> ## Documentation Index
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> Use this file to discover all available pages before exploring further.
# Bash Bitwise NOT
The `~` (tilde) operator triggers **tilde expansion**, a built-in shell parsing mechanism where Bash replaces a tilde-prefixed string with a specific absolute directory path. This expansion occurs strictly before variable expansion, command substitution, and pathname expansion, provided the tilde is unquoted and positioned at the beginning of a word or immediately following an unquoted `=` or `:` in a variable assignment.
During parsing, Bash identifies an unquoted `~` and reads all subsequent characters up to the first unquoted slash `/` (or the end of the word). This extracted string is evaluated as the **tilde-prefix**. If the tilde-prefix matches a known expansion rule, the shell substitutes the prefix with the corresponding path. If the prefix is invalid or unresolvable, the string is left completely unchanged.
## Syntax and Expansion Variants
**Current User Home Directory**
```bash theme={"dark"}
~
~/path
```
An un-suffixed tilde expands to the value of the `$HOME` shell variable. If `$HOME` is unset, Bash falls back to the home directory of the user executing the shell, resolved via the operating system's password database (e.g., `getpwuid()`).
**Specific User Home Directory**
```bash theme={"dark"}
~username
~username/path
```
Expands to the absolute home directory associated with the specified `username`, resolved directly from the system's password database (e.g., `getpwnam()`).
**Current Working Directory**
```bash theme={"dark"}
~+
~+/path
```
Expands to the value of the `$PWD` shell variable, representing the current working directory.
**Previous Working Directory**
```bash theme={"dark"}
~-
~-/path
```
Expands to the value of the `$OLDPWD` shell variable, representing the previous working directory. If `$OLDPWD` is unset, the expansion fails and the string remains literal.
**Directory Stack Expansion**
```bash theme={"dark"}
~+N
~-N
```
Expands to specific entries within the shell's directory stack (managed via `pushd` and `popd`).
* `~+N` expands to the string that would be displayed by the `dirs +N` command (the *N*th directory from the left of the stack, zero-indexed).
* `~-N` expands to the string displayed by `dirs -N` (the *N*th directory from the right of the stack, zero-indexed).
## Assignment Context Parsing
Bash applies special parsing rules for tilde expansion during variable assignment. The expansion is triggered immediately following an unquoted `=` sign, and subsequently after any unquoted `:` characters within the assigned value. This is specifically designed to support `PATH`-like variable structures.
```bash theme={"dark"}
# Tilde expansion occurs after '=' and ':'
VAR=~/dir1:~+/dir2
```
## Quoting and Suppression
Tilde expansion is strictly dependent on the `~` character remaining unquoted. If the tilde, or any character within the tilde-prefix, is enclosed in single quotes, double quotes, or escaped with a backslash, the expansion is suppressed and the tilde is treated as a literal character.
```bash theme={"dark"}
# Expansion suppressed
\~
"~"
'~'
~"username"
```
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