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The purpose of this assignment is to learn to develop multi-process programs. You are expected to extend the myshell.c program and add pipelines and I/O redirections. In particular, your shell program should recognize the following:
- > – Redirect standard output from a command to a file. Note: if the file already exists, it will be erased and overwritten without warning. For example,COP4338$ ls > 1COP4338$ sort myshell.c > 2Note that you’re not supposed to implement the Unix commands (ls, sort, …). You do need to implement the shell that invoke these commands and you need to “wire” up the standard input and output so that they “chain” up as expected.
- >> – Append standard output from a command to a file if the file exists; if the file does not exist, create one. For example,COP4338$ sort myshell.c >> 1COP4338$ echo “Add A Line” >> 1
- < - Redirect the standard input to be from a file, rather than the keyboard. For example,COP4338$ sort < myshell.cCOP4338$ sort < myshell.c > 1COP4338$ sort > 1 < myshell.cThe second and third commands are the same: the sort program reads the file named myshell.c as standard input, sorts it, and then writes to file named 1.
- | – Pass the standard output of one command to another for further processing. For example,COP4338$ ls | sortCOP4338$ sort < myshell.c | grep main | cat > output
Make sure you have your parent process fork the children and ‘wire’ them up using pipes accordingly. There will be spaces separating tokens in the command line. Your program should be able to run with all the Unix commands as expected. Don’t assume your program will only be tested by the above commands. Also, there could be more than two commands chained by pipes, like the second example above.
Your program needs to provide necessary sanity-check from the user input. Prompt meaningful errors accordingly as what you’d experience with your shell.
Hint: use strtok() functions to separate the tokens and use strstr() to locate sub-strings.
