I have written a C program to get all the possible combinations of a string. For example, for abc, it will print abc, bca, acb etc. I want to get this output in a separate file. What function I should use? I don't have any knowledge of file handling in C. If somebody explain me with a small piece of code, I will be very thankful.
Using function fopen (and fprintf(f,"…",…); instead of printf("…",…); where f is the FILE* obtained from fopen) should give you that result. You may fclose() your file when you are finished, but it will be done automatically by the OS when the program exits if you don't.
If you're running it from the command line, you can just redirect stdout to a file.
On Bash (Mac / Linux etc):
./myProgram > myFile.txt
or on Windows
myProgram.exe > myFile.txt
Been a while since I did this, but IIRC there is a freopen that lets you open a file at given handle. If you open myfile.txt at 1, everything you write to stdout will go there.
You can use the tee command (available in *nix and cmd.exe) - this allows output to be sent to both the standard output and a named file.
./myProgram | tee myFile.txt
Related
I'm new to c.
Is there any simple way to redirect all the console's output (printfs etc.) to a file using some general command line \ linkage parameter (without having to modify any of the original code)?
If so what is the procedure?
Use shell output redirection
your-command > outputfile.txt
The standard error will still be output to the console. If you don't want that, use:
your-command > outputfile.txt 2>&1
or
your-command &> outputfile.txt
You should also look into the tee utility, which can make it redirect to two places at once.
On unices, you can also do:
your-command | tee output file.txt
That way you'll see the output and be able to interact with the program, while getting a hardcopy of the standard output (but not standard input, so it's not like a teletype session).
As mentioned above, you can use the > operator to redirect the output of your program to a file as in:
./program > out_file
Also, you can append data to an existing file (or create it if it doesnt exit already by using >> operator:
./program >> out_file
If you really want to learn more about the (awesome) features that the command line has to offer I would really recommend reading this book (and doing lots of programming :))
http://linuxcommand.org/
Enjoy!
In Unix shells you can usually do executable > file 2> &1, whch means "redirect standard output to file and error output to standard output"
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 have a C program which gives some output. I am compiling the C program via the shell but I need the output from the run C program and store in shell.
Edit.
Save the output to a shell Variable.
I assume that you want to store the output of the program in a variable. Unix shells offer a facility that's called command substitution to do just that. Depending on your shell, you can do either :
output=$(./run)
or
output=`./run`
Bash supports both. If, however, you want to save the output to a file, you will need to redirect the standard output stream to a file. You can do this like this :
./run > output.txt
Or, if you want to see the output while the program is running and save it to an output file as well, you can use the tee utility and pipe the output of your program to it.
./run | tee output.txt
You can redirect your output into a file like this:
./run > file
If you want to store it into a variable, you have to decide which shell we're talking about. It depends whether you have a windows shell, or linux bash..
I have a ton of printf statements and I would like to write all of them to a text file. I realize I could just add fprintf statements after each one but is there a better way, such as a function, or should I just write my own function? I feel like this is probably a standard procedure, I just don't know what it's called, so it's hard to find an answer by googling.
EDIT: Just for clarity, I'd like the output to keep going the terminal like normal, but also be printed to a file simultaneously. Several people are suggesting bash commands. When should those be executed? After the program is run?
If you're on a UNIX type box, you can pipe the output through tee, which will deliver it to standard output and a file:
myProg | tee /tmp/myProg.out
There are also ways to do the same thing with standard error as well:
( myProg 2>&1) | tee /tmp/myProg.out_and_err
This of course depends on whatever shell you're using but that should work on the most common ones.
When the program is run, use the > bash operator to redirect the programs output to a text file:
IE
myprogram > myfile.txt
That will save everything normally outputted to the screen with printf to a file named myfile.txt.
How about to redirect the stdout to a file, like
int main ()
{
freopen ("myfile.txt","w",stdout);
printf ("This sentence is redirected to a file.");
fclose (stdout);
return 0;
}
code sample comes from c++ Reference for freopen
You can do: ./executable > outputfile 2>&1 or ./executable &> outputfile
This will enable you to redirect both the standard output and the standard output to the outputfile.
I am trying to redirect the output of a c program to file, even when it generates some errors because of problems with the input data. I can send the output but the error messages to a file.
Does somebody know how to do it?
From within C source code, you can redirect outputs using freopen():
General outputs:
freopen("myfile.txt", "w", stdout);
Errors:
freopen("myfile_err.txt", "w", stderr);
(This answer applies to bash shell, and similar flavors. You didn't specify your environment and this sort of question needs that detail.)
I assume you know about basic redirection with ">". To also capture STDERR in addition to STDOUT, use the following syntax:
command > file-name 2>&1
For some more background on standard streams and numbers:
http://en.wikipedia.org/wiki/Standard_streams#Standard_input_.28stdin.29
This depends on what you mean and what platform you are using. Very often you can accomplish this from the command line, which has been covered in another answer. If you use this method to accomplish this you should be aware that FILE * stderr is typically written immediately (unbuffered) while FILE * stdout may be buffered (usually line buffered) so you could end up with some of your error messages appearing to have been printed earlier than some other messages, but actually the other messages are just being printed late.
From within a C program you can also do something similar within the stdio system using freopen, which will effect the FILE *, so you could make fprintf(stderr, "fungus"); print to something besides what stderr normally would print to.
But if you want to know how to make a program redirect the actual file descriptors under a unix like system you need to learn about the dup and dup2 system calls. They allow you to duplicate a file descriptor.
int fd = open("some_file", O_WRONLY);
dup2(2,fd);
close(fd);
This code will make "some_file" the new stderr at the OS level. The dup2 call will close and replace file descriptor 2 (stderr, which is usually used by FILE * stderr but not necessarily if you call freopen(x,y,stderr) since that may make FILE *stderr use a different file descriptor).
This is how shell programs redirect input and output of programs. The open all of the files that the new program will need, fork, then the child uses dup2 to set up the files descriptors for the new program, then it closes any files that the new program won't need (usually just leaving 0, 1, and 2 open), and then uses one of the exec functions to become the program that the shell was told to run. (some of this isn't entirely accurate because some shells may rely on close on exe flags)
Using a simple linux command you can save the output into the file. here is a simple linux terminal command.
ls > file.txt
The output of this command will be stored into the file.
same as you can store the output of the program like this suppose, object file name is a, run the following command to save output in a file:
./a > file.txt