scd

Index paths recursively for the very first run

$ scd -ar [path/to/directory]

$ scd [path/to/directory]

$ scd "[pattern1 pattern2 ...]"

$ scd -v

$ scd --alias=[word]

$ scd [word]

scdaemon [options]

--version
    Display the scdaemon version information.

--help
    Show a comprehensive help message and exit.

--server
    Run in server mode, typically for client communication.

--daemon
    Run scdaemon in the background as a daemon process.

--multi-server
    Allow multiple clients to connect when in server mode.

--homedir DIR
    Specify the GnuPG home directory for configuration files.

--reader-port N
    Specify which smart card reader port to use (0, 1, etc.).

--disable-locking
    Disable file locking for specific operations (use with caution).

--list-hotplug-devices
    List detected hotplug smart card devices.

The command 'scd' is not a standard, standalone executable commonly found in most Linux distributions. However, it is often used as a colloquial or shorthand reference for scdaemon, the Smart Card Daemon, which is an integral component of the GnuPG (GNU Privacy Guard) suite.

scdaemon serves as an intermediary between GnuPG applications (such as gpg-agent) and physical smart card readers, along with the smart cards themselves. Its primary function is to manage the low-level communication protocols required to perform cryptographic operations directly on the smart card, including digital signing, decryption, and user authentication. It abstracts the complexities of various smart card types, like OpenPGP cards, providing a unified interface for GnuPG. While typically running as a background daemon managed by gpg-agent, it can be manually invoked for diagnostic purposes or specific smart card management tasks. It plays a crucial role in enhancing the security of cryptographic keys by enabling them to reside on tamper-resistant hardware.

The command scd is not a standard, general-purpose Linux command. This analysis focuses on scdaemon, a GnuPG component commonly referenced as scd.
It requires the GnuPG suite to be installed and typically relies on a functional smart card reader and compatible smart card hardware.
Usually, scdaemon runs implicitly in the background, managed by gpg-agent; direct manual invocation is primarily for diagnostics or advanced administration.

scdaemon rarely needs direct user interaction as it's primarily managed by gpg-agent, handling smart card communications transparently on behalf of GnuPG applications.

For low-level communication with smart card readers, scdaemon often depends on the PC/SC Lite daemon (pcscd), which provides a standardized interface for accessing card readers.

It specifically supports smart cards compliant with the OpenPGP Card specification, allowing cryptographic keys to be securely stored on and used from a physical smart card.

Part of the GnuPG project, scdaemon was developed to integrate smart card support for cryptographic operations. Its evolution closely tracks GnuPG's development and advancements in smart card technology, enabling secure key storage on hardware tokens.

gpg(1), gpg-agent(1), gpgconf(1), pcscd(8)