Linux fundamentals — Summary:
Locate :
When it’s not locating the temp.txt then go and update the DB.
The command for updating the DB is
sudo updatedb
Then start typing locate temp.txt it will locate the file.
Man:
The manual command is used to describe the other commands, like how it’s working and how to use it.
man ls
Which:
Check whether the software is installed or not.
It will look into what directory particular software is located.
which python3
Whatis:
The command is meant to describe what the particular is about.
whatis ls
Alias:
Before explaining what is this command about I will like to give an example of it.
alias temp=’ls -a’
History:
It is used to list out what is the previous command type in the terminal.
History
note: The history command does not work while you close the currently active terminal.
Some more Linux commands:
0 - Stdin (Input Stream)
1 - Stdout (Output Stream)
2 - Stderr (Error Stream)
Where 0 denotes input that is pointed towards the file descriptor 0. 1 denotes output is pointed to the file descriptor 1. 2 meant for error is pointed to the file descriptor 2.
Standard error descriptor example:
cat log.txt
Mahesh 2 > cat log.txt
Then checkout log.txt using cat
It will show output like
Mahesh command not found
note: We are familiar that number 2 is a standard error descriptor it would show
command not found error
Mahesh 2 >> log.txt
The above “>>” show displays the content without overriding already existing contents.
example: while you view cat log.txt this results in
Mahesh: command not found
Mahesh: command not found
without overriding the contents.
rm:
rm is used to remove the file or folder(empty).
rm log.txt
To remove the folder which is not empty.
rm -rf foldername
Echo:
The echo command is used to print the contents in the file.
echo “hello” > log.txt 2>&1
It will print hello in the log.txt without any error (2 - is the error stream and 1 is the output stream).
Find:
This command shows a list of directories with a permission denied statement list.
find / -type f -name log.txt
/ - Signifies root directory
f - Search for file
name - Name of the file
While typing the above command it would list out all permission denied statements and it’s tough to find the particular file we need.
So we use /dev/null for concatenation of the find command of the log.txt
less:
The less command displays content one screen at a time.
ls -l | less
cut:
In UNIX, the cut command is used to cut out sections of files and write the results to standard output. It can be used to cut sections of a line based on byte position, character, and field. In log.txt having content “hi”.
cat log.txt | cut -c 1 (“|” pipeline used to perform more than one operations simulataneously)
output is h (display the first letter of the string)
cat log.txt | cut -c 2
output is i (display the second letter of the string)
cat log.txt | cut -c 3
It will display the empty character because there is no character after i
cat log.txt | cut -c 1–2
output: hi (total number of strings is from the range of 1 to 2)
ls -l | cut -d “ “ -f 1
This will list out the permission present for a particular file
output:
total
— rw — r — — r — —
— rw — r — — r — —
Display the first column of ls -l and followed by space which means it prints the first field only
ls -l | cut -d “ ” -f 2
Likewise, it will display the second fields of a long listing of directories.
ls -l | cut -d “ “ -f 3
It will display the second field of long listings of directories.
Mahesh
Mahesh
NL: Numbering lines
Before using nl just concatenate the contents without overriding the contents.
echo “hello world” >> log.txt
echo “ironman” >> log.txt
echo “superman” >> log.txt
echo “spiderman” >> log.txt
nl log.txt
output is
- hi
- hello world
- ironman
- superman
- spiderman
head:
Head display the contents from the first line
head -n 3 log.txt
output:
hi
hello world
ironman
tail -n 2 log.txt
output:
superman
spiderman
Displays the last two contents of the file.
file permission:
ls -l it is used to display the long listing of a file.
output:
— rw —r — — r — — 1 Mahesh Mahesh 59 Aug 09:23 log.txt
The first dash denotes the text file type of “r” reading and “w” writes permission for the user to have given( — rw — ).
“r” followed by double dashes specifies read permission only given for groups (r — — ).
Final read-only permission is given to others (r — — ).
If drwxr — xr — x specifies directory type read write execute permission is given (“d” directory).
To change the permission, we need to use the chmod command.
Sort:
sort log.txt
Contents get sorted in ascending order.
output:
hi
hello world
ironman
superman
spiderman
sort -r log.txt
It helps to sort the contents in reverse order.
output:
spiderman
superman
ironman
hello world
hi
unique:
It checks when 2 lines are identical and limited to one line.
sort log.txt | unique
WC:
word count (wc)
It will calculate the number of words in the file.
wc log.txt
It would print
5 5 59 — Five lines , Five Words and 59 characters of bytes.
wc -l log.txt — “l” specifies number of lines present in the text file.
wc -w log.txt — “w” specifies number of words present in the text file.
wc -c log.txt — “c” specifies number of characters present in the text file.
Grep command:
Grep command matches the pattern given and prints only the line that matches the pattern.
cat log.txt | grep “hello”
hello world will be the output.
cat log.txt | grep -v “hello”
it will print all except the word hello or world including hello.
chmod:
Changes the permission of the text file.
4 - read
2 - write
1 - execute
for example:
chmod 777 log.txt
When you give this command it gives full permission to users, groups, and others with all read-write, and execute permission.
chmod 766 log.txt
It gives users all permissions and groups, others only have read-write permission.
— rwxrw — rw —
If you created a new file that file has default permission.
— rw — r — — r — — new.txt
umask:
If you give
umask 777
This permission was added to the new.txt
Special permissions:
SUID,SGID,Sticky bits
If you want to execute the binaries it asks for sudo permission the most highly privileged permission.
To run the binary every time sudo permission is required to avoid this special permission required.
chmod u+s temp
After that permission would be changed into
-rwSr — —r — — (S comes in user level permission which setuid)
To enable execute permission we need to chmod
chmod +x temp
permission becomes
-rwSr-xr-x
To make setgid
touch temp2
chmod g+x temp2
Then the permission be like
-rw-r-Sr — —
Sticky bit:
The purpose of a sticky bit is useful when sharing resources in the network of your computer.
chmod a+x temp2
output is -rwxr-Sr-t
It will prevent any other user to delete the contents of the folder except the owner or root user. It’s a safe and secure way to host your directories in the network.
setuid binaries -> establishing the setuid binaries are called SUID binaries.
These binaries are very prone to privilege escalation.
One way to find suid binaries is:
find / -perm -u=s (This is to find userbit is equal to set).
find / -perm -u=s -type f 2 >/dev/null
Linux file Structures:
/ -> indicate root
bin -> we have all binaries in bin directories.
dev -> you find the list of all device files.
lib64 ->contains libraries of 64-bit shared
mnt -> if any other partition or any Pendrive inserted you will be mounted at mnt point.
root -> root folder contains root files
snap -> if you wanted to install any packages snap is used.
tmp -> temp folder contains temporary files.
Boot -> contains a file required for booting Linux.
etc -> contains configuration files of applications and users.
opt -> additional software added to the opt.
cdrom -> mount cd or DVD ROM and have folder contains for many other users.
sbin -> system binaries which give specific information about the system.
var -> we have backup files
sub-directories present in var are
- > cache (cache is local cache)
- > lib contains library data
- > you can install one software at a time
- > snap contains snap files
- > temp contains temporary files.
How to add users to the Linux system: (user management)
sudo adduser Mahesh
How to delete users present in the Linux system:
sudo deluser Mahesh
It would delete the Mahesh user
cat etc/passwd - It is used to view the configuration of the passed file.
cat etc/shadow - It is used to view unencrypted passwords.
env:
env is environment variables that tell all environmental command variables.
One environment among the environmental variable is PATH.
If you type “ls” it would check “ls” binaries available in the path environmental variable.
If you type “Mahesh” as a command in the terminal it shows the “Mahesh” command not found.
To access what is in the environmental variable use
echo $HOME -> /home/Mahesh
echo $USER -> Mahesh
export:
The export command is used to assign a value of the command to a variable.
export $ip = google.com
export PATH = $PATH:/home/Mahesh/Desktop
Software Management in Linux:
To install any software (or) tools in Linux use the command
“sudo apt install software_name”
Before installing any software we need to update the system
“sudo apt update”
while updating the system it must be saved on repositories. Each software package is also stored in repositories.
To check whether the application or software is installed or not.
“apt-cache search chrome”
To install Debian files without error there is a command for that:
“sudo apt install gdebi”
“sudo gdebi example_amd64.deb”
To remove any software installed:
“sudo apt remove”
Cron jobs:
If you want to run a task on a particular day a particular time or a particular month automatically schedule jobs by cron jobs.
“cronjob -l”
m h dom mon dow - specifies minute, hour, day of the month, day of the week.
export EDITOR = /bin/nano
crontab -e (It’s used to edit the crontab contents)
Then we can add
- *** echo “hi” >> ~/Desktop/temp.txt
- Once you typed the content in the crontab editor it would execute the command.
- **** echo “hi” >> ~/Desktop/temp.txt
- 30 2 **** echo “hello” >> ~/Desktop/temp.txt
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