Miscellaneous Commands -1

Super User & File Permissions

Linux Users

The root user, also known as the superuser or administrator, has access to all commands and files.

Root User

sudo

command temporarily elevates the privileges allowing users to complete sensitive tasks without logging in as the root user.

1

sudo command

Linux Commands

Executable Path

which

command is used to identify the location of a given executable path.

Syntax :

1

which command

Example :

1

2

$ which sudo

/usr/bin/sudo

Create New Users

useradd

is used to create a new user with the given username.

Syntax :

1

sudo useradd username

Example :

1

sudo useradd abhi

Set/Change User Password

passwd

is used to set or change password of a given user.

Syntax :

1

sudo passwd username

Example :

1

sudo passwd abhi

Execute Command as Another User

su

is used to execute command as another user.

1

su user

1

su -c command user

File Permissions

Authorization Levels

Multi-user operating systems like linux provide two levels of authorization in securing the files

• Ownership

• Permission

User Ownership

Users accessing a file/ directory can be categorized into 3 types

Type	Description
Owner	User who is considered as owner
Group	A group of users, who are assigned a specific permission
Others	Any other users who doesn’t belong to the above two types

Ownership and Permissions

Permissions	Description
Read	Open and read a file
Write	Modify the contents of a file
Execute	Authority to run as a program

File Permissions in Linux

Changing Permissions

Syntax :

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chmod permissions filename

Example :

1

chmod 764 sample.txt

"chmod" stands for change mode

Number	Binary	Symbol	Permission Type
0	000	---	No Permission
1	001	--x	Execute
2	010	-w-	Write
3	011	-wx	Execute + Write
4	100	r--	Read
5	101	r-x	Read + Execute
6	110	rw-	Read +Write
7	111	rwx	Read + Write +Execute

Changing Ownership

• For changing the user of a file/directory

Syntax :

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sudo chown user filename

Example :

1

sudo chown root sample.txt

• For changing the user and group of a file/directory

Syntax :

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sudo chown user:group filename

Example :

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sudo chown root:root sample.txt

Packaging

Package & Repository

• A package file is a compressed collection of files that comprise the software package

• Packages are stored in repositories to make them accessible to users

Package Managers

apt

• APT stands for the Advanced Packaging Tool

• It is used to install, upgrade or remove software packages in linux

Syntax :

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sudo apt

Installing Package

Installing a package from a repository

Syntax :

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sudo apt install package_name

brew

• It is a Package Manager for macOS.

  • With 
    brew
     command you can install the packages in macOS.

Installing Package

Syntax :

1

brew install package_name

yum

yum

is a graphical based package management tool for RPM (RedHat Package Manager) based Linux systems.

Installing Package

Syntax :

1

yum package_name

Downloading Files From Web

wget

wget

is a command-line utility for downloading files from the web.

To install

wget

:

• apt

1

sudo apt install wget

• brew :

1

sudo brew install wget

• yum:

1

sudo yum install wget

wget

will download the resource specified in the url to the current directory.

Syntax :

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wget "URL"

Example :

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wget "https://www.lifewire.com/uses-of-command-wget-2201085"

curl

curl

is a command-line utility for transferring data from or to a server designed to work without user interaction.

To install

curl

:

• apt:

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sudo apt install curl

• brew :

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sudo brew install curl

• yum :

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sudo yum install curl

curl

prints the contents of the URL to the output.

Syntax :

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curl "URL"

Example :

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curl "wttr.in"

Searching a Package

apt-cache

command is used to searches for a particular package.

Syntax :

1

sudo apt-cache search package_name

Example :

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apt-cache search google

Updating Packages

• System checks against the repositories.

• If newer version available of the program it will update the information about the existing packages and their versions available.

Updating a package from a repository:

upgrade

upgrade

will upgrade all the applications to latest version.

1

sudo apt upgrade

update

update

will simply update the information about the existing packages and their versions available

1

sudo apt update

Listing Installed Packages

dpkg -l

: It is used to list all installed packages

1

sudo dpkg -l

Adding Repository

PPA (Personal Package Archive) is an application repository that can be used to upgrade and install packages from third parties.

add-apt-repository

: It is used to add repository

Syntax :

1

sudo add-apt-repository repository_link

Adding the Private PPA's security key

Your Linux device can use that signature to check the authenticity of the packages.

apt-key

: Add PPA security key

Syntax :

1

sudo apt-key add - KEY_ID

Removing a Package

remove

: removes the installed packages

Syntax :

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sudo apt remove package_name

Summary

Command	Description
sudo	Temporarily elevates the privileges
which	Used to identify the location of a given executable path
useradd	Used to create a new user with the given username
passwd	Used to set or change password of a given user
su	Used to execute command as another user
chmod	Used to change the access permissions of file
chown	Changing the user of a file/directory
apt	Used to install, upgrade or remove software packages in linux
wget	Used for downloading files from the web
curl	Used for transferring data from or to a server
apt-cache	Searches for a particular package
add-apt-repository	Used to add repository
apt-key	Used to add PPA security key

Warning

Under the installation of an

apt

package from a PPA, there is a change in one of the MongoDB installation steps, due to the changes made by the MongoDB website.

Update command from

1

curl https://www.mongodb.org/static/pgp/server-4.4.asc | sudo apt-key add -

to

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curl https://pgp.mongodb.com/server-4.4.asc | sudo apt-key add -

for a seamless monogodb installation.