Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN,
SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from sending the SIGHUP signal to a particular job, it should be removed from the jobs table with the disown builtin (see
Section 7.2 [Job Control Builtins], page 98) or marked to not receive SIGHUP using disown
-h.
If the huponexit shell option has been set with shopt (see Section 4.3.2 [The Shopt
Builtin], page 62), Bash sends a SIGHUP to all jobs when an interactive login shell exits.
If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait builtin, the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.
3.8 Shell Scripts
A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the -c nor -s option is supplied (see
Section 6.1 [Invoking Bash], page 80), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.
When Bash runs a shell script, it sets the special parameter 0 to the name of the file, rather than the name of the shell, and the positional parameters are set to the remain- ing arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset.
A shell script may be made executable by using the chmod command to turn on the execute bit. When Bash finds such a file while searching the $PATH for a command, it spawns a subshell to execute it. In other words, executing filename arguments is equivalent to executing bash filename arguments if filename is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of hash in Section 4.1
[Bourne Shell Builtins], page 41) are retained by the child.
Most versions of Unix make this a part of the operating system’s command execution mechanism. If the first line of a script begins with the two characters ‘#!’, the remainder of the line specifies an interpreter for the program. Thus, you can specify Bash, awk, Perl, or some other interpreter and write the rest of the script file in that language.