1. Notes

1.1. Table of Contents

Table of Contents
Colorized Logs
Notes
Common Commands
1. Shell
2. Home
3. Files
4. History
5. Users
6. Groups
7. Permissions

1.2. Colorized Logs

setup_env.sh installs lnv package that enables tailing and colorizing logs, searching etc. Run the following commands after running the setup to set up a custom log viewer for python logs.

This enables colorized viewer for the following python log format:

[%(asctime)s] %(levelname)s %(name)s - %(message)s

% mkdir -p ~/.lnav/formats/ 
% cp config/lnav_format_python.json ~/.lnav/formats/
% lnav -i ~/.lnav/formats/lnav_format_python.json
✔ installed -- /Users/O60774/.lnav/formats/installed/pythonlogger.json
%    

1.3. Notes

Hard Link	Symbolic Link
associate 2 or more files with same inode	a small file that is a pointer to another file
share same data blocks on hard disk. That is why hard links show the same size as the original file	contains the path to the target file instead of a physical location on the hard disk. That is why they are always small in size
can not span partitions because inode numbers are only unique within a given partition	since inodes are not used in this system, soft links can span across partitions
`ln sfile1file link1file`	`ln -s targetfile linkname` is used to create symbolic link

1.3.1. `alias` list all aliases currently set for your shell account

1.3.1.1. `ls` default scheme color

Color	File type
blue	directories
red	compressed archives
white	text files
pink	images
cyan	links
yellow	devices
green	executables
flashing red	broken links

1.3.1.2. `ls` default suffix scheme

Character	File type
nothing	regular file
`/`	directory
`*`	executable file
`@`	link
`=`	socket
`\\|`	named pipe

1.3.1.3. What is umask?

umask setting plays a big role in determining the permissions that are assigned to files that you create
The default permissions when creating a new dir is octal 777 (111 111 111), and a new file is octal 666 (110 110 110). We set the umask to block/disable certain permissions.
A mask bit of 1 means to block/disable that permission (put masking tape over that bit).
A mask bit of 0 will allow the permission to pass through (no masking tape over that bit).
So an octal 022 (000 010 010) mask means to disable group write and others write, and allow all other permissions to pass through.
umask is a setting that directly controls the permissions assigned when you create files or directories. Create a new file using a text editor or simply with the touch command, and its permissions will be derived from your umask setting.
The umask setting for all users is generally set up in a system-wide file like /etc/profile, /etc/bashrc or /etc/login.defs — a file that’s used every time someone logs into the system

1.3.1.3.1. What is my default umask setting?

Ignore the first character/zero

rishik@rishik-computer:~$ umask -S
u=rwx,g=rwx,o=rx
rishik@rishik-computer:~$ umask
0002

1.3.1.4. /dev/null, /dev/random, and /dev/zero

The /dev file system does not just contain files that represent physical devices. Here are three of the most notable special devices it contains:

/dev/null – Discards all data written to it – think of it as a trash can or black hole. If you ever see a comment telling you to send complains to /dev/null – that’s a geeky way of saying “throw them in the trash.”
/dev/random – Produces randomness using environmental noise. It’s a random number generator you can tap into.
/dev/zero – Produces zeros – a constant stream of zeros.

1.4. Common Commands

1.4.1. Shell

1.4.1.1. Known shells to Linux system

rishik@rishik-computer:~/ws$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/bin/rbash
/bin/dash

1.4.1.2. Which shell am I using?

rishik@rishik-computer:~/ws$ echo $0
/bin/bash
rishik@rishik-computer:~/ws$ echo $SHELL
/bin/bash

1.4.1.3. What is the default shell set for each user?

rishik@rishik-computer:~/ws$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
...
rishik:x:1000:1000:Rishik,,,:/home/rishik:/bin/bash

1.4.2. Home

1.4.2.1. What is my home dir?

rishik@rishik-computer:~/ws$ echo $HOME
/home/rishik

1.4.3. Files

1.4.3.1. Guess the file type!

rishik@rishik-computer:~/Downloads$ file ideaIC-2018.3.2.tar.gz 
ideaIC-2018.3.2.tar.gz: gzip compressed data, from FAT filesystem (MS-DOS, OS/2, NT)

rishik@rishik-computer:~/ws$ file datasets/citibikenyc/JC-201709-citibike-tripdata.csv 
datasets/citibikenyc/JC-201709-citibike-tripdata.csv: ASCII text

rishik@rishik-computer:~$ file /dev/sda
/dev/sda: block special (8/0)
rishik@rishik-computer:~$ file /dev/sda1
/dev/sda1: block special (8/1)
rishik@rishik-computer:~$ file /dev/null
/dev/null: character special (1/3)

1.4.3.2. Find executable file

which searches the user’s search PATH. Good for troubleshooting Command not found problems.

rishik@rishik-computer:~$ which -a java
/usr/bin/java
rishik@rishik-computer:~$ which docker
/usr/bin/docker

1.4.3.3. Check if a command is an alias for another command!

rishik@rishik-computer:~$ alias ls
alias ls='ls --color=auto'
rishik@rishik-computer:~$ alias ltr
alias ltr='ls -ltr'

1.4.3.4. Find all files whose filename has “readme” in it

rishik@rishik-computer:~$ find /usr -name "*readme*"
/usr/share/snmp/mib2c-data/mfd-readme.m2c
/usr/share/snmp/mib2c-data/syntax-DateAndTime-readme.m2i
/usr/share/mime/text/x-readme.xml
/usr/share/icons/elementary/mimes/128/text-x-readme.svg
/usr/share/icons/elementary/mimes/64/text-x-readme.svg
/usr/share/icons/elementary/mimes/32/text-x-readme.svg
/usr/share/icons/elementary/mimes/16/text-x-readme.svg
/usr/share/icons/elementary/mimes/24/text-x-readme.svg
/usr/share/icons/elementary/mimes/48/text-x-readme.svg
/usr/share/games/assaultcube/packages/maps/official/official_readme.txt
/usr/share/games/assaultcube/packages/maps/preview/readme.txt
/usr/share/games/assaultcube/packages/maps/servermaps/readme.txt
/usr/share/games/assaultcube/packages/textures/kurt/dummyfiles_readme.txt
/usr/share/doc/assaultcube-data/docs/cube_bot-readme.txt.gz
/usr/share/doc/aufs-tools/examples/uloop/00readme.txt.gz
/usr/share/doc/p7zip/DOC/readme.txt.gz
/usr/share/lintian/checks/debian-readme.pm
/usr/share/lintian/checks/debian-readme.desc

1.4.3.5. Find all files bigger than 100MB

rishik@rishik-computer:~$ find . -size +100M
./Downloads/ideaIC-2018.3.2.tar.gz
./.config/epiphany/gsb-threats.db

1.4.3.6. Find all files whose filename has “readme” in it

locate is fast as its output is based on file index database. But it is refreshed only once every day.

rishik@rishik-computer:~$ locate readme
/home/rishik/ws/datasets/machine-learning-a2z/readme.txt
/usr/share/doc/aufs-tools/examples/uloop/00readme.txt.gz
/usr/share/doc/p7zip/DOC/readme.txt.gz
/usr/share/icons/elementary/mimes/128/text-x-readme.svg
/usr/share/icons/elementary/mimes/16/text-x-readme.svg
/usr/share/icons/elementary/mimes/24/text-x-readme.svg
/usr/share/icons/elementary/mimes/32/text-x-readme.svg
/usr/share/icons/elementary/mimes/48/text-x-readme.svg
/usr/share/icons/elementary/mimes/64/text-x-readme.svg
/usr/share/lintian/checks/debian-readme.desc
/usr/share/lintian/checks/debian-readme.pm
/usr/share/mime/text/x-readme.xml
/usr/share/snmp/mib2c-data/mfd-readme.m2c
/usr/share/snmp/mib2c-data/syntax-DateAndTime-readme.m2i
rishik@rishik-computer:~$ 

1.4.4. History

history lists all previously ran commands
!! runs the last command
!2 runs the command at index 2 from the output of history command

1.4.4.1. Where is my history stored?

rishik@rishik-computer:~$ echo $HISTFILE
/home/rishik/.bash_history

1.4.5. Users

1.4.5.1. What is my username?

rishik@rishik-computer:~$ echo $USER
rishik
rishik@rishik-computer:~$ whoami
rishik

1.4.6. Groups

1.4.6.1. What is my default group and other groups I belong to?

rishik@rishik-computer:~$ id
uid=1000(rishik) gid=1000(rishik) groups=1000(rishik),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),118(lpadmin),127(sambashare)

:warning:

User private group scheme: In order to allow more flexibility, most Linux systems follow the so called user private group scheme, that assigns each user primarily to his or her own group. This group is a group that only contains this particular user, hence the name “private group”. Usually this group has the same name as the user login name, which can be a bit confusing.

1.4.6.2. What other groups do I belong to?

:warning: groups is deprecated in lieu of id -Gn

rishik@rishik-computer:~$ groups rishik
rishik : rishik adm cdrom sudo dip plugdev lpadmin sambashare docker
rishik@rishik-computer:~$ id -Gn rishik
rishik adm cdrom sudo dip plugdev lpadmin sambashare docker

1.4.6.3. How can I log in to other groups I belong to? For e.g. docker

rishik@rishik-computer:~$ id
uid=1000(rishik) gid=1000(rishik) groups=1000(rishik),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),118(lpadmin),127(sambashare)

rishik@rishik-computer:~$ newgrp docker

rishik@rishik-computer:~$ id
uid=1000(rishik) gid=999(docker) groups=999(docker),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),118(lpadmin),127(sambashare),1000(rishik)

rishik@rishik-computer:~$ newgrp rishik

rishik@rishik-computer:~$ id
uid=1000(rishik) gid=1000(rishik) groups=1000(rishik),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),118(lpadmin),127(sambashare),999(docker)

1.4.6.4. How to change user and group ownership on a file or directory?

sudo chown ownerName:groupName [dir | fileName] - change owner and group of a folder or file
sudo chgrp - change only group permissions
Both chown and chgrp can be used to change ownership recursively, using the -R option

1.4.7. Permissions

Use chmod to change access modes for user, group or others.