Linux Fundamentals

Linux Command Syntax:

Commands typically have the syntax:

command option argument or

ls [OPTION]... [FILE]…

for example:

ls -ltr linux.txt

OPTION: Modify the way that a command works

usually consists of a hyphen or dash followed by a single letter
Some commands accept multiple options which can usually be grouped after a single hyphen

ARGUMENT:

Most commands are used together with one or more arguments
Some commands assume a default argument if none is supplied
Arguments are optional for some commands and required by others

These are the three main commands that you are going to use when working in any company:

whoami

hostname

pwd

File Permissions

UNIX is a multi-user system. Every file and directory in your account can be protected from or made accessible to other users by changing its access permissions. Every user has the responsibility for controlling their files.
Permissions for a file or directory may be restricted to by types
There are 3 types of permissions
- r - read
- w - write
- x - execute = permission to run a program
Each permission (rwx) can be controlled at three levels:
- u - user =yourself
- g - group = can be people in the same project
- o - other = everyone on the system
File or Directory permission can be displayed by running ls -l command
- rwxrwxrwxrwx
man chmod → to check more about this ‘chmod’ command
chmod : chmod changes the file mode bits of each given file according to mode, which can be either a symbolic representation of changes to make or an octal number representing the bit pattern for the new mode bits.

File permission using numeric mode:

Permission to a file and directory can also be assigned numerically
```
  chmod ugo+r file_name
```
or
```
  chmod 444 file_name
```
Below you can see the table of these numeric values

File Ownership commands (chown, chgrp)

There are 2 owners of a file or directory
- User and Group
Command to change file ownership
- chown changes the ownership of a file
- chgrp changes the group ownership of a file
Recursive ownership change option (Cascade)
- -R (know more about this!)

Access Control List (ACL):

The access control list provides an additional more flexible permission mechanism for file systems. It is designed to assist with UNIX file permission. ACL allows you to give permission for any user or group to any disc resource.

Use of ACL

Think of a scenario in which a particular user is not a member of the group created by you but still, wants to give some read or write access, how can you do it without making the user a member of the group, here ACL comes into the picture, ACL helps us to do this trick
ACLs are used to make a flexible permission mechanism in Linux.
From Linux man pages, ACLs are used to define more fine-grained discretionary access rights for files and directories.
commands to assign and remove ACL permission are:

setfacl and getfacl

List of commands for setting up ACL:

To add permission for user:
```
 setfacl -m u:user:rwx /path/to/file
```
To add permission for a group:
```
 setfacl -m g:group:rwx /path/to/file
```
To allow all files or directories to inherit ACL entries from the directory it is within
```
 setfacl -Rm “entry” /path/to/file
```
To remove a specific entry:
```
 setfacl -x u:user /path/to/file
```
To remove all the entries:
```
 setfacl -b path/to/file
```

Note: As you assign the ACL permission to a file/directory it adds a + sign at the end of the permission

Help Commands

There are 3 types of help commands
- whatis command
- command - - help
- man command

TAB completion and Up Arrow:

Hitting the TAB key completes the available commands, files, or directories
- chm TAB
- ls j<TAB>
- cd Des<TAB>

Adding text to files (Redirects)

3 Simple ways to add text to a file
- vi
- Redirect command output > or >>
- echo > or >>
  - ‘>’ This will enter your data in the first line
  - ‘>>’ This will enter your data in the next line
ls -ltr > filename: it will copy all the data that are present in ls -ltr
date >> filename: copy the date in the filename

Input and output Redirects:

ls -la : a is append here and will show the hidden files starting with ‘.’
for example:
.git, .bashrc, .profile, etc
There are 3 redirects in Linux:
- stdin(Standard Input): It has a file descriptor number as 0
- stdout( Standard output): It has a file descriptor number as 1
- stderr(Standard Error): It has file descriptor number as 2
Output(stdout) - 1
- By default when running a command its output goes to the terminal
- The output of a command can be routed to a file using > symbol
```
  ls -l > file_name
```
```
  pwd > file_name
```
- If using the same file for additional output or to append to the same file then use >>
```
  ls -la >> file_name
```
```
  echo “Hello Linux” >> file_name
```

Input (stdin) - 0

input is used when feeding the contents to a file.

  cat < file_name

  mail -s "Office memo" allusers@abc.com < memoletter

Error (stderr) - 2
- When a command is executed we use a keyboard and that is also considered (stdin -0)
- That command output goes on the monitor and that output is (stdout-1)
- If the command produces any error on the screen then it is considered (stderr- 2)
- We can use redirects to route errors from the screen.
```
  ls -l /root 2> file_name
```
```
  telnet localhost 2> error_file
```

Standard Output to a file (tee)

The ‘tee’ command is used to store and view (both at the same time) the output of any command.
The command is named after the T-splitter used in plumbing. It breaks the output of a program so that it can be both displayed and saved in a file. It does both tasks simultaneously, copies the result into the specified files or variables, and also displays the result.

echo "Hello Linux bro" | tee file_name

We can store it in multiple files:

echo "Hello Linux bro" | tee file1 file2 file3

Here ‘ -a ‘ command is for adding a new line or appending to a file:

echo "Hello Linux bro" | tee -a file_name

This command will display how many characters you have in your file:

wc -c file_name

Check more about tee:

tee --version

Pipes ( | ):

A pipe is used by the shell to connect the output of one command directly to the input of another command.
The symbol for a pipe is the vertical bar | The command syntax is:

command1 [arguments] | command2 [arguments]

File Maintenance Commands:

cp - copy files and directories

cp file_name_you_want_copy new_file_name

or

cp file_name /directory_name

rm - remove the files.

rm file_name

mv - move the file and rename it but do not change the content.

mv file_name new_file_name

mkdir - to make a new folder/directory

mkdir folder_name

rmdir - to remove the directories

rmdir directory_name

rm -r - will forcefully remove sub-directories and its contents as well

rm -r directory

using a path:

rm -r /path/to/directory

force delete:

rm -r -f directory_name

chgrp - used to change the group name, to do so first go into the root directory

chgrp root file_name or directory_name

chown - used to change ownership

chown suraj linux.txt

This command will change the owner and group of the specified file

chown suraj:suraj linux.txt

File display commands

cat - displays the content of the file

cat file_name

more - It will give the information page by page and hit the space bar to go page by page. And to quit this just simply type ‘q’.

more file_name

less - ‘less’ and ‘more’ are very similar just the less has a lot more options in its navigation. you can search here

less file_name

head - this will display by default the first 10 lines of your content

head file_name

Now this will display the first 5 number of lines of your content because we have specified it using the ‘-n’ flag

head -n 5 file_name

tail - this will display by default of last 10 lines of your content

tail -n 5 file_name

Filters/text processors commands

cut - cut is a command line utility that allows you to cut parts from specified files or piped data and print the result to standard output. It can be used to cut parts of a line by delimiter, byte position, and character

cut file_name              = does not work
cut --version              = check version
cut -c1 file_name          = List one character
cut -c1,2,4                = Pick and choose character
cut -c1-3 filename         = list range of character
cut -c1-3,6-8 file_name    = list specific range of character
cut -b1-3 file_name        = list by byte size
cut -d: -f 6 /etc/passwd   = list first 6th column separated by :
cut -d: -f 6-7 /etc/passwd = list first 6th and 7th column separated by :
ls -l | cut -c2-4          = only print user permissions of files/dir

awk: awk is a utility/language designed for data extraction. Most of the time it is used to extract fields from a file or an output

awk --version                                     = Check version
awk '{print $1}' file_name                        = List 1st field from a file
ls -l | awk '{print $1,$3}'                       = List 1 and 3rd field of ls -l output
ls -l | awk '{print $NF}'                         = Last field of the output
awk '/suraj/ {print}' file_name                   = Search for a specific word
awk -F: '{print}' /etc/passwd                     = Output only 1st field of /efc/passwd
echo "Hello World" | awk '{$2="suraj"; print $0}' = Replace words field words
cat file | awk '{$2="suraj"; print $0}'           = Replace words field words
awk 'lenth($0) > 15' file_name                    = Get Lines that have more than 15 byte size
ls -l | awk '{if($9 == "suraj") print $0;}'       = Get the field matching suraj in directory you are in
ls -l | awk '{print NF}'                          = Number of fields

grep and egrep:
What is grep: the grep command which stands for “global regular expression print,” processes text line by line and prints any lines which match a specified pattern.

grep --version
grep keyword file_name
grep -c keyword file_name
grep -i KEYword file_name
grep -n keyword file_name
grep -v keyword file_name
grep -vi keyword file_name
grep keyword file_name | awk '{print $1}'
ls -l | grep Desktop

egrep uses- egrep is a little more powerful than grep see the example below:

egrep -i "keyword|keyword2" file_name

sort: Sort command sorts in alphabetical order**:**

sort --version OR man sort OR sort --help = Check version or help
sort file                                 = Sorts file in alphabetical order
sort -r file                              = Sort in reverse alphabetical order
sort -k2 file                             = Sort by field number, and 2 here is the field number

uniq: The Uniq command filters out the repeated or duplicate lines:

uniq file
sort file | uniq     = Always sort first before using uniq their line numbers and it'll not show the duplicates
sort file | uniq -c  = Sort first then uniq and list count
sort file | uniq -d  = Only show repeated lines.

wc( Word Count): The command read either standard input or a list of files and generates: newline count, word count, and byte count

wc --version OR wc --help = check version or help
wc file                   = Check file line count, word count, and byte count
wc -l file                = Get the number of lines in a file 
wc -w file                = Get the number of words in a file
wc -c file                = Get the number of bytes in a file
wc Directory_name         = Not Allowed
ls -l | wc -l             = Number of files
grep keyword | wc -l      = Number of keyword lines.

Compare files:

diff (line by line)

diff file_name1 file_name2

cmp ( byte by byte)

cmp file_name1 file_name2

Compress and un-Compress File

tar: used to make a zip file

The below command is to zip your file with tar extension in the current directory

tar cvf your_username.tar .

to unzip the file:

tar -xvf filename.tar

gzip: used to make a file compress

to compress this tar file:

gzip file_name.tar

gzip -d OR gunzip : used to make a file un-compress

to un-compress

gzip -d file_name.tar

Truncate file size (truncate):

The Linux truncate command is often used to shrink or extend the size of a file to the specified size

truncate -s 10 file_name

By running the below command the file size will be of 60 characters:

truncate -s 60 file_name

Combining and Splitting Files

Multiple files can be combined into one and
One file can be split into multiple files
- cat file1 file2 file3 > file4
- split file4
```
  split -l 300 file.txt childfile
```
  after running the above command we can see our file countries here split into parts you can see the below image.
- split file.txt into 300 lines per file and output to childfileaa, chilfileab, and childfileac

Linux Vs Windows Commands:

We should be aware of Windows CLI commands also.

Hope this blog will be helpful for you and I'll come back with another blog. Thanks for reading.