# The Shell

About Shell and bash

### What is the shell?

Shell is a textual interface. Terminal is the virtual interface to a shell. In linux and macos, it is called bash (Bourne Again SHell).

### Navigating in the shell

* The prompt we see in a terminal when we open it for the first time looks like below,
    
    `rashid:~$`
    
    Here rashid the machine and the current directory is ~ (short for “home”). The $ tells you that you are not the root user. ~ is called tilde.
    
    Eg:
    
    `rashid:~$ date`  
    `January 8, 2026 1:15:24 AM`
    
    `rashid:~$ echo hello`  
    `hello`
    
* The command “cd” means change directory. The command “pwd” means present working directory.
    

### What is available in the shell?

* When the shell is asked to execute a command, it consult an environment variable called $PATH. The command “ls” lists all the files in the directory.
    
* cat - content of the file
    
* echo is the argument parsing.
    
* “man” (manual) is the program that explain how to use a program. or we can use -- help after writing the program name. for eg: date --help.
    
* tool: ZOxide tool which remember all the paths we cd into.
    
* Double tapping the tab key after enetering the first letter , it will show all the possible options.
    
* `rashid@LAPTOP-G15TMEFH:~$ which -a sh   /usr/bin/sh   /bin/sh`
    
* head -n1 data , for print 1st line.
    
* `grep` is a powerful command-line utility used for searching plain-text data for lines that match a specific pattern, which is typically a regular expression. Its name stands for "global regular expression print".  
    `grep [OPTION...] PATTERNS [FILE...]`
    
* `sed`
    
* glob is for path, like \*/\*.md
    
* `find` for finding the files. For eg:  
    `rashid@LAPTOP-G15TMEFH:~/assignments$ find -type f -mtime +15`  
    `./test2.txt`  
    `./New Rich Text Format.rtf`  
    `./test1.txt`  
    here, we are trying to find files which are more than 15 days old.
    
* `rashid:~$ find ~/Downloads -type f -name "*.zip" -mtime +30`  
    Finds ZIP files in the download directory that are older than 30 days.
    
* `awk` program is used to parse data
    
* `ls -l file.sh` give more details about the file including its permission.
    
* `chmod +x file.sh` means, change the mode, make it executable.
    

### The shell language (bash)

The pipes ( | ) let you string together output of one program with the input of another.

The command `>file` let you take the output and write it to `file`. The command `»file` let you take the output and append it to `file`.

`#!/bin/bash` is the shebang and specifies that the script should be executed using the Bash shell located at the path `/bin/bash`.