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Linux Commands
Yakinur Rahman
July 2023
Introduction
• Can know background details using the command line.
• Before diving into the command lines we need to know two things first.
1. Shell:Is a program that takes command from the keyboard and gives
them to the operating system to perform.
2. Terminal:Is a tool where you can pass your shell command.
Type wsl in cmd to go to linux terminal.
• File System
1. Hierarchical Directory Structure
2. It uses Root directory.
3. Root folder starts from ’/’
• Commands
1. pwd Command: To know where I am.
2. clear Command: To clear the terminal.
3. cd Command: To change the directory.
If use cd you will change to home directory.
To go to root folder use cd /.
To change to home directory use cd ˜
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To get back to the previous folder or one folder
above use cd ..
To go to a directory use cd [path]
When going to a directory named with white
space like ”My books” use cd ”My books” or
cd Mybooks or cd ’My books’ but not cd
My books.
4. ls Command: Shows the list of all the files and
directories in the current folder.
The syntax is ls [options][files or directory].
If we write ls filename/ we will see the directory
content in that file. For the root folder, it is ls /.
For the list of files and directories in the home
folder use ls ˜.
To go back to the previous directory we use ls
... To go back two directory we use ls ../...
To get the list of directories in long format(detail
of the files and directories) use ls -l.
To know the hidden files also use ls -a. To get
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the hidden files in the long format use ls -al.
To get the file list in long format and in sorted
order according to size use ls -lS.
And if you want sorted according to name use
ls -ls.
To get all the .c files in home/yakin use ls home/yakin/*.c
And to get all the files with whatever extension
in home/yakin use ls home/yakin/*.*
If I want to save the output of ls command in
a file I use ls -lS > out.txt
To get only the directories, use: ls -d */
T see a directory structure use : ls -R
To know more about ls command use: man ls
5. cat command: It has three relative functions
with regard to the text files.They are:
(a) Displaying the text files
(b) Combining copies of text files
(c) Creating new text file
Syntax of using cat command is : cat options
file1 file2 ...
If we use cat and then type hello world the
output will be hello world
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To get out of cat command use ctrl+d
To display the contents of a text file use cat file-
name.txt
Adding line number to each non-blank line in the
file use cat -b filename.txt
To add line number to all the lines in the file
use cat -n filename.txt
To display all the lines in a file with a single line
gap between the lines(if had multiple blank lines)
use cat -s filename.txt
To add dollar symbol at the end of each line use
cat -E filename.txt
To know more about cat command use man cat
To write to a file use cat > filename.txt, then
type whatever you want in the file.Use ctrl+d
to get out of cat command. Then use cat file-
name.txt to be sure what you wrote were per-
sistent or not.
If we write anything that overrides the previous
contents again, use cat > filename.txt.
If you don’t want to override but append then
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use cat >> filename.txt.
To copy the contents of files to another file use
cat filename1 filename2 ... > destination-
filename
To append files to a file use cat filename1 file-
name2 ... >> destinationfilename
6. mkdir command:
It is make directory command.
To create a directory, use mkdir directoryName
To create a subdirectory in a directory, use mkdir
dirName/subDirName. If dirName don’t ex-
ist than it will show ”No such file or directory”.
To creat directory with subdirectory use mkdir
-p dirName/subDirName.
To create a list of subdirectory in a directory use
mkdir -p dirName/{subDirName1,subDirName2,...}
.Remember subdirectory names should be sepa-
rated by ’,’(comma) and there can’t be any space
before and after comma.
7. rmdir command:
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Remove directory command.Syntax of rmdir com-
mand is :
rmdir options directoryName
To remove a directory, use rmdir dirName
To remove a subdirectory use rmdir dirName/subdir1/.../subdirN.
Using the command you will remove subdirN. To
check use ls -R
To remove the whole directory structure, use rmdir
-p dirName/subdir1/.../subdirN. Check the
result using ls -R.
To know how rmdir command is working in the
background on a directory structure like a/b/c/d/e
use rmdir -pv a/b/c/d/e
Now for your kind information rmdir command
will not work if the directory is not empty. Like,
if the directory contain any .txt, .c, .html file,
etc. And to accomplish it use rm command.
8. rm command:
It is remove command.
To delete a directory together with its files use
rm -r structureOfTheDirectory.Suppose, a
directory is a/b/c/d/e and b contains a text file
like test.txt then using rmdir -p a/b/c/d/e
will remove c,d, and e from the directory struc-
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ture. To remove b and its file use rm -rv a/b, it
will also who what’s happening behind. You will
have only directory a. Instead of using rm -rv
a/b you could have used rm -rv a.To delete the
structure altogether.
9. cp command:
It is copy command. Used for copying files and
directories.The syntax of this command is
cp options source destination
If used cp file1.txt file2.txt, then it will copy
file1 to file2. If file2 does not exist, then it will
create file2.
If I want to have a copy of a file in a directory,
then use cp fileName1 fileName2 directory-
Name.Now fileName1 and fileName2 will have a
copy in the directoryName. It will override any
existing file.
To know is the cp command overriding anything
or not use cp -i fileName1 fileName2 direc-
toryName. If you enter y it will override, else
if you enter n then it will not override.
If we are in a subdirectory, but want to copy file
from the directory, use cp ../fileName . . cp is
the command. ”..” is used to go up the directory.
And the last ”.” indicates the current directory.
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To copy file to a directory cp filePath direc-
toryPath.
To copy the contents of a directory to another
directory and to create the destination directory
use cp -R direct1 direct2. The contents of di-
rect1 will be copied to direct2.
If the direct2 already exists a copy of direct1 will
also be in direct2.To observe use cp -vR direct1
direct2
For more details use man cp
10. mv command:
Used to change the name of a file or directory.
Syntax is
mv options source destination, source and
destination are files or directories.
If you want to move a file named test.txt to yakin/
directory use mv test.txt yakin/
If test.txt already exists in yakin/ directory, then
using mv test.txt yakin/ will override the al-
ready existing one.
If you want to prevent the overriding, use mv
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-i test.txt yakin/
If you want to move a directory suppose yakin/
into another directory, suppose family/ then use
mv yakin family. But the condition is both
directory exists. Otherwise, it will just rename
yakin/ directory to family/ directory.
To know more about this command use man mv
11. less command:
Used to find something (word|pattern) in a file.
When having a large size of text, if you want
to read from the start use less big.txt, if the
file name is big.txt.Press the up arrow and down
arrow to read.
If you want to read page by page instead of line
by line, use ’space’ to down instead of down-
arrow and ’b’ instead of up-arrow.
To reach the END of the file, use Shift+G.
To reach the TOP press ’g’.
To search for something, press ’/’ and then type
the word. After typing the word use ENTER.It
will get you the first occurrence of the word. To
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go to the next occurrence, press ’n’.
If you want to search the word from the END
of the file, press ’?’, then type the word. If you
press ’n’ you will get the occurrences from bot-
tom to top.
To exit less command press ’q’.
To know more use man less
12. touch command:
Used to create new empty files in Linux. Also
used to change the timestamps of directories.
To create an empty file, use touch filename-
WithExtension You can create files only using
touch command.
To change the timestamp of the already existing
file to the current time, use touch filename-
WithExtension
13. nano command:
Nano is a useful and friendly text editor.It has
many features.
To create a new file : nano filename.extension
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File will not be created, until saved.
You can write programs too. For example, nano
hello.cpp
14. sudo command:
sudo command provides us administrative privi-
leges. When we want to install any third-party
software we need this command.
To install gcc , use sudo apt-get install gcc
To install g++ , use sudo apt-get install g++
To get to super user mode, use sudo -s
15. top command:
Provides dynamic view of the running system.
Used to know which program is using more re-
sources.
To know which program is using how much re-
sources, enter top. PID refers to process id.
By default, after every 3 seconds the view is re-
freshed.
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To change the refreshing time, press ’s’ when the
view after top is screening.
To filter out idle processes or to screen active
processes, press ’i’.
To kill a process, press ’k’. Then input the pro-
cess id.Or, PID.
An alternative is ps -ux.
If you want to know all the processes under a
userName then use ps -U userName
16. kill command:
It is used to kill processes. The syntax is:
kill -flags PID
To know the PID of a process, use pidof pro-
cessName
Then use kill PID to kill the process.
If you were not able to kill use this command
instead kill -KILL PID
Or, you can use kill -9 PID too.
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17. echo command:
Displays anything you write after the echo com-
mand.The echo command is important when script-
ing.
To do the echo use echo statement Or, echo
”statement”
var=”Hello”, no space before and after = sign.Then
use echo $var, and you will be displayed Hello.
Or, x=10 then use echo ”The value of x is
$x. This will display what you are thinking.
If you use echo -e ”Some text ” then the
escape character will be enabled. And this will
print: Some ext. If used echo ”Some
text” then it would have printed: Some text.
18. chmod command:
Here instead of filename, you can use directory-
Name. The function is the same.
chmod o+[x|r|w] filename will grant to ex-
ecutable or read or write permission to others on
a file.
chmod o-[x|r|w] filename will sack executable
or read or write permission of others on a file.
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Use ls -l to confirm these operations.
chmod g+[x|r|w] filename these will grant ex-
ecutable or read or write permission to groups on
a file.
chmod g-[x|r|w] filename these sack executable
or read or write permission of groups on a file.
chmod ug=[x|r|w] filename used to give per-
mission to both user, group, and everyone on a
file. Remember, no space before and after of ’=’.
To remove all the permissions use chmod a-
[x|r|w] filename
To give permission to all, use chmod a+[x|r|w]
filename
To specifically give permission to user,group and
others, use chmod u[+|-][x|r|w],g[+|-][x|r|w],o[+|-
][x|r|w] filename.
Octal and Numerical Permissions: Remem-
ber (u,g,o) can be of combinations like([r|w|x],[r|w|x],[r|w|x]).
So, u can be expressed as a 3-bit combination.
100 means granting reading privileges to the user,
and 111 means granting all privileges to the user.
The same is applied to groups and others. 100 in
binary represents 4 in octal and 111 represents 7
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in octal. Now, if you use chmod 000 filename
then you give no permissions to anyone. But if
you use chmod 700 filename then you provide
all permissions to the user but nothing to groups
and others.
• I/O Redirection
Redirection simply means capturing output from a file or command or
program and sending it into another file or command or program as an
input. If we want to send output to a file use output > file . Now use
cat5 command to do it.
• File/Directory Permission:
The ’r’ stands for read, ’w’ stands for write, ’d’ stands for directory, and
’x’ stands for executable. Use chmod18 command to change the file per-
mission.
• Bash Scripting:
A script is a text file that contains a sequence of commands for an op-
erating system. Create a script file using nano myscript1.sh. And we
need to know where our bash is. Then type #! [location of the bash].
To know where the bash is, use which bash. So, now write the com-
mands you want in the myscript1.sh file.If permission is already given,
use ./myscript1.sh to do the execution.
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