B

Bash Intro

23.03.20188 Min Read — InLinux

Easy Shell is a collection of useful boilerplate linux commands for the daily life of every linux user!

All credit goes to lucasviola on GitHub!

Welcome

This is a basics guide intended to help newcomers and experienced Linux users alike get more comfortable with Bash.

This is not a tutorial or anything of that nature, jjjjkust a collection of useful commands, materials, tips and tricks gathered from around the internet.

Introduction

This is probably the most important lesson you will ever learn on Linux: man pages are your best friend! Learn to love them.

To access a man page on any linux program just type:

man <program>

For example:

$ man ps
PS(1)                User Commands                     PS(1)

NAME
       ps - report a snapshot of the current processes.

SYNOPSIS
       ps [options]

DESCRIPTION
       ps displays information about a selection of the active processes.
       If you want a repetitive update of the selection and the displayed
       information, use top(1) instead.

       This version of ps accepts several kinds of options:

       1   UNIX options, which may be grouped and must be preceded by a dash.
       2   BSD options, which may be grouped and must not be used with a dash.
       3   GNU long options, which are preceded by two dashes.
  • To view currently running programs / processes:

ps -aux

  • To view a brief summary on any program:

whatis <program>

  • You can find out where a file is located by typing:

whereis <program> or locate <program>

  • To change user:

sudo -u <user>

  • To change user to superuser (root):

sudo su

This is not recommended! As a superuser you have the rights modify any file, allowing you to fuck your system if you don't know what you're doing.

  • To see your session history:

history

  • | is the pipe character and is used to 'pipe' the output from one command into another as the input.

Remember ps lists running processes. That list can be huge, so if we want to search only for processes running which include the word 'init' (hint grep filters output):

ps -aux | grep init

  • Similarly > and >> are called angle brackets and are used to redirect output. For example, you can write the output of one command into a file by doing:

echo "any random sentence" >> any-random-file.txt or ps -aux | grep chrome >> any-random-file.txt

There is an important difference between using one and two angle brackets like this. The double angle brackets (>>) will concatenate, or append to the target file, while the single angle bracket (>) will overwrite anything that was previously in the target file.

File System Basics

  • / represents the root directory.
  • ~ is an alias for your user's home directory, which usually is: /home/youruser/.
  • . represents the current directory

Compression

  • To decompress a tarball:

tar -xvf <compressed-file.tar.gz>

Note: Flag -v is for verbosity.

  • You can call a previously used command pra typing "!" plus the initials of the said program.

For example, let us say you want to edit .bashrc and then open it to edit again:

vim ~/.bashrc
exec bash
!vim #This will execute the last command on history with the "vim" initials
  • uname -a will tell you information about your system
  • df -h will tell you about your file system disk space

Network

Utils commands

Show all network interfaces

ifconfig

Configure a wireless network interface

iwconfig

Get more information about wireless interface

iwlist <your_interface_here> scan

Check hardware information include about your network, this shows PCIs drivers which is installed or not

lspci

Show who is connected in your network

nmap 192.168.0.*

Verify if you have any open port

nmap <your_ip>

Custom Bash

Customize your bash cursor

The PS1 environment variable contains the style for the bash cursor:

Export it to your ~/.bashrc file:

export PS1='\u@\h \$'

This will print the following as a cursor: user@host $

Some formatting options can be:

\h - The hostname, up to the first ' . '

\H - The hostname.

\n - A newline.

\t - The time, in 24-hour HH:MM:SS format.

\T - The time, in 12-hour HH:MM:SS format.

\@ - The time, in 12-hour am/pm format.

\u - The username of the current user.

\w - The current working directory, with $HOME abbreviated with a tilde.

\W - The basename of $PWD, with $HOME abbreviated with a tilde.

Further reading:

PS1 Generator

Git info on bash PS1

Bash manual

Text File Manipulation

Bit bucket (as known as the black hole, one of the linux's special files):

/dev/null

Standard input stream:

STDIN

Standard outpout streams:

STDERR and STDOUT

Tailing files (getting the last lines)

tail <file>

Outputing whole file content

cat <file>

Reading from STDIN to a file (stop with CTRL + C)

cat > <file>

Reading from file to another file

cat <file1> > <file2>

Reading all lines of file with line numbers

cat -n <file>

Saving output to file (using nano editor, allowing you to edit before saving)

<command> | nano file_name

Searching through the output

<command> | grep <term>

Displaying output in a file-like style (less allows searching by pressing /)

<command> | less or <command> | more

Continue running after command. This will not block further running while the program waits on something (network, user input, etc.)

<command>&

[!] Ignoring file output completely (note that 2 = STDERR and 1 = STDOUT)

<command> > /dev/null 2 > &1

File Permissions

  • To change the owner of a directory (recursively):

sudo chown -R newowneruser:newownergroup

  • To view actual files permissions

ls -l

  - Example: `drwxr-xr-x`:
    - First letter is the file type:

      d          | b          | c            | p    | s      | \-
      :---------:|:----------:|:------------:|:----:|:------:|:-----------:
      directory  | block file | special file | pipe | socket | regular file

    - Second, third and fourth letters are the **user** permissions
    - Fifth, sixth and seventh letters are the **group** permissions
    - Eighth, ninth and tenth letters are the **others** permissions
    - Permissions

      r    | w     | x       | \-
      :---:|:-----:|:-------:|:------:
      read | write | execute | disable

- To change the files permissions:
  - Using letters:
    - Which users:

      u    |   g   |    o   | a
      :---:|:-----:|:------:|:--:
      user | group | others | all

    - Operators:

      \+             |        \-         |                  =
      :-------------:|:-----------------:|:----------------------------------:
      add permission | remove permission | changes permissions to the inserted

    - Permissions:

      r    | w     | x
      :---:|:-----:|:------:
      read | write | execute

    - Example: `$ chmod a+w file` add **write** permission for **all** users
  - Using numbers
    - Permissions:

      read | write | execute
      :---:|:-----:|:------:
        4  |   2   |    1

    - Example: `$ chmod 754 file` set permission to file:

       user                  |      group     |others
      :---------------------:|:--------------:|:------:
       7                     |       5        |  4
      read + write + execute | read + execute | read

Managing Processes

To list all processes on your system

ps -aux

To list all processes running as root

ps -U root -u root u

To list all processes owned by you

ps x

Searching processes by keyword

ps -aux | grep '<keyword>'

Listing it Tree style

ps -aux --forest

Killing a specific process

sudo kill -9 <PID>

Killing all processes except for kill and init

sudo kill -9 -1

Working with environment and variables

Listing local variables

set

Listing global variables

env

Printing variable content

$ foo='This is a variable!' $ echo $foo

Looking for a local variable

set | grep foo

Exporting it to env

$ export foo $ env | grep foo

Useful environment variables

  • $PS1 your prompt setup
  • $PATH your path setup
  • $USER your current user

The most important files regarding your environment are:

  1. $ ~/.profile
  2. $ ~/.bashrc

Both of them are shell scripts and contain instructions which are executed when you log in.

Permanently exporting variables to your PATH:

echo 'export $PATH="$PATH:/path/to/file/"' >> ~/.bashrc

Working with files

  • Where am I?

pwd

  • Creating Files

touch filename

  • Changing directories

cd path/to/directory

  • Moving things around

mv -v <filename> /another/path/

  • Deleting things FOREVER

sudo mv -v <filename> /dev/null/

  • Copying stuff

cp -v <filename> <another-filename>

  • Deleting files

rm -v <filename>

  • Deleting folders

rm -vR /path/to/<folder>

  • Updating timestamp:

touch <filename>

  • Listing files and directories

ls -al

Notes:

  • -a tells ls to list all files.
  • Files starting with . are hidden files. (Remember this).
  • Printing the content of a file:

cat <filename.txt>

  • Printing the last 2 lines of a file:

tail -n 2 <filename.txt>

  • Printing the first 2 lines of a file:

head -n 2 <filename.txt>

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