Linux Commands

These LINUX commands should be common to most implementations and variants of the operating system. Your LINUX version may not support all the commands, and will almost certainly have extra commands not mentioned here. If you use UNIX then most of these commands will be familiar. If you are entering a long command that needs to span more than one line, then finish each line off with a backslash as this indicates the command will continue on the next line, without an Enter separating one line from the other.

File commands

cat

Used to view the content of a file, the content is usually sent to your monitor Syntax:

 >cat filename1 filename2 ... 

cp

used to copy data Syntax:

 >cp [-R] sourcefile targetfile	

The source and target files can be directories, then the –R option will copy all files and subdirectories under the source directory.

file

Used to determine the format of a file Syntax:

 >file [-z] filename	

The results should not be taken as absolute, but more of an educated guess. The –Z option lets ‘file’ investigate compressed files.

grep

looks inside a file, and prints out all lines that contain a search pattern. Syntax:

 >grep PATTERN file	

Grep is often combined with other commands to process output.

Special characters
 
‘.’ Exactly one character
‘*’ 0 or more characters
‘?’ 0 or 1 instance of the character preceding the ? for example cm? matches ‘c’ or ‘cm’
[………] range for example [1.6] = 123456
‘^’ the start of a line
‘$’ the end of a line
‘/’ is the escape character and is used to nullify special characters when placed before them, for example ‘why/?’ will be parsed as ‘why?’ It can also give a special meaning to non-special characters, like ‘(’ and ‘)’ for example grep ‘search\(ing)\?’ will look for ‘search’ and ‘searching’
To search for the ‘example’ abbreviation e.g. use grep e\.g\. filename

head

Look at the beginning of a file Syntax:

 >head [-n number] filename

the –n option specifies how many lines to display

less

The GNU file viewer version of the ‘more’ command Syntax:

 >less filename

ln

Define a link or a shortcut from your directory to a file in another directory Syntax:

 >ln [–s] target_file link_name	

The –s parameter means that this is a symbolic link, and they can span partitions. Without the –s the link is a hard link and they cannot span partitions. A soft link is just a pointer, which a hard link is the same size as the original file. is particularly interesting for beginning users: they are fairly obvious to see and you don't need to worry about partitions.

locate

Print all accessible files matching the search pattern. This command requires that the slocate database has been set up. If it has not, you can do this by entering the slocate –u command. Locate will only return files that you have access rights to. Syntax:

 >locate searchstring	

ls

Lists the contents of a directory Syntax:

 >ls –l 

is probably most the often used but there are lots of other parameters which can be combined as ls –liap for example. Some parameters are –
 
–a Shows you all files including hidden files that begin with a dot
–c Sort list by last modification of the i-node
–C Multi-column output (usually provided by default)
–d list directories only, not contents
–i print the i-node number in the first column of the report for each file
–l long list, displays most available information including permissions on a file, file size, inode number, creation date and time, owners and amount of links to the file
–L list the link references for symbolic links, rather than the link itself.
–m display the list in csv format
–p display a slash ( / ) in front of all directories.
–R display the contents of subdirectories.
–u Sort by last access instead of last modification

ls normally displays file types by colours, if it is aliased to color-ls . The standard colour scheme is

Color	File type
blue	directories
red	compressed archives
white	text files
pink	images
cyan	links
yellow	devices
green	executables
flashing red	broken links

ls -l provides a lot of information about a file which is confusing if you do not know how to interpret it. Typical output looks like

-rwxrw-r-- 1   ax0002  1500   Mar 06 11:26   file_name.ext

The first character, a ‘-‘ indicates that this is a file
The next 9 characters are the file permissions, broken down into 3 groups of 3 characters. The first three are the permissions for the owner, the middle three are permissions for the group which has access to the file and the last three are the permissions for everybody else.
The ‘1’ is the number of links to this file
‘ax0002’ is the file owner
‘1500’ is the file size
‘Mar 06 11:26’ is the date and time the file was last modified
‘file_name.ext’ is the file name

mkdir

Creates a new directory. Syntax:

 >mkdir directory_name	

You can create a directory and a sub directory below it the -p option. Say you have a directory called reports, and you want to create a path for /reports/2006/jan/ but 2006 does not exist yet. Just use

 >mkdir -p reports/2006/jan/

mv

used to move or just rename a file Syntax:

 >mv oldfile_name newfile_name	
or
 >mv oldpath\file_name  newpath\file_name

rm

deletes files syntax:

 >rm file_name	

rmdir

deletes directories syntax:

 >rmdir directory_name

tail

displays the end of a file syntax:

 >tail [–f] –n filename	

-n specifies the number of lines to display. –f will refresh the display as more data is added, and is handy when displaying log files

wc

Counts the number of lines, words and characters in file. Syntax:

 >wc file_name	

Navigation commands

cd

used to navigate to a different directory Syntax:

 >cd directoryname

find

Find searches for files based on a string in the file name, but it does not look inside files. Find can also be used to find files greater than a supplied size, which can be useful if you have space problems. Syntax:

 >find path -name searchstring 

Look for files in the supplied path that have the searchstring in their name

 >find . -size +5000k

Look for files bigger than 5 MB
You can string commands after the find to take action on the results, for example

 >find . -name "*.tmp" -exec rm {} \;

will delete all files ending in tmp. I’m NOT saying you should do this, just that it is possible.

pwd

Displays the current working directory Syntax:

 >pwd

which

displays the full path to command. Syntax:

 >which command	

If a command is not working as expected it could be that you are picking up a different command from an earlier path in the path variable. The which command can eliminate this as a possibility.

Security commands

chmod

security commands used to change the access permitted to files. Syntax:

 >chmod [-r] permissions filenames	

Permissions can be expressed as a number, but is easier to understand as a list of permissions by type. There are four main types;

u - User who owns the file.
g - Group that owns the file.
o - Other.
a - All.

These types can be strung together in the chmod command and can be granted three levels of permissions;

r - Read the file.
w - Write or edit the file.
x - Execute or run the file as a program.

Finally the + and - operators are used to grant or deny a given right, so for example to grant the owner and group read, write and execute to file file.txt, and remove the execute right from other and all, use the command

 >chmod ug+rwx oa-x file.txt

The -R option is used to change the access in all the underlying files and subdirectories of a specified directory.

chown

Change to owner of a file
Syntax:

 >chown [-R] new_owner file_name

The -R option is used to change the owner of all the underlying files and subdirectories of a specified directory.

chgrp

changes the group that has access to a file or directory.
Syntax

 >chgrp [-R] [-h] newgroup file_name

The -R option is used to change group of all the underlying files and subdirectories of a specified directory. the -h option will change the group of the symbolic link. If -h is not specified then the group of the file referenced by the symbolic link is changed.

id

displays the groups a userid belongs to. Syntax:

 >id userid

Utility commands

df

the ‘disk full’ command reports on disk space usage in active partitions. df is a default GNU command, and may be replaced by more comprehensive commands in different Linux variants. Syntax:

 >df –h    will show space usage and mount points on all active non-swap partitions
 >df –h .   will just show the partition the current directory belongs to, for example

 >df –h .
Filesystem	Size  	Used 	Avail 	Use% 	Mounted on
/dev/hda4 	980M  	163M  	767M  	18% 	/

echo

Displays a line of text or the contents of a variable Syntax:

 >echo string	
 >echo $path displays the contents of the PATH variable

info

Displays command help Syntax:

>info command

man

Display the full manual page for a command Syntax:

 >man command	

quota

If you have quotas set to limit disk space usage this command will show the quota size and current usage. Syntax:

 >quota –v

back to top

Copyright © Lascon Storage Ltd. 2000 to present date. By entering and using this site, you accept the conditions and limitations of use