Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
This shell builtin replaces the current process with a specified command. Normally, when the shell
encounters a command, it forks off a child process to actually execute the command. Using the exec
builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script,
therefore, it forces an exit from the script when the exec'ed command terminates. For this reason, if an
exec appears in a script, it would probably be the final command.
Example 11−20. Effects of exec
#!/bin/bash
exec echo "Exiting \"$0\"."
# Exit from script here.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# The following lines never execute.
echo "This echo will never echo."
exit 99
# This script will not exit here.
# Check exit value after script terminates
#+ with an 'echo $?'.
# It will *not* be 99.
Example 11−21. A script that exec's itself
#!/bin/bash
# self−exec.sh
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
# Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl−C to exit ===================="
sleep 1
exec $0
# Spawns another instance of this same script
#+ that replaces the previous one.
echo "This line will never echo!" # Why not?
exit 0
An exec also serves to reassign file descriptors. exec