Hello I am currently trying to printf a certain part of the command line when i run my program. I currently run my program as follows
zcat gcc | sim
What i am trying to accomplish is print out to a seperate file the 'gcc' part of the command line. Is there any way to accomplish this. Thanks for the help
I forgot to mention this is in c not C++
The 'sim' program only knows that it has data from stdin. It doesn't know what is handing the data to stdin - it may be another program, or redirected input, or from a device.
From 'sim' POV, there's only data from stdin.
IF you can guarantee that you will always receive data from another process via a pipe, you can start looking for process headers matching the 'sim' process, and from then extracting process info from associated processes. However, I suspect that this is more than you're willing to go.
You are passing output of zcat (through pipe) to your program, it doesn't know details of arguments passed to zcat.
You can explicitly pass it as a part of input stream though,
(echo 'zcat gcc'; zcat gcc ) | sim
but that is a hack. It may be Ok if you are planning to run your
program this way only.
Related
I know how to get the stdout into a file using dup/dup2 system calls, but how do I get the entire output that would be normally shown on my terminal(including the prompt that says my username along with the $ symbol and the current working directory) to a file?
Yes you can, but this may be difficult in many details (depending on your expert level). For the shell to behave normally (I would mean exactly as in a terminal), then it needs to interact with a terminal (special system object). So you need to create a program that behave like a terminal, this what pseudo-terminals devices (/dev) are intended for. Read documentation about this to implement it but roughly, your application should behave like the user so should be connected to the slave side of the pseudo-terminal, and the shell to the master side of the pseudo-terminal. Then you can easily log real inputs made by the user and catch outputs made by the shell.
Can't comment cause of low reputation.
I would say there is no way to do that inside a code in C. Instead, you could use bash for example to redirect everything to a file, and leave the code in C as it is.
In this way you have all the info you want to save: prompt, current directory, call to the program (including flags), and of course the output of the program.
Well, you can do:
-For bash prompt PS1: Echo expanded PS1 (in case you want it expanded, if not there is a simple way to do it just echong PS1)
- For executed command: https://unix.stackexchange.com/questions/169259/how-to-capture-command-line-input-into-logfile-and-execute-it-at-the-same-time
- Standard output and error output: Redirect stderr and stdout in a Bash script
And that's all you want to capture, I think.
Look up the script command in Unix systems. If you want to capture all keyboard and std in/out for a command, use the script executable. If you want to see how it's done, look up the source.
I'm now working on a small C program in Linux. Let me explain you what I want to do with a sample Linux command below
ls | grep hello
The above command is executed in the below passion (Let me know if I've got this wrong)
ls command will be executed first
Output will be given to grep command which will again generate output by matching "hello"
Now I would like to write a C program which takes the piped output of one command as input. Means, In the similar passion of how "grep" program was able to get the input from ls command (in my example above).
Similar question has been asked by another user here, but for some reason this thread has been marked as "Not a valid question"
I initially thought we can get this as a command line argument to C program. But this is not the case.
If you pipe the output from one command into another, that output will be available on the receiving process's standard input (stdin).
You can access it using the usual scanf or fread functions. scanf and the like operate on stdin by default (in the same way that printf operates on stdout by default; in the absence of a pipe, stdin is attached to the terminal), and the C standard library provides a FILE *stdin for functions like fread that read from a FILE stream.
POSIX also provides a STDIN_FILENO macro in unistd.h, for functions that operate one file descriptors instead. This will essentially always be 0, but it's bad form to rely on that being the case.
If fact, ls and grep starts at the same time.
ls | grep hello means, use ls's standard output as grep's standard input. ls write results to standard output, grep waits and reads any output from standard input at once.
Still have doubts? Do an experiment. run
find / | grep usr
find / will list all files on the computer, it should take a lot of time.
If ls runs first, then OS gives the output to grep, we should wait a long time with blank screen until find finished and grep started. But, we can see the results at once, that's a proof for that.
I am using tcpstat in a linux environment. I want to capture its output in a C program even though it has not finished. I tried using the popen() function, but it can only process the output after the program has finished. I want to process the output of tcpstat on the fly as and when it prints it on standard output. How do i do so?
For example,
$ tcpstat -i wlan0 1
Time:1297790227 n=2 avg=102.50 stddev=42.50 bps=1640.00
Time:1297790228 n=11 avg=86.36 stddev=19.05 bps=7600.00
Time:1297790229 n=32 avg=607.97 stddev=635.89 bps=155640.00
Time:1297790230 n=13 avg=582.92 stddev=585.55 bps=60624.00
The above output keeps going on till infinity. So I want to process the output in a C program as and when tcpstat outputs something onto stdout.
Thanks and Regards,
Hrishikesh Murali
Run tcpstat -i wlan0 -a 1 | your_program and read from the standard input in your program. This way the shell will take care of the piping.
The popen library function and the pipe system call can be used to achieve the same result at a lower level. You may want to take a look at named pipes too - they appear like files in userspace and can be manipulated in the same way.
Run tcpstat with the -F option, this will cause it to flush its output on every interval. (instead of using the default block buffering for stdout)
In addition, you may want to explicitly disable the buffering on your popen FILE handle using setbuf, eg.
setbuf(popen_fd, NULL);
Alternately, you can set it to be line buffered, using setlinebuf
setlinebuf(popen_fd);
I am trying to control ftp client from C program (OS X). I did fork and execve - process is started ok. The problem is with pipes - I can send command to ftp client process and get feedback from it just fine (If i send "help\n" i get back help output) but what I never get in pipe is "ftp> " prompt. Any ideas?
Ivan
Your ftp client is probably behaving differently if stdin/stdout is a terminal or something else (lots of program do, for a start the C library does buffering in a different way...) If you want to control that, search information about pseudo-terminals, that's a little too technical to be explained here. (And looks first at programs like expect, it's possible you won't have to write yours).
A program can examine stdin to find out whether it's a terminal or a pipe. In your case, the FTP program probably does that (for example to know whether it can use escape sequences to render progress bars or offer command line editing).
If you really need the prompt, you have to look into PTYs (pseudo terminals) which emulate a console.
wild guess: isn't the "ftp>" prompt written to STDERR ?
I need to run a script and have access to the default stdin (terminal input) in my program. I could do ./program "script", opening and parsing the script through the program, but I want to make it POSIX style, accepting input from pipes or from redirection.
I mean, since my program is a parser, I could run ./program, type the script and still use stdin (in a scanf, for example). But I'd like to run ./program < script and still be able to use stdin (in a scanf).
My program is a simplified Pascal interpreter, that's why I need to run read(x) and write(x) in my scripts.
Yes, it's homework (the intepreter), but the doubt just popped up in the brainstorming process.
The current controlling terminal can be accessed using /dev/tty, even if stdin has been redirected.
ttyname(0) will return the filename of the current terminal associated with stdin. You can then open that and read from it.
If I understand what you're asking, you're asking for the ability to take in interactive input from a user when using file redirection, like the ./program < script bit above.
I don't believe there's a way to do that. A POSIX system will feed the script in via stdin and that's that. No interaction from the user.
It's also worth noting that you don't have to do anything special to realize that. Just treat stdin like you normally would. You don't have to think about whether it's coming in interactively or from a file, which is really quite nice.