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..
Related
I have a C program that takes an output file name and dumps data into it. Most probably the program uses FILE* pointers. Is it possible to pass terminal stdout to that C program? I do not have access to the code.
What I mean, the program works as follows --
> ./program out.txt # --> dumps data into out.txt
What I want to do is something like --
> ./program &1 # --> dumps data on the terminal.
Is it possible? How can it be done?
Use /dev/stdout or /dev/fd/1.
./program /dev/stdout
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'm having difficulty writing a function in C that checks whether a user inputed file (via stdin) exists. For instance if the program is run as ./a.out <myfile.txt, I want it to return false if this file does not exist. I can do this by passing the file as an argument (i.e ./a.out myfile.txt)using fopen(), but not sure how to do this using 'stdin' (i.e ./a.out <myfile.txt)
Ok to clarify:
The larger program is supposed to take the contents of a text file and perform actions on it. The program must be run in the command line as ./a.out arg1 arg2 <myfile.txt. If user ran the program as ./a.out arg1 arg2 or ./a.out (i.e not specifying the file to perform actions on), I want to prompt the user to include a file (using stdin <, not passed as an argument).
Stdin might not be coming from a file at all. Even if it is, when the user types "< myfile.txt" at the command line, the shell swallows that part of the command, and never passes it to the program. As far as the program is concerned, it's an anonymous stream of bytes that might be from a file, a device, a terminal, a pipe, or something else. It is possible to query which of these you have, but even if you know it's a file you won't get the name of the file given on the command line, only an inode.
Since the shell is responsible for opening the file for redirection, it will refuse to execute the command if the file doesn't open.
Input redirection is something done by the shell, not your program. It simply attaches the file to standard input.
Hence, if you try to redirect input from a non-existent file, the shell should complain bitterly and not even run your program, as shown in the following transcript:
pax> echo hello >qq.in
pax> cat <qq.in
hello
pax> cat <nosuchfile.txt
bash: nosuchfile.txt: No such file or directory
In any case, your program generally doesn't know where the input is coming from, since you can do something like:
echo hello | cat
in which no file is involved.
If you want your program to detect the existence of a file, it will have to open the file itself, meaning you should probably give the filename as an argument rather than using standard input.
Or, you could detect the file existence before running your program, with something like the following bash segment:
fspec=/tmp/infile
if [[ -f ${fspec} ]] ; then
my_prog <${fspec}
else
echo What the ...
fi
The OS prevent calling your program since it can provide a valid stdin if myfile.txt does not exists. You program will not run thus there is no way you can signal the file is missing, and this diagnostics is done at the OS level.
If user ran the program as ./a.out arg1 arg2 or ./a.out (i.e not specifying the file to perform actions on), I want to prompt the user to include a file (using stdin <, not passed as an argument).
You could use OS-specific functions to check whether stdin is terminal. Checking whether it's file is a very bad idea, because it's very useful to pipe into stdin ... in fact, that's a major reason that there is such a thing as stdin in the first place. If you only want to read from a file, not a terminal or pipe, then you should take the file name as a required argument and not read from the orginal stdin (you can still read from stdin by using freopen). If you insist that you don't want to do it that way, then I will insist that you want to do it wrong.
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