An example where it does change is the following script
$ cat sig-example
#!/bin/sh
stty all
trap 'echo "signal: $v"' 1 2
v=1
while :
do
sleep 2
v=expr $v + 1`
echo "v = $v"
done
which you can run, hit your interrupt character (usually control-C) a
few times, and observe the output. To terminte the script, you should
use the suspend character (usually control-Z) to suspend the job and
then type in kill %1 (if it is job 1 -- shells with job
control print out the job number when you hit control-Z).
Signals are asynchronous notifications of special events. Every signal is associated with an unique integer pre-defined in UNIX systems to ensure uniformity throughout the system and the compatibility across the different systems. The C head file can be included in a C/C++ program by:
#include <signal.h>
A signal can originated from the kernel (e.g., SIGIO, SIGSEGV), sent from a process to itself using the _a(`_http_man`?kill(2)'',``kill system call''), sent from another process (likewise using kill(2)), or sent by the terminal driver on behalf of user (e.g., control-C). The syntax is:
$ kill -SIGNAL pidwhere pid is the process ID of the process. There are many different signals predefined in UNIX systems. The following is a brief summary of some common signals.
| Signal | Symbolic Name | Usage |
|---|---|---|
| 1 | SIGHUP | Hang-up signal is used to tell programs that the terminal disconnected from the host. For example, you might use a modem to log on to a Unix machine from a home computer or dialup terminal. Suppose you are in the middle of reading your email using pine. If the phone line suddenly disconnects, all of your processes, including pine, will receive a SIGHUP signal from the system. The default action for most programs is to exit. However, programs like pine catches the SIGHUP signal and performs graceful cleanup, e.g., saves your mail files. |
| 2 | SIGINT | Interrupt signal. This signal is generated from your terminal when you hit control-C. (Control-C is the default; it can be set to another character using _a(`_http_man`?stty'',``stty'')) An interrupt causes programs to terminate by default. |
| 3 | SIGQUIT | Quit signal is similar to SIGINT. In addition to cause program to quit, it will also force the running program to do a core dump (leaving a file called core in the current directory). "core" is a file contains an image of the memory used by this program right before it aborts. It is often used by programmers, who may interrupt a program at anytime and exam the execution memory. The default key combination for generating a SIGQUIT signal is CTRL-\ on most UNIX system. |
| 9 | SIGKILL | This is another commonly used signal. SIGKILL signal kills a process immediately. It cannot be captured by the target process. Target process die instant upon receiving of SIGKILL, meaning you will not be able to use trap command with this signal. You could use this signal to kill any program that will ignore all other signals. syntax: kill -9 pid . |
| 15 | SIGTERM | Termination signal. This signal instructs a process to terminate itself. This is the default signal sent by kill when no signal is specified. (i.e. $ kill pid). When UNIX system is instructed to shutdown, it will first send SIGTERM signal to all the processes and wait for them to terminate. It will then send SIGKILL signal, forcefully kill any remaining processes. |
| 0 | N/A | Signal 0 is not a real signal. It is not defined in signal.h mentioned above. It is a shell specific signal represent normal exiting status(program exit normally.). You can not use this signal in C/C++. It allows you to specify an action, which will be taken when the script _a(`_http_man`?exit'',``exit'') normally. For example, You can specify actions that will be taken when the program received a Interrupt Signal,Hangup Signal or it just exits normally. |
Note: You may use a symbolic name in place of any signal numbers. For example:
$ kill -1 1234
is equivalent to:
$ kill -SIGHUP 1234Where 1234 is an random process id. Note: You may use these symbolic names in C/C++ too.
url_fetch will fetch the specified
_a(``http://www.ncsa.uiuc.edu/demoweb/html-rimer.html'',``HTML'')
page or file and write the file to standard output.
html_url is a
_a(``http://www.ncsa.uiuc.edu/demoweb/html-rimer.html'',``HTML'')
code parser that reads HTML code from standard input (stdin),
and filter out the URLs (Universal Resource Locators)
embedded in HTML and print them out line by line to the standard
output(stdout).
This line of code is a little complicated. It creates a new variable suffix and initialize it to the output of the commands in the backquotes. Every thing on the right side of "=" is wrapped with _backquote _backquote (back quotes). The shell will replace the quoted string by the output of the commands inside the quotes.
Let's take a look at whats inside of the back quotes:
- "$tmpf" (which contains the name of the temporary file created by url_fetch) will be expanded first. The file is then opened for reading, and the standard input of html_url is made to refer to it. html_url parses its input and send all URLs found to the next program in the pipeline, which is grep.
- grep archive will then exam the output of the html_url line by line, and prints out only the lines that contains the word "archive".
- head -1 reads its standard input and outputs only the first line. Its input coming from the grep, this means that only the first URL containing the word archive will be printed. This output is stored into shell variable suffix.
xv is a X-windows program that can display variety files (pictures and text file). "-", at the end instructs xv to read the input from stdin. In this case, stdin is the piped output of url_fetch. As we have mentioned in the previous class, $1 represent the first command line argument of the shell script. "+" here (in analog to "-" above) means, if $1 has a value, use "$@" instead; if $1 does not exist, the expression is removed.
The notation $@ expands into the list of all arguments to the shell script. There is a crucial distinction, however, when the $@ is quoted:
| Variable | Equivalent Notation | Effect |
|---|---|---|
| "$*" | "$1 $2 $3 ... $N" | one argument |
| "$@" | "$1" "$2" "$3" ... "$N" | many arguments |
${1+"$@"} simply pass all arguments given at the command line to program xv. This is useful for passing options to xv program.
[ CSE 80 | ACS home | CSE home | CSE calendar | bsy's home page ]
bsy@cse.ucsd.edu, last updated