Explain the principles of string manipulation and pattern matching in shell scripts.

String Manipulation:
String manipulation refers to the process of modifying, extracting, or manipulating strings in shell scripts. It allows you to perform various operations on strings, such as concatenation, substitution, trimming, searching, and more. Shell scripting provides several built-in mechanisms for string manipulation:

1. Concatenation:

* Concatenation combines multiple strings into a single string.
* Example:
```
bash`str1="Hello"
str2="World"
result="$str1 $str2"
echo $result`
```
Output: `Hello World`
2. Substring Extraction:

* Substring extraction allows you to extract a portion of a string based on a starting position and length.
* Example:
```
bash`str="Hello World"
substr=${str:6:5}
echo $substr`
```
Output: `World`
3. String Length:

* You can obtain the length of a string using the `${#string}` syntax.
* Example:
```
bash`str="Hello World"
length=${#str}
echo $length`
```
Output: `11`
4. Pattern Matching and Substitution:

* Shell scripting supports pattern matching and substitution using the `${string/pattern/replacement}` syntax.
* Example:
```
bash`str="Hello World"
replaced=${str/World/Universe}
echo $replaced`
```
Output: `Hello Universe`

Pattern Matching:
Pattern matching allows you to match and manipulate strings based on specific patterns or regular expressions. It is particularly useful for searching, validating, and manipulating strings based on patterns. Shell scripting provides pattern matching capabilities using various constructs:

1. Wildcards:

* Wildcards, such as `*` (matches any sequence of characters) and `?` (matches a single character), enable flexible pattern matching.
* Example:
```
bash`files=$(ls *.txt)`
```
2. Filename Expansion (Globbing):

* Filename expansion allows you to match multiple files based on a pattern.
* Example:
```
bash`for file in *.txt; do
echo $file
done`
```
3. Regular Expressions:

* Regular expressions are powerful patterns used to match and manipulate strings.
* Shell scripting provides pattern matching using the `grep` command with regular expressions.
* Example:
```
bash`str="Hello World"
if [[ $str =~ ^Hello.*$ ]]; then
echo "Match found"
fi`
```
4. Pattern Replacement:

* You can use pattern matching for string substitution and replacement.
* Example:
```
bash`str="Hello World"
replaced=$(echo $str | sed 's/World/Universe/')
echo $replaced`
```

These principles of string manipulation and pattern matching in shell scripts provide powerful tools for working with strings, performing various operations, and manipulating text data. Understanding and utilizing these techniques allows you to efficiently process and manipulate strings based on specific patterns and requirements.