I have a program that relies on a psuedo terminal that uses
term = posix_openpt()
grantpt(term)
unlockpt(term)
open(term)
to open a psuedo terminal. I'll call this program psuedoTerminal.bin
I want to open psuedoTerminal.bin from a bash script, in the following fashion (or similar)
#!/bin/bash
/bin/sh -c psuedoTerminal.bin &
The problem is when my program arrives to the posix_openpt() call, the behaviour is erratic, sometimes CPU consumption arrives to 100%, but it never works. I believe ssh, telnet, etc suffer from the same problems on opening psuedo terminals from inside scripts.
How can I run this program from inside a script? Thanks for your help.
Related
I am a newbee and looking for some info.
Thanks in advance.
What is difference between echo "Hello World!" and a c-program which prints "Hello World!" using printf.
How do shell commands get executed. For example if I give ls it lists all the files in the directory. Is there executable binary which is run when we enter ls in shell.
Please let me know if you guys have any links or source to get this clear.
There are two main types of "commands" that the shell can execute. Built-in commands are executed by the shell itself - no new program is started. Simply typing echo in a shell prompt is an example of such a built-in command.
On the other hand, other commands execute external programs (also called binaries) - and ls is an example of this kind of command.
So, if you run echo in a shell, it's executed by the shell itself, but if you write a C program that performs the same action, it wil be run as an external program. As a matter of fact, most Linux systems come with such a binary, located at /bin/echo.
Why does it sometimes make sense to have both a built-in command and a program to accomplish the same task? Built-in commands are faster to execute as there is some cost involved in running an external program. But built-ins have some drawbacks, too: they can't be too complex as this would make the shell big and slow; they can not be upgraded separately from the shell and from each other; finally, there are situations where an external program which is not your shell would like to run an application: it can run external programs but it can't execute shell built-ins directly since it's not the shell. So sometimes it makes sense to have it both ways. Apart from echo, time is another example of this double approach.
The shell is just a user level way of interacting with the operating system, or the kernel. That's one of the reasons it's called a shell. The shell itself (sh, csh, tcsh, ksh, zsh, bash, etc...) is essentially just a binary the operating system executes to allow you to execute other binaries.
It generally gives a lot of other functionality though like built in functions (echo, fg, jobs, etc...), an interpreted language (for x in ..., if then, etc...), command history, and so on...
So, any text entered into the shell (like echo), the binary (or process) interprets and runs the corresponding functions in its code. Built in functions (like echo) don't need to create a new process, but if the text is interpreted as a request to execute a binary (vim, emacs, gcc, test, true, false, etc...) the shell will create a new process for it (unless you prefix it withexec), and execute it.
So, echo "Hello World! just runs code in the shell (process). A printf("Hello World!") would be in seperate binary that the shell would create a new process for (fork), and have the operating system execute (exec).
I have a C Sockets application, different executables of which must run at same time all at once, preferably in different terminals. How do I do it?
For example, there are four exes, ./one, ./two, ./three, ./four.
I want them to be run in different linux terminals without slightest of time difference. How can I do it?
There will always be at least a "slightest of time difference".
Just have your exe's agree on a time to proceed and just sleep until that time before doing whatever it is that they need to do.
You will never run several process at the same time. If you need just to start them nearly at the same time for testing purposes, then, probably, easiest way is to start terminal emulator to run your each of your executable. If you have xorg installed, then you can open xterm and run there something like this:
xterm -e "./app1; /bin/bash" & xterm -e "app2; /bin/bash" & ...
where the /bin/bash is called to prevent closing new xterms after your application will be finished.
I'm not bash expert so this works on my machine and I can't be sure about exactly this string will work with older or newer bash or sh pr with another terminal emulator. But you always can write something similar.
To run simultaneously you can do something like this:
"declare -a cmd=(./first ./second ./third); for i in ${cmd[#]}; do exec $i wait & done"
This can help you to automate your task. Keep your commands on array, exec them. It'll run all the processes on background and will start trying to run them simultaneously.
I have written a program which I run after connecting to the box over SSH. It has some user interaction such as selecting options after being prompted, and usually I wait for the processes it carries out to finish before logging out which closes the terminal and ends the program. But now the process is quite lengthy and I don't want to wait whilst being logged in, so how could I implement a workaround for this in C please?
You can run a program in the background by following the command with "&"
wget -m www.google.com &
Or, you could use the "screen" program, that allows you to attach-deattach sessions
screen wget -m www.google.com
(PRESS CTRL+D)
screen -r (TO RE ATTACH)
http://linux.die.net/man/1/screen
The process is sent the HUP signal when the shell exits. All you have to do is install a signal handler that ignores SIGHUP.
Or just run the program using nohup.
The traditional way to do this is using the nohup(1) command:
nohup mycmd < /dev/null >& output.log &
Of course if you don't care about the output you can send it to /dev/null too, or you could take input from a file if you wanted.
Doing it this way will protect your process from a SIGHUP that would normally cause it to exit. You'll also want to redirect stdin/stdout/stderr like above, as you'll be ending your ssh session.
Syntax shown above is for bash.
you can use screen command. here is a tutorial. note you might need to install it to your systems.
There are many options :-) TIMTOWTDI… However, for your purposes, you might look into running a command-line utility such as dtach or GNU screen.
If you actually want to implement something in C, you could re-invent that wheel, but from your description of the problem, I doubt it should be necessary…
The actual C code to background a process is trivial:
//do interactive stuff...
if(fork())
exit(0);
//cool, I've been daemonized.
If you know the code will never wind up on a non-linux-or-BSD machine, you could even use daemon()
//interactive...
daemon(0, 0);
//background...
I've been trying to open a new terminal window from my application and execute a command on this second window as specified by the user. I've built some debugging software and I would like to execute the user's program on a separate window so my debugging output doesn't get intermixed with the programs output.
I am using fork() and exec(). The command I am executing is gnome-terminal -e 'the program to be executed'.
I have 2 questions:
Calling gnome-terminal means the user has to be running a gnome graphical environment. Is there a more cross-platform command to use (I am only interested in Linux machines though)?
After the command finishes executing the second terminal also finishes executing and closes. Is there any way to pause it, or just let it continue normal operation by waiting for input?
You probably want something like xterm -hold.
1) gnome-terminal should work reasonably also without the whole gnome environonment, anyway the old plain "xterm" is enough.
2) you can execute a short bash script that launch your program and at the end reads a line:
bash -c 'my program ... ; read a'
(or also 'xterm -e ...')
I have written a program which calculates the amount of battery level available in my laptop. I have also defined a threshold value in the program. Whenever the battery level falls below threshold i would like to call another process. I have used system("./invoke.o") where invoke.o is the program that i have to run. I am running a script which runs the battery level checker program for every 5 seconds. Everything is working fine but when i close the bash shell the automatic invocation of invoke.o is not happening. How should i make the invoke.o to be invoked irrespective of whether bash is closed or not??. I am using UBUNTU LINUX
Try running it as: nohup ./myscript.sh, where the nohup command allows you to close the shell without terminating the process.
You could run your script as a cron job. This lets cron set up standard input and output for you, reschedule the job, and it will send you email if it fails.
The alternative is to run a script in the background with all input and output, including standard error output, redirected.
While you could make a proper daemon out of your program that kind of effort is probably not necessary.
man nohup
man upstart
man 2 setsid (more complex, leads to longer trail of breadcrumbs on daemon launching).