Key identifiers

File Descriptors

• Description: A number that identifies an input/output stream such as an open file, socket, pipe, etc.
• Example: 0 (standard input), 1 (standard output), 2 (standard error).

# command 결과 표준에러 제거후 출력
$command 2>/dev/null

# command 결과 표준에러만 출력 (표준출력 제거)
$command 2>&1 1>/dev/null

Process ID (PID, Process Identifier)

• Description: A unique number that identifies each process running on the system.
• Example: Each process has a unique PID, which can be checked with the ps command.

User ID (UID, User Identifier)

• Description: A number that identifies the user account on the system. Each user account is assigned a unique UID.
• Example: Superuser (root) has UID 0 by default.

Group ID (GID, Group Identifier)

• Description: A number that identifies the user group. Each group has a unique GID.
• Example: Each user account belongs to one or more groups by default, identified by a GID.

Session ID (SID, Session Identifier)

• Description: A number that identifies the terminal or remote login session.
• Example: Used when managing logged in terminal sessions.

Network interface index

• Description: A number that identifies the network interface (e.g. Ethernet, WiFi).
• Example: Used when managing a specific network interface in the system.

Other identifier

• File System Inode Number: An identifier that uniquely identifies a file in a file system.
• Port Number: A number that identifies a specific service (e.g. HTTP, SSH) in network communication.
• Signal Number: A number that identifies a specific signal delivered to the process (e.g. SIGKILL, SIGTERM).