- Bash (AKA Bourne Again Shell) is a type of interpreter that processes shell commands.
- A shell interpreter takes commands in plain text format and calls Operating System services to do something.
- Bash is the improved version of Sh (Bourne Shell).
- A shell scripting is writing a program for the shell to execute and a shell script is a file or program that shell will execute.
- Terminal is the interface to the shell interpreter.
- A shell script is a fully-fledged programming language in itself. It can define variables, functions and we can do conditional execution of shell commands as well.
- When we enter a command like ls or touch, the shell interpreter first checks for an alias present in .bash_profile file in wer home directory. If alias is not present, then it looks for a binary executable file in the PATH.
- Normally, a Bash script file has .sh extension to make it clear that it is a shell script file.
- we can also directly execute it like a binary but we need to put a shebang or hashbang line at the top of the file to declare the interpreter.
#! /usr/bin/env bash- When we run a script file in the terminal, terminal issues a new session for the script to run.
- To declare a variable and assign with a value, use
VARIABLE_NAME=VALUEexpression (with no spaces in between) - Bash does not have a type system, it can only save string values. Hence internally, Bash saves them as a string.
- However, based on operations, Bash might convert them to a suitable data type on the fly.
- echo adds a space automatically between two adjacent arguments (in the output) by default.
- read user input in the terminal using 'read' command. When the user input the text and hit enter, that entire text will be saved in a variable.
- echo command with -n flag, will prevent adding a newline to the terminal so that user can input text on the same line.
- read command blocks the script execution until the user presses enter in the terminal.
VARIABLE_NAME=$(cmd)orVARIABLE_NAME=<backtick>cmd<backtick>- The newline in Bash is a command separator but we can also use
;. If we want to write multiple commands on a single line, then use;as the command separator. For example,echo $PWD; echo $BASH_VERSION.
- We can concatenate two variable by just placing them side by side.
FULL_NAME=$FIRST_NAME$LAST_NAME; echo $FULL_NAME- Bash also supports
+=operator to concatenate two strings. NUMBER=1; NUMBER+=2; NUMBER+=$NUMBER; echo $NUMBER- We can also substitute a variable in a string defined by double-quotes by directly putting it in with
$prefix as we would use outside or with${VARIABLE_NAME} FIRST_NAME='JOHN' ; LAST_NAME='DOE' ; echo Hello ${FULL_NAME}!- Bash supports both single-quotes (‘) and double-quotes (“) to define a string. But only double quotes are capable of string interpolation.
- But if we need to put single or double quote as a character in a string, we need to escape it with a backslash character.
FIRST_NAME='JOHN' ; echo "Hello ${FIRST_NAME}!" ; echo 'Hello ${FIRST_NAME}!'- When we reference a string variable and pass it as an argument to a command like
echo, Bash breaks it into different words (separated by a space) and pass them as individual arguments. argscommand prints the number of arguments passed to it.- Use string interpolation by wraping a variable in double-quotes.
- We can perform arithmetic operations in Bash even though Bash does not support number data type.
let RESULT=1+1; echo $RESULTletwould evaluate the expression in a string. So if we want to use interpolation to generate a string, it’s possible.- We can use any arithmetic operation like
+,-,*or/.+=,-=,*=,/=and%= - We can also calculate the modulus using
%operator. We can also increment or decrement a variable usingVAR++,++VAR,VAR--or--VARexpression. NUMBER=1; let RESULT="++NUMBER"; echo $RESULT; echo $NUMBER- We can also declare a variable inside the expression using
letor perform an arithmetic operation. Using this, we can also add spaces in the expression.