Bash notes operators

Bash operators are symbols or keywords that instruct the shell to perform specific actions, such as mathematical calculations, comparisons, or logical evaluations, to control the flow of a script. They are essential for dynamic scripting.
Operators are categorized into several types:
Arithmetic Operators
Used for mathematical calculations within a (( )) or $(( )) construct:
+: Addition
-: Subtraction
*: Multiplication
/: Division
%: Modulus (remainder)
**: Exponentiation
++, --: Increment and decrement operators
Comparison Operators
Used to compare values. Numeric comparisons use specific operators within [[ ]] or [ ] (or with the test command), while string comparisons use different ones:
Numeric Comparison (e.g., [ $a -eq $b ])
-eq: Equal to
-ne: Not equal to
-lt: Less than
-le: Less than or equal to
-gt: Greater than
-ge: Greater than or equal to
String Comparison (e.g., [[ "$a" = "$b" ]])
=: Equal to (use for POSIX compatibility)
==: Equal to (Bash specific)
!=: Not equal to
<: Less than (ASCII alphabetical order)
>: Greater than (ASCII alphabetical order)
=~: Regular expression match operator (only within [[ ]])
Logical Operators
Combine multiple conditional expressions:
&&: Logical AND. Executes the second command/expression only if the first succeeds (returns an exit code of 0).
||: Logical OR. Executes the second command/expression only if the first fails (returns a non-zero exit code).
!: Logical NOT. Inverts the result of a condition.
File Test Operators
Check various characteristics of files and directories (e.g., [ -e filename ] checks if a file exists):
-e: Checks if the file exists.
-f: Checks if it is a regular file.
-d: Checks if it is a directory.
-r, -w, -x: Checks for read, write, or execute permissions.
-s: Checks if the file is not empty.
Input/Output Redirection and Piping
Control the flow of data between commands and files:
>: Redirects standard output (stdout) to a file, overwriting the file if it exists.
>>: Appends stdout to a file.
<: Redirects standard input (stdin) from a file.
|: Pipes the stdout of one command to the stdin of the next command.
<<<: "Here String"; supplies a word/variable's content as standard input to a command.
$(...) or `...`: Command substitution; the output of the enclosed command replaces the expression itself.