Signal Trap

The trap command is a POSIX-compliant shell builtin that allows a Bash script to intercept system signals and shell-specific events, executing a predefined command string or function (the handler) when the specified signal is delivered to the shell process. It provides a mechanism for asynchronous event handling within the synchronous execution flow of a script.

trap [-lp] [[arg] signal_spec ...]

Syntax Breakdown

• arg: The command string or function name to execute when the signal is caught.
• signal_spec: The target signal. This can be a standard OS signal name (e.g., INT, TERM), a signal number (e.g., 2, 15), or a shell pseudo-signal (e.g., EXIT, ERR). For standard OS signals, the SIG prefix is optional (e.g., SIGINT and INT are equivalent). The SIG prefix must not be used with shell pseudo-signals (e.g., SIGEXIT is invalid).
• -p: Prints the currently installed traps in a format that can be reused as input.
• -l: Prints a list of all supported signal names and their corresponding numeric values (maps to the OS signal.h).

Handler Evaluation and Quoting

The command string provided to arg is parsed twice by the shell: once when the trap is registered, and again when the signal is caught and the handler is invoked.

• Single Quotes ('...'): Delays variable expansion until the signal is caught.
• Double Quotes ("..."): Expands variables immediately at the time the trap is registered.

# Evaluates $VAR when SIGINT is received
trap 'echo $VAR' SIGINT


# Evaluates $VAR immediately; the handler executes the static string
trap "echo $VAR" SIGINT

Modifying Signal Disposition

Beyond executing commands, trap alters the default disposition of signals:

Ignoring a Signal: Passing an empty string ('' or "") as the arg instructs the shell to completely ignore the specified signal.

trap '' SIGTERM

Restoring Default Behavior: Passing a dash (-) as the arg resets the specified signals to their default OS or shell disposition. Omitting arg entirely also resets the signal, but its behavior depends on the arguments provided. If the first argument is a valid signal number (e.g., trap 2 15), Bash treats it as a signal_spec and resets all specified numeric signals. However, if multiple arguments are provided without a dash and the first argument is a signal name (e.g., trap INT TERM), Bash interprets the first argument (INT) as the arg (the command to execute) and applies it to the subsequent signals (TERM). Using a dash is the safest way to reset multiple signals by name.

# Resets a single signal by name
trap INT


# Resets multiple signals by number
trap 2 15


# Resets multiple signals by name safely using a dash
trap - INT TERM

Pseudo-Signals

The shell supports intercepting internal state transitions as pseudo-signals. While EXIT is a standard POSIX feature, Bash extends this functionality with additional pseudo-signals:

• EXIT (or 0): A POSIX-standard pseudo-signal invoked when the shell process terminates, regardless of whether the termination was successful or caused by an error.
• ERR: A Bash extension invoked whenever a pipeline, list, or compound command returns a non-zero exit status. Its behavior is strictly bound by the same rules as the set -e (errexit) option.
• DEBUG: A Bash extension invoked immediately before every simple command, for command, case command, select command, or arithmetic for command is executed.
• RETURN: A Bash extension invoked when a shell function or a script executed with the . or source builtins finishes executing.

Execution Context and Inheritance

• Scope: The handler executes in the current shell environment. If the handler is a function, it shares the variable scope and execution context of the script at the exact moment of interruption.
• Subshells: Standard signal traps defined in a parent shell are not inherited by subshells (e.g., command substitutions, background processes).
• Pseudo-Signal Inheritance: By default, the Bash-specific ERR, DEBUG, and RETURN traps are not inherited by shell functions, command substitutions, or subshells. To force inheritance into these contexts, specific shell options must be enabled:
  • set -E (or set -o errtrace): Inherits the ERR trap.
  • set -T (or set -o functrace): Inherits the DEBUG and RETURN traps.
• Ignored Signals: Unlike active traps, signals that are explicitly ignored (trap '' SIG) are inherited by child processes.
• Signal Queuing: Standard POSIX signals are tracked via a bitmask in the kernel, meaning they are not queued. If multiple signals of the same type are delivered while the shell is busy executing a foreground command, the kernel simply sets the corresponding pending flag. Once the foreground command completes, the shell detects that the pending flag is set and executes the trap handler exactly once, regardless of how many times the signal was sent.

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