Linux-Command-Line

Bash Shell Command & Scripting

Shell

Shell is a program that is capable of understanding shell commands. Bash shell is one of the most propular shell programs.

Terminal

Terminal is just a graphical interface between the shell program and command. This is where we write commands.

Commands

Command are instruction that are written in the terminal and submitted to the shell for interpretation.

GNU Project (http://www.gnu.org/)

Free software is open source but not all open source is free software
Richard Stallman began development of GNU operating system in 1983
The freedom is provided by GNU public license (GPL)
In 1991 Linus Torvalds created the Linux Kernel and published it under GPL v2.0

Installing free software (for example: coreutils)

wget https://ftp.gnu.org/gnu/coreutils/coreutils-8.28.tar.xz
tar -xvJf coreutils-8.28.tar.xz
cd coreutils-8.28/
bash configure # configure compuler and create Makefile
make # compilation probably
sudo make install install the software

Check your ubuntu distribution codename

lsb_release -a

Ubuntu software repositories (https://help.ubuntu.com/community/Repositories/Ubuntu)

is like a library of software
Used for Installing softwares
Used for Updating softwares to updated version
Four different software repositores:

Main: Cononical supported free and open source software
Universe: Community maintained free and open source
Restricted: Proprietary drivers for devices (can be open source)
Multiverse: Software restricted by copyright or legal issues (can be open source)

Note: Fedora only supports free softwares

Checksum

Ref: How-To Geek

Checksums may also be called “hashes.”
The algorithm uses a cryptographic hash function that takes an input and produces a string (a sequence of numbers and letters) of a fixed length

Ubuntu package manager

APT (Advanced Package Tool)

deals with installing dependencies and makes life easier
list of all available packages is located at var/lib/apt/lists (this folder is called apt-cache)
sudo apt-get update # updates the cache of packages and dependencies
sudo apt-get upgrade # upgrades every software in the local machine to the newest version
sudo apt-get install x11-apps # install x11-apps in the local machine

Downloading and modifying source code

nano /etc/apt/sources.list and uncomment all of the lines in front of deb-src
sudo apt-get update # updates the cache
sudo apt-get install dpkg-dev
sudo apt-get source <package_name> # downloads source code in pwd

Uninstallting package

sudo apt-get remove <package_name> # uninstall package_name from the system but configuration files are left in the system
sudo apt-get purge <package_name> # uninstall package_name and all of the configuration files. (preferred way)
sudo apt-get autoremove# remove packages those were previously installed as dependencies but no longer required by any packages
sudo apt-get clean # deletes all compressed package archives
sudo apt-get autoclean # deletes package archives that are no longer accessible

Uninstalling deb packages (installed with dpkg)

Search the package name ex: 'foliate'
If installed it will give fullname with ii
Use the fullname to uninstall: sudo dpkg -P com.github.johnfactotum.foliate

Note: package manager downloads archive files (package archives) prior to installing the package into the system. These are located at /var/cache/apt/archives

Snap packages

Install with snap: sudo snap install <package_name> Example: sudo snap install rambox
Update packages installed with snap: snap refresh
Uniinstall packages: sudo snap remove rambox

Basics

0 = standard input; 1 = standard output; 2 = standard error
redirection of standard output breaks the pipleline
tee command helps continue flowing the output whereas output redirection halts the output upto that stage
pipe | is used for connecting starndard output of one command to standard input of anohter command
all command doesn't accept standard input, they only accept command line arguments, xargs are used in these cases
~ is home directory for current user i.e /home/utshaw
directories or files starting with dot . are hidden files
single dot . is current directory
double dot .. is previous directory
press double tab to show all completion possible with current amount of text
file extension doesn't matter in linux unlike windows
In order to determine the file type linux reads piece of code inside the top of each file known as header
directory/* means everything inside the directory but not itself
directory/ means directory itself + everything inside the directory

EUID vs UID

EUID: Effective User ID
UID: User ID
When the setuid bit is set, the file will execute with the permission of the owner of the file & only then EUID=owner's id, UID=user executing the script; other times EUID=UID
This is rarely seen as modern linux OS doesn't allow scripts to be setuid
root's UID=0
if a command/program/script exits successfully then the exit status is 0 otherwise, the exit status is non-zero

User ID of accounts

system/application accounts have lower uids (<1000)
normal user has id>=1000
ids are set in file /etc/login.defs

Shell built-in

Stackoverflow post about shell-builtin

builtin commands are part of the shell, and are implemented as part of the shell's source code.
see the help page by help <shell_built-in>; ex: help read # read is a shell-built in

nano text editor

^ symbol = ctrl
M- symbol = alt
Nano configuration file location: /etc/nanorc

bash script file

Note: bash scripts can be used for scheduling

name the file as script.sh
the first line will be #!/bin/bash (Shebang line); the rest of the script will be interpreted by bash script
chmod +x script.sh
In order to run the script from anywhere we need to add the path to $PATH environment variable
In the end of the ~/.bashrc file add this line: PATH="$PATH:$HOME/bin"
The above line will cause all of the files in $HOME/bin location to be avialable from terminal
bash shell keeps hash to remember all program locations to execute. hash -r resets that hash for all remembered locations

Arithmetic Expression

NUM=$(( 1 + 2 ))
echo "${NUM}" # 3
NUM=$(( 6 * 8 ))
echo "${NUM}" # 48
NUM=$(( 6 / 4 ))
echo "${NUM}" # 1 integer division; doesn't support floatin point arithmetic
REM=$(( 6 % 4 ))
echo "${REM}" # 2 -> remainder of 2
DICEA='3'
DICEB='6'
NUM=$(( DICEA + DICEB ))
echo "${NUM}" # 9
MY_NUM=9
(( MY_NUM++ ))
echo "${MY_NUM}" # 10
(( MY_NUM-- ))
echo "${MY_NUM}" # 9
(( MY_NUM+=5 ))
echo "${MY_NUM}" # 14
OTHER_NUM=$(( MY_NUM+25 ))
echo "${OTHER_NUM}" # 39

read

It is a bash shell built-in

basename

strip directory and suffix from filenames

basename /farhan/tanvir/utshaw/file.txt
# Output: file.txt

dirname

strip last component from file name

dirname /farhan/tanvir/utshaw/file.txt
# Output: /farhan/tanvir/utshaw

scheduling scripts

cron

cron - daemon to execute scheduled commands (Vixie Cron)
Each user gets a cron tab that lists which commands or scripts will be automatically run by the user

crontab -e # edit crontab of current user
format: m h dom mon dow command
<minute[0-59]>
<hour[0-23]>
<day_of_the_month[1-31]>
<month[[1-12][JAN-DEC]]>
<day_of_the_week[[0-6][SUN-SAT]]>

20 11 * * * <command> means it will run at 11:20AM every day,month, year
20 11 10 JUN SUN <command> this command will run at 11:20AM of JUN only if 10th JUN is Sunday
* * means any minute any day
* * * * * <command> # the command will run every minute every hour
0,15,30,45 * * * * echo "Hello23" # evry 15 minutes run this command
*/15 \* \3 \* \* echo "Hello23" # evry 3 days every 15 minutes run this command
59 23 * JAN,DEC * echo Hello # run the command only on JAN and DEC each day at 23:59
**Note: adding to environment variable $PATH won't let cron command know the command. **

59 23 * * FRI bash ~/bin/script # run the script file named /home/<user_name>/script

aliases

Note: aliases can't be used for scheduling aliases are mainly used as shorter form of the original longer command steps:
create a .bash_aliases file in home
alias aliasName="<long_command>" i.e alias getPythonVersion="python --version"
save the file
restart terminal
enter aliasName i.e getPythonVersion

wildcards

* means abything i.e *.txt matches any file with .txt extension
? means any character occuring once i.e ?,txt means any file named 'A.txt' or 'B.txt'
[] means choose any one from inside of the square (must choose exactly one) file[123].txt matches file1.txt , file2.txt, file3.txt but doesn't select file12.txt or file.txt

brace expansion

echo {1..10} outputs 1 2 3 4 5 6 7 8 9 10
echo {A,B,C}-{1,2,3} outputs A-1 A-2 A-3 B-1 B-2 B-3 C-1 C-2 C-3
mkdir {jan, feb, mar, apr, may}_{2019,2020} # creates 5*2 directories

Archiving file

Make tarball from the files using tar command
Compress tarball using gzip or bzip2

Commands

tty

command to show current terminal file location

grep

grep searches for PATTERN in each FILE.
grep returns lines that contain a piece of text wildcards also works
grep [OPTIONS] PATTERN [FILE...]
options:

-C NUM, -NUM, --context=NUM Print NUM lines of output context.
-v, --invert-match Invert the sense of matching, to select non-matching lines.
-i, --ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

grep command is canse-sensitive
grep -rnw './' -e '<text_to_find>' # search for all files that contain a specific text
grep -rnwi './' -e '<text_to_find>' # search for all files that contain a specific text case-insensitive
grep e hello.txt # outputs the line(s) containing letter e in hello.txt
grep you -c gadsby.txt # outputs number of references of word 'you' in gadsby.txt file
grep -i gadsby gadsby.txt # case insensitive search
grep -i "our boys" gadsby.txt # sentence search
grep -vi e gadsby.txt # outputs those lines without the letter e
grep -ic "e" hello.txt gadsby.txt # multiple file search
How do I find all files containing specific text on Linux?

man

search for specific keyword: man -k "your keyword here"

man -k "list directory contents"

Output:

dir (1) - list directory contents

ls (1) - list directory contents

ntfsls (8) - list directory contents on an NTFS filesystem

vdir (1) - list directory contents

cat

reads from standard input and writes into standaed output

standard output redirection:

cat 1> output.txt # redirects standard output; cleans the taget file and writes into it

cat > output.txt # redirects standard output; cleans the taget file and writes into it

cat >> output.txt # redirects standard output; appends into the target file

standard input redirection:

cat 0< input.txt # redirects standard input; reads from the input.txt and loads them into cat program

cat < input.txt # redirects standard input; reads from the input.txt and loads them into cat program

cat 0< input.txt 1> output.txt # redirects standard input; reads from the input.txt and redirects them into output.txt

cat 0< input.txt > /dev/pts/3 # redirects std input and std output; reads from input.txt and sends them to another terminal

cat file1.txt file2.txt file3.txt > compile.txt # concatenating all of the outputs and puts them into compile.txt
cat file[1-3]txt > compile.txt # concatenating all of the outputs and puts them into compile.txt

File Descriptor

Number that represents an open file
Every new process starts with three file descriptor
FD 0 STDIN
FD 1 STDOUT
FD 2 STDERR
Linux represents everything as file
Resources like keyboard, montior are treated like a file
FDs are like pointers of sources of data or where data is displayed

tac

tac - concatenate and print files in reverse
tac file.txt # outputs the lines of the file in reverse order. the last line will be first. the first line will be last horizontally same vertically reversed

rev

rev - reverse lines characterwise
rev file.txt # vertically same horizontally reversed each line

less

less - opposite of more

head

head - output the first part of files
by default shows the first ten lines
head -n 5 file.txt # outputs first 5 lines of file.txt
find . | head -n 2 # outputs only first 2 results

tail

tail - output the last part of files

cut

remove sections from each line of files

example:

date | cut -d " " -f 1 | cat > date.txt # pipes from date to cut then get the first field based on space delimiter and then pipes to date.txt

tee

read from standard input and write to standard output and files

example:

date | tee fulldate.txt | cut -d " " -f 1 # two pipes from date one gets the output to be written to fulldate.txt , the other goes to cut command

xargs

build and execute command lines from standard input

example: echo doesn't support standard input only supports command line argument

date | xargs echo # prints the date in standard output cat fileToDelete.txt | xargs rm # delete file with filename written in fileToDelete.txt

file

determine file type
file [file_name]
Example:
file .bash_aliases
Output:
.bash_aliases: ASCII text

rm

remove files or directories
rm -r delFolder # deletes delFolder
rm -r delFolder/del{1,2,3} # # deletes del,1 del2, del3 folders
rm -r delFolder/ # deletes delFolder and everything inside of it
rm -r delFolder/* # deletes everything inside delFolder but not itself
rm -ri delFolder/ # interactive deletion: asks before deletion of each file or folder
rm delFile # deletes delFile

rmdir

rmdir - remove empty directories
rmdir delFolder # removes delFolder only if it is empty

mkdir

mkdir - make directories
mkdir -p aFolder/bFolder/cFile # creates the full path along the way of creating folders and files

touch

touch - change file timestamps
generally used to create empty file[s]

touch aFolder/file{1,2,3}.txt # creates three empty files inside aFolder

cp

cp - copy files and directories
cp [OPTION]... [-T] SOURCE DEST
cp [OPTION]... SOURCE... DIRECTORY
cp file1.txt file2.txt # file1 is source file2 is destination
cp file1.txt file2.txt directory/ # file1 , file2 are sources directory is destination directory
cp directory/* . # copy everything from directory to current folder
cp -r sourceDir/* destDir # copy everything from sourceDir to destDir

sort

sort - sort lines of text files
oprtions:

-n, --numeric-sort compare according to string numerical value
-M, --month-sort
-r, --reverse reverse the result of comparisons
-h, --human-numeric-sort compare human readable numbers (e.g., 2K 1G)

Note: -M and -n options can't be used together

sort file.txt # sorts the content of file.txt alphabetically
sort -r file.txt # sorts the content of file.txt in reverse alphabetical way
sort -n numbers.txt # sort based on value of the numbers
sort -u numbers.txt # sort and outputs only unique
ls -l | head -n 20 | sort -k 5 -n # sorts files based on size
ls -lh | head -n 20 | sort -k 5 -hr # human readable format sort ls -lh /etc | head -n 20 | sort -k 6 -M # sort by month

su

temporarily substitute users
command -options arguments

which

which -a python3 # prints all matching executables in the PATH

mv

mv - move (rename) files
mv oldName.txt newName.txt # renames to newName.txt
mv oldFolder newFolder # renames to newFolder
mv directory/* . #moves everything from directory folder to current directory

locate

locate - find files by name
locate command uses a database which gets updated once a day. this can show outdated result
in order to update the database use: sudo updatedb
options: -e: Print only entries that refer to files existing at the time locate is run.
locate utshaw # searches for file or directories with case sensitive name utshaw in their path
locate -i uTsHaW # searches for file or directories with case insensitive name utshaw in their path
locate -c uTsHaW # count the number of occurrences of utshaw found
locate -e utshaw # outputs actually existing file
sudo updatedb # updates the database locate command uses for searching
locate --existing --follow utshaw # checks if the file really exists and omit broken symbolic links

find

find - search for files in a directory hierarchy
find command doesn't require database to operate it's always upto date
locate only outputs files whereas find outputs files and folders
find # outputs every files and directories from the current directory tree recursively
find . -maxdepth 1 # det maximum depth for search
find . -type f # outputs only files
find . -type d # outputs only directories
find -type f -size +100k # outputs files with size > 100k
find -type f -size -100k # outputs files with size < 100k
find -type f -size +100k -size -5M # outputs files with size > 100k & <5M
sudo find -maxdepth 3 -type f -size +10k -size -12k -exec cp {} ~/Desktop/test/ \; # copies each of those files to test folder
find -type f -name "test.txt" # case-sensitive searches for test.txt from current directory tree
find -type f -name "test*" # case-sensitive searches for files starting with test from current directory tree
find -type f -iname "test*" # case-insensitive searches for files starting with test from current directory tree
find -type f -name "*.py" # finds all files with extension py
find -type f -mmin -10 # finds all files those were modified less than ten minutes ago
find -type f -mmin +1 -mmin -10 # finds all files those were modified greater than one minute ago and less than ten minutes ago

tar

tar - an archiving utility
options:

-c, --create Create a new archive.
-v, --verbose Verbosely list files processed.
-f lets tar command accept files
-t List the contents of an archive.
-C, --directory=DIR Change to DIR before performing any operations.This option is order-sensitive,
i.e. it affects all options that follow.
tar -xvzf ideaIU-2019.2.4.tar.gz -C ~/ # extracts to home folder
tar -cvf ourArchive.tar file[1-3].txt # create a new archive named ourArchive.tar
tar -tf ourArchive.tar # outputs contents of ourArchive.tar
tar -xvf ourArchice.tar # extracts ourArchive.tar
tar -cvzf ourArchive.tar.gz diary.txt utshaw.txt diary.txt # create gzip file in one command
tar -cvjf ourArchive.tar.bz2 diary.txt utshaw.txt diary.txt # create bz2 file in one command
tar -xvzf ourArchive.tar.gz # unzips from gz file
tar -xvjf ourArchive.tar.bz2 # unzips from bz2 file
tar -xvJf ourArchive.tar.xz # extract from tar.xz file

gzip

gzip is faster but with less compression power means file size won't be that small
gzip ourArchive.tar # compress the tar file with gz extension
gunzip ourArchive.tar # un-compress the gz file and give back tar file

bzip2

bzip2 is slower but has higher compression power can compress to very small files
bzip2 ourArchice.tar # compress the tar file with .bz2 extension
bunzip2 ourArchice.tar.bz2 # un-comress the bz2 file and gives back tar file

zip

zip - package and compress (archive) files
zip ourZip.zip file1.txt file2.txt # creates zip file named ourZip.zip from file1.txt and file2.txt

wc

wc - print newline, word, and byte counts for each file
wc -l # outputs number of lines the command received in the standard input

tmux

sudo apt-get install tmux

Create new pane: Ctrl+B+%
Switch between pane: Ctrl + B + -> / <-
Quit a pane: exit
Create new window: Ctrl + b c
Go to windows 1 : Ctrl + b 1
Go to windows 0 : Ctrl + b 0
Rename window: Ctrl + b , then rename enter
Detach session: Ctrl + b, d
View tmux sessoins: tmux ls
Attach to running session: tmux attach -t <session_name>
Ex: tmux attach -t 0
Rename tmux session: tmux rename-session -t <session_name> <renamed_session>
Ex: tmux rename-session -t 0 git
Create a session with name tmux new -s <name>
Killing session: tmux kill-session -t <session_name>

Use case scenario

tmux # start a session 0
source venv/bin/activate
export GOOGLE_APPLICATION_CREDENTIALS=/home/gcp_key.json
cd python; sudo python3 driver.py
Ctrl + b , d # detaching from session 0
tmux attach -t 0 # check the running session 0

rsync

https://youtu.be/qE77MbDnljA Usage:

rsync -av get-docker.sh [email protected]:~/Desktop/

xclip

xclip -sel c < file

Download YouTube videos

Folow instructions youtube-dl
Install ffmpeg sudo apt-get install ffmpeg

Resources

Cheat sheet: Linux Training Academy
man page: https://www.youtube.com/watch?v=uJnrh9hAQR0
crontab: https://crontab.guru/

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