You've made a very good start actually. You just didn't know about open() and write().
And actually, there are a few arbitrary numbers for fd; when your program is run, it inherits a few from the terminal you ran it from. You could do write(1, "abcd\n", 5); to print "abcd" and a newline to the terminal for example. FD 0 is standard input(terminal keyboard if not redirected), FD 1 is standard output(terminal screen if not redirected), FD 2 is standard error(again, the terminal screen).
Standard error exists so that you can redirect standard output into a file, and still get sensible error messages printed to your terminal, or avoid printing error messages into the middle of sensible data.
You can refer to these more sensibly with STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO, defined in unistd.h
If you use a number that doesn't match any opened file, unlike stdio the program won't crash; the system calls will just return error.
Hello guys, thank God that I found this forum.
I hope that someone can help me because I don't have any idea on how to start it. I know that for some of you this is a very simple task but I'm not as advance on shell scripting like many people out there.
I got this file with a permanent... (10 Replies)
Hi
My requirement is to read the contents of a fixed length file and validate the same.
But am not able to read the contents of the file and when i tried it to print i get <blank> as an output...
I used the below satatements for printing the contents
... (3 Replies)
Hi Friends,
I am new to this forum. Just struck up with a logic.
I have a csv file seperated by ":" (colons).
This csv file contains hostname and groups as follows:
HOSTNAME:VT Group
SGSGCT2AVPX001:Team1
SGSGCT2AVPX003:Team2
SGSGCT2AVPX005:Team2
PHMNCTTAVPX001:Team3
I want to... (2 Replies)
Hi,
Need help in writing a script to read the contents of this file test
Test
00a
00b
00c
00d
00e
00f
where it need to read each line to give a display such as
form meta from dev 00a , config=Striped; add dev 00b:00f to meta 00a
Can any one help me in writing this script (2 Replies)
My file is in this format :
username : student information : default shell : student ID
Eg :
joeb:Joe Bennett:/bin/csh:1234
jerryd:Jerry Daniels:/bin/csh:2345
deaverm: Deaver Michelle:/bin/bash:4356
joseyg:Josey Guerra:/bin/bash:8767
michaelh:Michael Hall:/bin/ksh:1547
I have to... (1 Reply)
Use and complete the template provided. The entire template must be completed. If you don't, your post may be deleted!
1. The problem statement, all variables and given/known data:
I have to read the contents of each field of a file creating user accounts.
The file will be of format :
... (6 Replies)
I have made a script something like this. I want it to read the contents of either file or directory but 'cat' and 'ls' is not working. Can anyone help me? I am a newbie in scripting so dont know much about it. I also dont know how can i put my code separatly on this forum
#!/bin/bash
echo... (9 Replies)
Hi,
I am kind of new at awk programming, so any help would be great !
I am trying to read a date from a file into a variable and a count into another variable and display both these variables.
The file looks like the attached file...
I tried this but it doesn't work ...
... (6 Replies)
Hi,
I am facing issue while reading data from a file in UNIX. my requirement is to compare two files and for the text pattern matching in the 1st file, replace the contents in second file by the contents of first file from start to the end and write the contents to thrid file.
i am able to... (2 Replies)
Discussion started by: seeki
2 Replies
LEARN ABOUT REDHAT
stderr
STDIN(3) BSD Library Functions Manual STDIN(3)NAME
stdin, stdout, stderr -- standard I/O streams
SYNOPSIS
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
DESCRIPTION
Under normal circumstances every Unix program has three streams opened for it when it starts up, one for input, one for output, and one for
printing diagnostic or error messages. These are typically attached to the user's terminal (see tty(4)) but might instead refer to files or
other devices, depending on what the parent process chose to set up. (See also the ``Redirection'' section of sh(1) .)
The input stream is referred to as ``standard input''; the output stream is referred to as ``standard output''; and the error stream is
referred to as ``standard error''. These terms are abbreviated to form the symbols used to refer to these files, namely stdin, stdout, and
stderr.
Each of these symbols is a stdio(3) macro of type pointer to FILE, and can be used with functions like fprintf(3) or fread(3).
Since FILEs are a buffering wrapper around Unix file descriptors, the same underlying files may also be accessed using the raw Unix file
interface, that is, the functions like read(2) and lseek(2). The integer file descriptors associated with the streams stdin, stdout, and
stderr are 0, 1, and 2, respectively. The preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are defined with these values
in <unistd.h>.
Note that mixing use of FILEs and raw file descriptors can produce unexpected results and should generally be avoided. (For the masochistic
among you: POSIX.1, section 8.2.3, describes in detail how this interaction is supposed to work.) A general rule is that file descriptors
are handled in the kernel, while stdio is just a library. This means for example, that after an exec, the child inherits all open file
descriptors, but all old streams have become inaccessible.
Since the symbols stdin, stdout, and stderr are specified to be macros, assigning to them is non-portable. The standard streams can be made
to refer to different files with help of the library function freopen(3), specially introduced to make it possible to reassign stdin, stdout,
and stderr. The standard streams are closed by a call to exit(3) and by normal program termination.
SEE ALSO sh(1), csh(1), open(2), fopen(3), stdio(3)CONSIDERATIONS
The stream stderr is unbuffered. The stream stdout is line-buffered when it points to a terminal. Partial lines will not appear until
fflush(3) or exit(3) is called, or a newline is printed. This can produce unexpected results, especially with debugging output. The buffer-
ing mode of the standard streams (or any other stream) can be changed using the setbuf(3) or setvbuf(3) call. Note that in case stdin is
associated with a terminal, there may also be input buffering in the terminal driver, entirely unrelated to stdio buffering. (Indeed, nor-
mally terminal input is line buffered in the kernel.) This kernel input handling can be modified using calls like tcsetattr(3); see also
stty(1), and termios(3).
CONFORMING TO
The stdin, stdout, and stderr macros conform to ANSI X3.159-1989 (``ANSI C89''), and this standard also stipulates that these three streams
shall be open at program startup.
Linux 2.0 March 24, 1998 Linux 2.0