Introduction-to-Bash-Shell-Scripting

Open cli(terminal) from gui or using ctrl + alt + t to execute commands.
echo $SHELL - returns the shell running on the terminal
- Generally in linux you will find bin/bash which is the bash shell. Bash - Bourne Again Shell
  There are many other shells like c-shell, z-shell, korn-shell.
ls - to list directories
- ls -al list with options. Here by - we are saying that we are starting with some options, a - stands for all (shows hidden files), l - long listing format.
```
total 68
drwxr-xr-x  2 rhyme rhyme  4096 May 25 05:03 .
drwxr-xr-x 27 rhyme rhyme  4096 Sep 22 22:19 ..
-rwxr-xr-x  1 rhyme rhyme  4693 Jul 10  2018 exo-terminal-emulator.desktop
-rw-------  1 rhyme rhyme 50417 May 25 05:03 rhymeemail.png
```
- Here the first column denotes the type and permissions for that files. - If starts with d it is directory, elseif it starts with - then it is a file. - r read, w - write, x - execute permissions.
- 2nd column and 3rd column are user and group respectively.
- 4th column shows the size in bytes and the time last accessed.
- Last column shows the Name of the file or directory
man ls to know more about ls command or any other command at anytime. To quit man page press q.
pwd to see the current directory you are in. Generally terminal starts with the user's directory
Type cd to change directory to user's home directory from any directory.
- You can also use cd ~ to get to the user's home directory
- cd - to move to the previous directory

touch, mv, rm
- rm removes files permanently.
cat - concatinate. Used to view file contents and create file too cat old.txt>new.txt
more - to view contents of a file, click space for moving down the page. q to exit
less - to view contents of a file, and move line by line using arrows and space bar to pagedown. q to exit
You can use any editors like vim, nano to create and edit files from terminal.

rmdir removes empty directory
rm -r removes files and folders recursively. rm -ir interactive delete.
find / -name "filename" - Find with name from /(root directory). This will show some permission errors.
- find / -name "filename" 2>/dev/null - 2 tells the terminal to redirect the error to /dev/null (where it is deleted) and show results.
  This command will look for files with name filename in the specified directory.
- find / -name "*filename*" 2>/dev/null - will show files containing filename text in its name.
grep tells terminal to search for regular expressions
- find / -name "*backup*" 2>/dev/null - This will return long list of files from many directories
  We can use | (pipeline) to pass output of a command as input of other command.
- find / -name "*backup*" 2>/dev/null | grep $USER - This will pass the output of find command to grep, Then grep will search for outputs with username. Learn regular expressions to use grep with more features
Bash keeps history of the commands we have executed
- history - to see the list of commands executed. (Even after shutdown)
- !ls (! - Bang) - To execute the last executed ls command.
- history | grep cat - To find the history of usage of grep command

You can setll as an alias of ls -laF by `alias ll='ls -la'
alias - To see the list of all aliased commands
We can create a file .bash_aliases to create list of all aliases you need to use.
To let terminal know our alias commands run source .bash_aliases
There are other files like .bash_history, .bash_logout Checkitout
To keep the aliases whenever you need, add the file to startup.

#!/bin/bash - This is a note to the interpreter that this is a bash script
# - is comment in bash
VAR=date %d%m%Y - will save the date to VAR variable ( enclose within ` to run the command and return the output)
To get value of stored variables use $VAR.
To run a file ./file(only if file has executable permission) or source file
- chmod to manage file permissions
  - chmod u=rwx filename - to give set permission for user for that particular file
  - chmod u-x filename - to remove the permission to execute from the user
  - chmod u+x filename - to add the permission to x
Refer the mybackup file for the script to create a backup

crontab - crontable to schedule tasks. crontab -l to see the tasks we have set
crontab -e to edit the crontab file to add tasks.
crontab -r to remove tasks in the current user account
crontab shortcuts to add tasks in crontab -e
@yearly /path/to/job or @annually /path/to/job
@monthly /path/to/job
@weekly /path/to/job
@daily /path/to/job or @midnight /path/to/job
@hourly /path/to/job
@reboot /path/to/job - This is useful for adding custom scripts

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