exec

Replace the shell with another command.

Source: src/execution/builtins.f90:2541-2654

Synopsis

exec [-cl] [-a name] [command [arguments]]
exec redirections

Description

The exec builtin replaces the current shell process with a new command. If no command is given, redirections are applied to the current shell.

Options

Usage

Replace Shell

exec /bin/bash
# Current shell is replaced; no return

With Arguments

exec python3 script.py arg1 arg2

Apply Redirections

exec > output.log 2>&1
# All subsequent output goes to log
echo "This goes to output.log"

Examples

Wrapper Script

#!/usr/bin/env fortsh
# Wrapper that sets up environment then runs real command

export LD_LIBRARY_PATH=/custom/lib:$LD_LIBRARY_PATH
export CONFIG_PATH=/etc/myapp

exec /usr/bin/myapp "$@"
# Shell replaced; myapp gets our PID

Login Shell

exec -l /bin/bash
# Starts bash as login shell

Custom argv[0]

exec -a custom_name /usr/bin/vim
# vim sees argv[0] as "custom_name"

Redirect All Output

#!/usr/bin/env fortsh
exec > /var/log/script.log 2>&1

echo "Starting..."    # Goes to log
ls -la                # Goes to log
echo "Done."          # Goes to log

Close File Descriptor

exec 3>&-     # Close fd 3
exec 0<&-     # Close stdin

Open File Descriptor

exec 3> output.txt   # Open fd 3 for writing
echo "data" >&3      # Write to fd 3
exec 3>&-            # Close fd 3

Redirections Only

When no command is given, exec applies redirections to current shell:

# Redirect all stderr to file
exec 2> errors.log

# Read from file as stdin
exec < input.txt

# Copy stdout to fd 3
exec 3>&1

Common Patterns

Daemonization

#!/usr/bin/env fortsh
# Redirect and exec for daemon

exec > /var/log/daemon.log 2>&1
exec /usr/bin/daemon --foreground

Container Entry Point

#!/usr/bin/env fortsh
# Setup then hand off to main process

setup_environment
load_secrets

exec "$@"   # Run container command

Script Runner

#!/usr/bin/env fortsh
# Determine interpreter and exec

case "$1" in
    *.py)  exec python3 "$@" ;;
    *.rb)  exec ruby "$@" ;;
    *.sh)  exec bash "$@" ;;
    *)     exec "$@" ;;
esac

Logging Wrapper

#!/usr/bin/env fortsh
# Prepend timestamp to all output

exec > >(while IFS= read -r line; do
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $line"
done)

exec 2>&1  # Merge stderr
exec "$@"  # Run command

Implementation

Source: builtins.f90:2541-2654

1. Parse options (-c, -l, -a)
2. Apply any redirections
3. If command given:
   • Search PATH for executable
   • Call execvp() to replace process
4. If no command:
   • Redirections persist for current shell

Exit Status

Notes

• After exec command, the shell no longer exists
• Trap handlers are not executed after exec
• Redirections persist for shell lifetime
• Use exec in wrappers to avoid extra process

Differences from Running Command

See Also

• eval - Execute string as command
• source - Run in current shell
• Redirection - I/O redirection