Linux and Shell Scripting-devops : 2

Linux is a free, open-source operating system (OS) based on the Unix operating system, designed to manage computer hardware and software resources. It acts as an intermediary between the user and the hardware, facilitating the execution of applications and services.

• Open source

• Free

• Secure

• Very Fast

Karnel : The core of the Linux operating system, managing hardware resources and communication between hardware and software.

Shell : The command-line interpreter that allows users to interact with the system by executing commands and scripts. Translates user commands into kernel-level instructions. Shells like Bash, Zsh, and others provide various functionality for command execution and scripting.

System Library : These libraries contain the essential functions and utilities that applications need to interact with the kernel without directly invoking system calls.

For example: GNU C Library (glibc), which provides functions like file handling, memory allocation, and process management.

Important commands

• pwd : Prasent working directory

• ls : List all file and folder

  • ls -a : List all including hidden file

  • ls -l : List with other permission and size related information

  • ls -lh : h use for human readable format

• cd directory_name : Change directory

  • cd ~ : Home directory

  • cd ../ : Back

• mkdir new_directory_name : Create directory

  • mkdir -p new_directory_name/new_directory_name : Create parent directory if parent not present

• rmdir directory_name : Remove directory but directory needs to be empty

  • rmdir -p directory_name/directory_name : Delete parent directory but all are needs to be empty.

• touch file_name : Create new file

• cat file_name : Show the content of file

• rm file_name : Remove file

• vi file_name : Edit file or create new file for write

• free : Check ram in the system

  • free -h

• nproc : Check number of cpu

• df -h : Check disk info

• top : Show the linux process.

• history : To see all the commands that you have used.

• ps : Process status

  • ps -ef : -e will select all processes . -f will provide a full-format listing for each process.

• | : Pipe is use to send the output of a first comand to second one as inout.

• set : The set command in Linux is a built-in shell command used to configure shell options and control the behavior of the shell environment.

  • set : Running set without any options will display all shell variables and functions that are currently set.

  • set -e : Exit immediately if a command exits with a non-zero status (useful in scripts for error handling).

  • set -x : Print each command to the terminal before executing it (debugging).

  • set +e : Disable the -e option.

  • set +x : Disable the -x option.

  • set -o pipefail : set -e does not catch any pipe failure. It use for detect pipefail.

• find : Find use to find folder or files . It works recursively.

  • find . -name *.sh : Find all the files that have .sh extension. -name is case sensitive. Use -iname for case insensitive.

  • find . -type d -name src : Find all the directory that name src. -type f for file and -type l for symbolic link.

• grep : Use to search in files. -i →case insensitive, -n → show line number, -w →Find exact matching word, -c →count the occurence, -v : invert match.

  • grep car car.txt : Search the car from car.txt.

  • grep -n car car.txt : Show the line number.

  • grep -c car car.txt : Show count

  • cat car.txt | grep -n taxi : Find taxi.

  • grep -nC 2 car car.txt : It tells grap to print two line before and after the matched line.

• curl [options] [url] : Curl use to transfer data using various protocol.

  • curl url : Fetch content from url.

  • curl -o directory/filename URL : Fetch and save data in specified directory.

• wget [options] [url] : Use to download file from server.

  • wget url : Download the content.

  • wget -b url : Download the content in background.

  • wget -o file url : Redirect the output message to the specified file.

• awk [options] [selection] inputFile : Use for text processing.

  • awk '{print $1}' inputfile.txt : Print frist column.

  • awk '{print NR " " $1}' inputfile.txt : Print frist column with serial number.

  • awk -F"," ‘{print $1}’ inputfile.txt : -F is use to tell that the data is separeted by specified delimiter.

  • awk '{if($2 > 10000)print $0}' inputfile.txt : Print all the data that 2nd col value grater than 10000. print $0 means print all the columns.

  • awk '{if($2=="Night"){$3 =100} print $0} inputfile.txt : If col 2 value is Night then col 3 will be 100.

  • awk '/Night/ {print $0}' inputfile.txt : Print the row that has text Night in any column.

  • awk '{if(NR == "4") print $0}' inputfile.txt : Print 4th row.

  • awk 'NR=="4"{print $0}' inputfile.txt : Print 4th row.

  • awk 'NR=="4",NR=="10"{print $0}' inputfile.txt : Print 4th to 10th row.

  • awk 'NF==0 {print NR}' inputfile.txt : Print empty row.

  • awk 'END {print NR}' fileinput.txt : Print number of row.

  • awk 'BEGIN {for(i=0;i<=10;i++) print i;}' : For loop.

  • awk 'BEGIN {while(i<10){ i++; print "Num is " i;}}' : While loop.

• kill [-SIGNAL] <pid> : The kill can send a variety of signals to a program. By default it send TERM signal if signal is not specify.

  • kill -HUP <pid> : HUP means hang up. It's sent automatically when a terminal window that started a process is closed before terminating the process.

  • kill -INT <pid> : INT means interrupt, and it sends the same signal used when we press ctrl-C in the terminal, which usually terminates the process.

  • kill -KILL <pid> : KILL is not sent to the process, but to the operating system kernel, which immediately stops and terminates the process.

  • kill -TERM <pid> : TERM means terminate. The process will receive it and terminate itself. It's the default signal sent by kill .

  • kill -CONT <pid> : CONT means continue. It can be used to resume a stopped process.

  • kill -STOP <pid> : STOP is not sent to the process, but to the operating system kernel, which immediately stops (but does not terminate) the process.

• trap [-options] "piece of code" [signal name or value] : It execute specific code in response to the specified signal.

  • trap "echo Hello World" SIGINT : Print Hello World everytime when you press ctrl+c .

Shell Scripting

Every .sh file (shell script) typically starts with #!/bin/bash (or a similar line like #!/bin/sh) . This is known as a shebang (also called hashbang).

• The line #!/bin/bash tells the operating system which interpreter to use to execute the script. In this case, it specifies the Bash shell as the interpreter.

• Without the shebang, the system might not know which interpreter to use, and it could default to a different shell, potentially causing the script to behave unexpectedly if it relies on Bash-specific features.

• #!/bin/sh: Uses the default shell, which might be a different shell like Dash on some systems.

• #!/usr/bin/env bash: This is another common variant that finds the Bash interpreter in your system's environment, making it more portable across different systems where Bash might be installed in different locations.

How to run .sh file : ./file or sh file

Shell Script Example :

#!/usr/bin/env bash
#####################################
# Author : 
# Date : 
# version :
# About the script
#####################################
set -x
set -e
set -o pipefail
# Print memory status
free
# Print disk status
df -h
# Print processes
ps -ef