> ## 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 Socket Check The `-S` operator is a unary file test operator in Bash used to determine whether a specified file path exists and is a socket (specifically, a Unix domain socket). When evaluated within a conditional expression, it returns a boolean exit status based on the file's type metadata in the filesystem. **Syntax** ```bash theme={"dark"} [ -S "$FILE_PATH" ] [[ -S "$FILE_PATH" ]] test -S "$FILE_PATH" ``` **Evaluation Rules** * **True (Exit Status `0`):** The path exists, the executing user has the necessary search permissions to resolve the path, and the target inode represents a socket (identified by the `S_IFSOCK` file type constant). * **False (Exit Status `1`):** The path does not exist, the path resolves to a different file type (e.g., regular file, directory, block device, FIFO), or the executing user lacks permissions to resolve the path. **Technical Characteristics** * **Symlink Resolution:** The `-S` operator follows symbolic links. If the target path is a symlink, the operator evaluates the ultimate target of the link, not the symlink itself. To test if a symlink itself is a socket (which is technically impossible as symlinks and sockets are mutually exclusive inode types), the `-h` or `-L` operators must be used instead. * **POSIX Compliance:** The `test -S` and `[ -S ]` constructs are defined in the POSIX.1 standard, ensuring cross-shell compatibility (e.g., `sh`, `dash`, `zsh`). The `[[ -S ]]` construct is a Bash-specific extended test keyword that executes the same underlying filesystem check but is parsed differently by the shell interpreter, preventing word splitting and pathname expansion. * **System Call Implementation:** Under the hood, Bash utilizes the `stat()` system call to retrieve the file's mode. It then applies a bitwise mask (typically via the `S_ISSOCK(m)` macro in C) against the `st_mode` field of the returned `stat` struct to verify the socket file type.

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