> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash String
A string in Bash is a contiguous sequence of characters treated as a single unit of data. Because Bash is fundamentally a macro processor and untyped by default, all variables are inherently treated as character strings unless explicitly declared with an integer attribute via `declare -i`. String manipulation in Bash is primarily handled through parameter expansion rather than dedicated functions or methods.
## Quoting Mechanisms
The parsing of a string depends entirely on the quoting mechanism used to define it.
* **Unquoted:** Subject to word splitting (based on `IFS`) and pathname expansion (globbing).
* **Strong Quoting (`'...'`):** Suppresses all interpolation. Every character within single quotes is treated as a literal.
* **Weak Quoting (`"..."`):** Suppresses word splitting and globbing but permits parameter expansion (`$`), command substitution (`$()`), and arithmetic expansion (`$((...))`).
* **ANSI-C Quoting (`$'...'`):** Evaluates backslash-escaped characters according to the ANSI C standard (e.g., `\n` for newline, `\t` for tab, `\xNN` for hexadecimal).
```bash theme={"dark"}
str_unquoted=HelloWorld
str_strong='Literal $var \n'
str_weak="Interpolated $var"
str_ansi=$'Line1\nLine2'
```
## Concatenation
Bash does not use a dedicated concatenation operator. Strings are concatenated via juxtaposition (placing them adjacent to one another).
```bash theme={"dark"}
str1="Hello"
str2="World"
concat_str="${str1} ${str2}!"
```
## Parameter Expansion (String Operations)
Bash provides built-in string manipulation capabilities through parameter expansion syntax `${...}`.
### Length
Returns the number of characters in the string.
```bash theme={"dark"}
${#string}
```
### Substring Extraction
Extracts a portion of the string starting at a zero-based `offset` for a specified `length`. If `length` is omitted, it extracts to the end of the string. Negative offsets must be separated by a space to avoid conflict with default-value expansions.
```bash theme={"dark"}
${string:offset:length}
${string: -offset:length}
```
### Substring Removal (Stripping)
Removes a substring matching a shell pattern (glob) from either the beginning (prefix) or the end (suffix) of the string.
* **Shortest Prefix:** `${string#pattern}`
* **Longest Prefix:** `${string##pattern}`
* **Shortest Suffix:** `${string%pattern}`
* **Longest Suffix:** `${string%%pattern}`
```bash theme={"dark"}
# Syntax visualization
${var#*.} # Removes shortest match of "*." from the front
${var##*.} # Removes longest match of "*." from the front
${var%.*} # Removes shortest match of ".*" from the back
${var%%.*} # Removes longest match of ".*" from the back
```
### Search and Replace
Replaces occurrences of a shell pattern with a replacement string.
* **First Match:** `${string/pattern/replacement}`
* **All Matches:** `${string//pattern/replacement}`
* **Prefix Match:** `${string/#pattern/replacement}` (Pattern must match the beginning)
* **Suffix Match:** `${string/%pattern/replacement}` (Pattern must match the end)
```bash theme={"dark"}
# Syntax visualization
${var/foo/bar} # Replaces first 'foo' with 'bar'
${var//foo/bar} # Replaces all 'foo' with 'bar'
```
### Case Modification (Bash 4.0+)
Modifies the alphabetic case of characters matching an optional pattern (defaults to `?`, matching any character).
* **Uppercase First Character:** `${string^pattern}`
* **Uppercase All Characters:** `${string^^pattern}`
* **Lowercase First Character:** `${string,pattern}`
* **Lowercase All Characters:** `${string,,pattern}`
```bash theme={"dark"}
# Syntax visualization
${var^} # Capitalizes the first letter
${var^^} # Converts entirely to uppercase
${var,,} # Converts entirely to lowercase
```
## String Comparison Operators
String comparison is performed within test commands (`[ ... ]` or `[[ ... ]]`). The extended test construct `[[ ... ]]` is preferred for strings as it prevents word splitting and supports advanced pattern matching.
* **Equality:** `==` or `=` (True if strings are identical)
* **Inequality:** `!=` (True if strings are not identical)
* **Lexicographical Less Than:** `<` (Evaluates based on the current locale's sorting order)
* **Lexicographical Greater Than:** `>`
* **Zero Length:** `-z` (True if string length is 0)
* **Non-Zero Length:** `-n` (True if string length is greater than 0)
* **Regular Expression Match:** `=~` (True if the string on the left matches the POSIX Extended Regular Expression on the right; only valid inside `[[ ... ]]`)
```bash theme={"dark"}
# Syntax visualization
[[ "$str1" == "$str2" ]]
[[ "$str1" < "$str2" ]]
[[ -z "$str1" ]]
[[ "$str1" =~ ^[A-Za-z]+[0-9]$ ]]
```
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