Introduction-to-Bash-Shell-Scripting

Video1: Navigation

• 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

Video2: Manipulating Files

• 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.

Video3: Finding Information

• 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

Video4: Aliases

• 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.

Video5: Writing a Shell Script

• #!/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

Video6: Bash Scripting and Creating a Cron Job With Crontab

• 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