X11R6/bin/Xnest       /usr/X11R6/bin/Xprt
/usr/X11R6/bin/Xmark       /usr/X11R6/bin/Xorg        /usr/X11R6/bin/Xvfb

mkdir

mkdir is a utility for creating a directory.

Examples

$ mkdir newdirectoryname

fdisk

fdisk, cfdisk and sfdisk are utilities for partition manipulation. cfdisk has a good menu options.

Examples

$ cfdisk -z /dev/hda
$ fdisk /dev/sda

Warning: Don't forget to make a backup of your data before using these tools. Otherwise you can lost your data.

cd

cd changes the current directory of the shell. This current directory will be used by other programs launched from the shell.

Because "cd" changes the state of the shell, it is a shell builting command. It contrast, most commands are separate programs which the shell starts.

Examples

Change to 'foobar' directory:

$ cd foobar

Change to you home directory, cd command used without an option will drop you back into you home directory.

$ cd

~ (tilde) stores the path to your home directory, this command has same effect as the previous one.

$ cd ~

Tips:

• The "CDPATH" might only work in some shells. For example, ksh has it.

By setting "CDPATH" environment variable in your shell you can take advantage of shell command completion facility.

$ echo $CDPATH
.:/usr/local:/usr/share/doc

If you have the $CDPATH set, then you press 'TAB' key and get possible path completions

$ cd bas [TAB]
base-config/  base-files/   base-passwd/  bash/         bastille/

pwd

pwd (for Print Working Directory) shows the current directory that you are in.

Though "pwd" is often available as an external program (like /bin/pwd), many shells offer an equivalent version as a shell builtin command. Like any external command, "pwd" would inherit the current directory from the shell or other program that starts it.

Examples

$ pwd
/home/username

You can change the directory, you can also

$ cd /usr
$ pwd
/usr

You can also use "pwd" in scripts. If you have enough experience with scripting, then you would know that the next line complains if the current directory is /home/username.

$ test "x$(pwd)" = x/home/username && echo wrong directory

chroot

chroot changes the root filesystem. The "chroot" page at the Linux questions wiki explains why you might want to do this.

Examples

To change the root filesystem so /mnt/usbdrive/ becomes / and files outside of it cannot be seen:

# chroot /mnt/usbdrive/

You must be root user to "chroot". Other users would be able to use "chroot" to gain root (superuser) priveleges, so their use of "chroot" is disallowed.

$ chroot /mnt/usbdrive/
chroot: /mnt/usbdrive/: Operation not permitted

cp

cp copies a file

Examples

$ cp -i debian  Debian

Copy all files from the current directory to a target directory:

Syntax:

$ cp -r -t <target> <source>

Example:

$ cp -r -t /opt/cwk/mysqldata/ .

mv

You can use the command mv to move or rename files and directoris.

Examples

Rename file 'unix' to 'Unix'
$ mv -i unix  Unix

Moving file Unix from your home directory to /tmp.
$ mv -i ~/Unix /tmp/Unix

The -i indicates that the command will be interactive. It prompts before overwrite.

To move and rename a directory to another place use the following command:

$ mv <source> <target>
$ mv mediawiki-1.6.7 /opt/www/wikilinux

rm

rm deletes a file from the filesystem, like the "del" command in DOS.

The GNU long options (like --directory) are available on Linux, but not most other systems.

Some useful options are:

-d, --directory 

unlink FILE, even if it is an empty directory (some systems let superuser unlink non-empty directories too)

-f, --force 

ignore nonexistent files, never prompt

-i, --interactive 

prompt before any removal

-P 

(*BSD only) overwrite file before deletion

-r, -R, --recursive 

remove the contents of directories recursively

-v, --verbose 

(GNU only) explain what is being done

--help 

(GNU only) display help and exit

--version 

(GNU only) output version information and exit

Examples:

The usage of "rm" is considered potentially more dangerous than equivalents in other operating systems because of the way the shell parses wildcards and names of special directories and in its non-verbose actions.

Here is a classic example. Instead of deleting files that end with .o ("*.o") it deletes all files in the directory ("*") and also a file called .o. There is an unwanted space between the asterisk and the period.

$ rm * .o
rm: cannot remove `.o': No such file or directory

To remove a file whose name starts with a `-', for example `-foo', use one of these commands:

$ rm -- -foo

$ rm ./-foo

To remove a directory:

$ rm -r <directory>

It might be useful to create an alias such as "remove" which moves the files to a local "trash" file so you can go there and recover files you accidentally "remove"d.

Secure deletion of files:

Note that if you use rm to remove a file, it is usually possible to recover the contents of that file since rm does not remove it from the hard disk. It simply removes the filesystems link to it.

On *BSD systems, the -P option overwrites the data with the file before removing it.

$ rm -P secretfile

However, as the NetBSD manual page explains it:

Recent research indicates that as many as 35 overwrite passes with carefully chosen data patterns may be necessary to actually prevent recovery of data from a magnetic disk. Thus the -P option is likely both insufficient for its design purpose and far too costly for default operation.

So while examining the data (using fsdb or making a disk image) will not reveal the secret data, other methods (such as laboratory examination of the disk) will reveal the data. In short, rm -P does not delete data securely. A program that attempts to delete data securely is GNU shred, available on Linux. But "shred" is not always successful in secure deletion; read its entry below.

rmdir

rmdir is a utility for deleting directories.

Examples

$ rmdir directoryname

shred

shred overwrites a file multiple times with special data patterns to make the old contents of the file unrecoverable from a disk, especially a hard disk. This command is part of GNU coreutils, so it is often only available on Linux systems.

Note that this actually is ineffective on most filesystems because they can keep old copies of data. Most popular Linux filesystems (including ext3) keep such copies through journaling. However, "shred" is very useful for destroying the data on entire partitions or disks.

Some useful options are:

-u, --remove 

unlink the file after removing it

-NUMBER, -n NUMBER, --iterations=NUMBER 

the number of iterations of overwriting the file; default is 25 iterations

Examples: Remove and completely destroy secretfile from a filesystem that overwrites data in place and does not use journaling (for example, the UFS filesystem of *BSD). For the last step, after the data is destroyed, the "-u" option unlinks the file from the filesystem.

$ shred -u secretfile

Note that if secretfile has multiple hard links (with ln for example), it will continue to exist with those other names, but will contain only random data.

touch

touch lets you change the date on a file. Can also be used to create a blank file.

Examples

This will change the access date and time of filename to the current time. If filename doesn't exist it will create a blank file.

$ touch filename

df

df reports the amount of free disk space available on each partition.

$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/md0               5763508    207380   5263352   4% /
/dev/md1              78819376  13722288  61093296  19% /home
/dev/md4              23070564   4309572  17589056  20% /usr
/dev/md2               5763508   1757404   3713328  33% /var
/dev/md3               2877756    334740   2396832  13% /tmp

Reports disk usage in human readable format with block-sizes in Kilo,Mega,Gigabytes.

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda1             2.3G  2.1G  133M  95% /
tmpfs                  61M  8.0K   61M   1% /dev/shm
/dev/hda2             2.0G  1.8G  113M  94% /usr

ln

ln creates links between files

Examples:

To make a soft (symbolic) link "hello" to the file "/home/alice/code/bin/world":

$ ln -s /home/alice/code/bin/world hello

chown

chown changes the owner and group of files. Normally, only root is allowed to do this, but if a user owns a file, then that user can change the group, but only to groups containing that user. On old systems, the ability of users to give files to other users caused abuses, so most systems prohibit non-root users from changing the owner of files.

Some useful options are:

-R 

recursively change owner/group on an entire directory tree

-h 

do not follow symbolic links i.e. changes owner of the link, not the target file

-f 

indicate no errors if change failed

Examples:

Root changes the ownership of "/etc/test.cfg" and "/etc/shadow" to user root, group wheel:

# chown root:wheel /etc/test.cfg /etc/shadow

The same, but only changing the owner:

# chown root: /etc/{test.cfg,shadow}

The same, but only changing the group:

# chown :wheel /etc/{test.cfg,shadow}

Root gives every file in "/etc/ssh", including files in subdirectories, to user root, group wheel:

# chown -R root:wheel /etc/ssh

The same, but excluding files in directories (and the invisible files "/etc/ssh/.*" will also be missed):

# chown root:wheel /etc/ssh/*

User "tux" changes the directory "/usr/local/src/xc" from group "tux" to group "wheel". Tux is a member of both groups.

$ ls -ld /usr/local/src/xc
drwxr-xr-x  11 tux  tux  512 Sep 30 16:19 /usr/local/src/xc
$ chown tux:wheel /usr/local/src/xc
$ ls -ld /usr/local/src/xc
drwxr-xr-x  11 tux  wheel  512 Sep 30 16:19 /usr/local/src/xc

chmod

chmod changes permissions of files. One must be familiar with Unix file permissions to understand this command. There are three permissions: read ("r"), write ("w"), and execute ("x"). There are three sets of permissions: for the owning user of the file ("u"), for the group of the file ("g"), and for other users ("o").

For a file, "execute" means to run it as a program. For a directory, "execute" permission is required to use anything in that directory tree, so doing anything with "/usr/share/doc/README" requires execute permissions on all of "/", "/usr", "/usr/share", and "/usr/share/doc".

If you are interested in more advanced topics like the set-uid, set-gid, sticky bits and octal numbers, try reading the FreeBSD manual page at http://www.FreeBSD.org/cgi/man.cgi (type "chmod" in the form and submit).

A useful option is:

-R 

recursively change or set permissions on an entire directory tree

Examples:

We wrote a shell script called "configure". We make it executable ("+x") and then execute it as a command. Usually, "+x" is the same as "u+x" or "ug+x", depending on the status of the file mode creation mask.

$ chmod +x configure
$ ./configure

Allow the owning user to run "configure":

$ chmod u+x configure

Deny the group and other users from running "configure":

$ chmod go-x configure

For all users except the owner ("gw"), disable all access to "~/mail" and "~/private" ("-rwx"). This way, the contents are private and only their owner (or root) can access them.

$ chmod go-rwx ~/mail ~/private

Note that in the previous example, "-R" was not specified. By disabling the execute bit ("-x"), all files inside ~/{mail,private} are protected even if their group and other read bits are enabled. Thus, simply moving some file from inside ~/{mail,private} to some public place like /tmp can make the files available to other users again.

The "root" user wants to set up /usr/local/src so that all users in group "wsrc" (including "tux") can create files there. Root will continue to own the directory. This is done by changing the group of /usr/local/src to "wsrc" and then by granting to the group ("g") the read, write, and execute permissions ("+rwx").

# chown :wsrc /usr/local/src
# chmod g+rwx /usr/local/src

All Unix-like systems should allow all users to create temporary files in "/tmp" and "/var/tmp". Thus root gives everyone ("a", short for "ugo") all permissions ("+rwx") on the files.

# chmod a+rwx /tmp /var/tmp

The problem with the above is that because all users have write access to /tmp and /var/tmp, every user can delete and rename files, even ones not created by them. For example, "tux" could create "/tmp/socket.3908" and another user could delete it or rename it to "/tmp/garbage", thus annoying Tux. To keep temporary files safe, we use the sticky bit called "t". This limits the deletion and renaming of files in /tmp to root, the owner of /tmp (also root), and the owner of the file (Tux for "/tmp/socket.3908"). It does the same for /var/tmp. So what we should do is:

# chmod a+rwxt /tmp /var/tmp

Finding Files

find

find searches a given path for a file or folder. The syntax is: find [path...] [expression...]

Examples: On some of the latest Unix-like OS's, -print options can be avoided as it is the default. The following command searches for the file 'grub.conf' starting at the root ('/') directory.

$ find / -name grub.conf
/etc/grub.conf

If you are not the administrator of the computer, you get error messages for all the directories you are not allowed to read. In this case do it like this for a bash shell:

$ find / -name grub.conf 2>/dev/null
/etc/grub.conf

Or like this for a csh/tcsh:

$ find / -name grub.conf >& /dev/null
/etc/grub.conf

The following command will search for all directories named 'local'.

$ find / -name local -type d
/usr/X11R6/lib/X11/fonts/local
/usr/local
/var/cache/man/local
/var/local

Tips: Using 'exec' option executes certain commands for each file found by find:

$ find . -name '*bak' -exec rm -i {} \;
rm: remove regular empty file `./file1.bak'? y
rm: remove regular empty file `./file2.bak'? y
rm: remove regular empty file `./file3.bak'? y

Using 'ok' has same effect except it will prompt for every file:

$ find . -name '*~' -ok rm {} \;
< rm ... ./RMAIL~ > ? y

• When using "-exec" or "-ok", a semicolon must be used to indicate the end of the arguments (to "rm" in the example). Because semicolon is special to the shell, it must be quoted. The above examples quote the semicolon with a backslash.

whereis

whereis searches the normal executable and man page locations for a specified file.

Examples:

$ whereis ls  
ls: /bin/ls /usr/bin/ls /usr/man/man1/ls.1.gz /usr/share/man/man1/ls.1.gz

which

which searches the locations in your PATH variable for a specified file. If you know a program is in your path (i.e you can run it) this is faster than whereis.

$ which pine
/usr/bin/pine

locate

locate finds all filenames that match the specified query.

Examples:

$ locate make.conf
/etc/make.conf
/etc/make.conf.orig
/etc/make.conf.example
/usr/qt/3/mkspecs/linux-g++/qmake.conf
/usr/share/man/man5/make.conf.5.gz

locate however, is a GNU software and the command is not a standard in traditional UNIX systems like Solaris. The locate command comes standard with Linux based systems.

grep

Examples:

1. Search for a string in all the files at /etc :

grep -R -B3 -A4 "a string" /etc/*

Network

Setting up the network configuration

Normally you can set up the network configuration using the command netcardconfig. Otherwise, you can set up the network configuration using low level commands. For example, to set up or change the IP address of a computer using a gateway with IP 192.168.178.1 you can use the following commands:

1) Bring down the interface:

$ ifconfig eth0 inet down

2) Set up the new IP address:

$ ifconfig eth0 inet up 192.168.178.30 \
               netmask 255.255.255.0 broadcast 192.168.178.255

3) Define the route through the gateway

$ route add default gw 192.168.178.1

4) Edit the file /etc/resolv.conf and add the string below:

nameserver 192.168.178.1

ifconfig

To see the actual network configuration use:

$ ifconfig -a

Others

loadkeys

loadkeys changes the keyboard layout. Example:

loadkeys /usr/share/kbd/keymaps/i386/qwertz/de-latin.map.gz

TAR

To extract the contents of a tar file, use the x (extract) flag in a command, like this:

tar xvf panda.tar

To pack a directory into a tar file, use the x (create) flag in a command, like this:

$ tar czf <file.tgz> <directory>

Example:

$ tar czf bkp-`date +%Y%m%d`.tgz wikidb

RPM

RPM stands for Red Hat Package Manager. For more information about the use of RPM see german website of SuSe Der Paket-Manager des SuSE Linux

Use the following command to install a package into the linux system:

$ rpm -i PAKET.rpm

It checks all the dependencies and show an error message if something goes wrong.

To remove a package use the following command:

$ rpm -e PAKET

To check if a package is installed and what is its status use the following command:

$ rpm -q -i PAKET

shutdown

To restart the system use the following command:

$ shutdown -r now