I want to check if user is entering something like ls& or ls & so that I will be able to set the bg_flag for background jobs in the shell. however the following code can't check against ls&&. because I don't want it to be considered a background job(I am not sure if it's the same in Linux shell) if user enters ls&& or anything with more than one & in the end.
if (strstr(args[arg_count-1],"&")!=NULL)
//if (strcmp(args[arg_count-1],"&")==0)
{
bg_flag=1;
printf("I am a background job %d ",getpid());
}
Please let me know what is the appropriate method for fixing this?
To do that reliably, you need to properly parse the command in the same way the shell does it. Usually you define a grammar for that and use a parser generator. Everything else is just guessing and will most likely fail.
For example, consider this:
some_program \&&
it ends with two ampersands (&&) but anyway it will be a background process because the first ampersand is escaped (\&).
However, marking the postfix \&& as "background" is also not correct because
some_program \\&&
would not be a background process (but an incomplete command instead).
And as long as you don't define a proper grammar, it's very likely that you will not correctly catch everything as my two examples may show.
Another sort of problems might be programs that detach themselves from the terminal (sometimes called daemonization). They are not backgrounded by the shell, they do the same by themselves.
Related
I am trying to code a little console chat-program in C on linux.
So far I coded it in a way that both chatting partners are only able to alternately send/recv, because these function calls are blocking by default.
Now I would like to modify that program, so that both are able to send and receive simultaneously.
The problem that I find is, that once you typed some input to the terminal, I don't know how to output received messages, without messing up the current input line of the terminal.
If there was a way to delete that current input line, you could temporarily save that line, print the new message and put the input line right back.
However, I was not able to find a solution for this problem on the internet.
Is it possible to delete the current input line, and if not, how else could I achieve what I want?
I think you should look into ncurses as Edd said in his comment.
It would allow you to easily manage contents in your terminal window, which sounds like a good idea for your chat program.
All you'd need to do is store your messages in 2 character arrays:
char incoming[MSG_MAX]
and
char outgoing[MSG_MAX]
Then you can output those messages wherever you want in your terminal window, since ncurses allows you to specify x,y coordinates on where to put your text.
Then a simple wrapper for one of ncurses erase() family functions would allow you to delete characters from specify x,y coordinates in your terminal window.
Edit: MSG_MAX is not an actual ncurses macro.
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 am wanting to make a terminal app that stores information about files/directories. I want a way to keep the information if the file is moved or renamed.
What I thought I could do is have a function execute before any command is run. I found this:
http://www.twistedmatrix.com/users/glyph/preexec.bash.txt
But I was wondering if this would be a good way to go about it. Or should I do something else?
I would like to call that function from a C program whenever mv is entered I suppose.
If what you're trying to do is attach some sort of metadata to files, there's a much better supported way to do that -- extended attributes.
Another solution might be to use the file's inode number as an index into a database you maintain yourself.
Can you alias the mv command? in .profile or .bashrc
alias mv=/usr/bin/local/mymv
where mymv is a compiled executable that runs your C code function and calls /usr/bin/mv.
precmd and preeexec add some overhead to every bash script that gets run, even if the script never calls mv. The downside to alias is that it requires new code in /usr/local and if scripts or users employ /usr/bin/mv instead of mv it will not do what you want. Generally doing something like this often means there is a better way to handle the problem with some kind of service (daemon) or driver. Plus, what happens if your C code cannot correctly handle interesting input like
mv somefille /dev/null
If you want to run command each time after some command was executed in the terminal, just put the following in ~/.bashrc:
PROMPT_COMMAND="your_command;$PROMPT_COMMAND"
If you want your command to be executed each time before mv is executing, put the following in ~/.bashrc:
alias mv="your_script"
Make sure that your script will execute real mv if needed.
You can use inotify library to track filesystem changes. It's good solution, but once user remove file, it's already gone.
You might be able to make use of the DEBUG trap in Bash.
From man bash:
If a sigspec is DEBUG, the command arg is executed before every
simple command, for command, case command, select command, every
arithmetic for command, and before the first command executes in
a shell function
I found this article when I was forced to work in tcsh and wanted to ensure a specific environemtn variable was present when the user ran a program from a certain folder (without setting that variable globally)
tcsh can do this.
tcsh has special alias, one of which is precmd
This can be used to run a script just before the shell prompt is printed.
e.g. I used set precmd 'bash $HOME/.local/bin/on_cd.sh'
This might be one of the very few useful features in csh.
It is a shame but I don't think the same or similar feature is in bash or other sh derivites (ash, dash etc). Related answer.
I created a little mini shell and it let's the user enter a command like 'ls' and it will list the contents of the directory like it's supposed to using execv() in my code, but that doesn't seem to work for when the user enters something like 'set name="bob"'. I've been looking all over the place for what I should use in my code to execute a set command when the user enters it and the best I can find is system(), but that still isn't working for me. Any ideas?
set is a shell-builtin command, not an external command (indeed it needs to be to have the intended effect, which is to modify a shell variable within the shell process itself).
This means that you need to look for and handle set within your shell itself, by adding the named variable to some internal data structure that tracks shell variables (or updating it if it already exists there).
Since you're doing a fork-and-exec or a system(), the command is really being run in a separate process. What happens in that process (like setting an environment variable) does not affect the parent's environment. (A separate issue is that set doesn't actually create an environment variable. You'd need export in [ba]sh or setenv in [t]csh to do that.)
So you need to code your mini-shell to handle the set command explicitly, rather than passing it off to another program.
You might want to look at setenv(3) and getenv(3). These are functions for changing and reading environment variables from within a C program.
From a C program I want to call a shell script with a filename as a parameter. Users can control the filename. The C is something like (initialization/error checking omitted):
sprintf(buf, "/bin/sh script.sh \"%s\"", filename);
system(buf);
The target device is actually an embedded system so I don't need to worry about malicious users. Obviously this would be an attack vector in a web environment. Still, if there is a filename on the system which, for example, contains backquotes in its name, the command will fail because the shell will perform expansion on the name. Is there any to prevent command substitution?
Well, you could always reimplement system() using a call to fork() and then execv().
http://www.opengroup.org/onlinepubs/000095399/functions/system.html
Try triggering "unalias " in the system function.
Since you tagged this as C I will provide you with a C answer. You will need to escape the filename -- create a new string that will be treated properly by the shell, so that things like This is a file name produces This\ is\ a\ file\ name or bad;rm *;filename becomes bad\;rm\ \*\;filename. Then you can pass that to the shell.
Another way around this would be to run the shell directly with fork and one of the exec functions. Passing arguments directly to programs does not result in shell command line expansion or interpretation.
As sharth said, you should not use system but fork and execv yourself. But to answer the question of how you make strings safe to pass to the shell (in case you insist on using system), you need to escape the string. The simplest way to do this is to first replace every occurrence of ' (single quote) with '\'' (single quote, backslash, single quote, single quote) then add ' (single quote) at the beginning and end of the string. The other fairly easy (but usually less efficient) method is to place a backslash before every single character, but then you still need to do some special quotation mark tricks to handle embedded newlines, so I prefer the first method.