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Project 2 2017 - a simple Unix shell

Suggested steps

Step 0. Develop a Makefile

Develop a Makefile, employing variable definitions and automatic variables, to compile and link the project's files.

Step 1. Execute simple external commands

1. execute commands without arguments, such as  /bin/ls
2. execute commands with arguments, such as  /usr/bin/cal -y

Helpful C99 and POSIX functions: fork(), execv(), exit(), wait().
DO NOT use the functions: system(), popen(), execlp(), or execvp() anywhere in your project.

Step 2. Search path

The global variable PATH points to a character string which is interpreted as a colon separated list of directory names.

If the user enters a command name which does not contain a '/' character, then the members of the search path list are considered as directories from which to attempt to execute the required command. Once the required command is found and executed, the search terminates.

Helpful C99 and POSIX functions: strchr().

Step 3. Execute internal commands

Internal commands are performed by myshell, itself, rather than by external commands. myshell does not create a new process, with fork(), before running an internal command. Three internal commands are to be supported:

1. exit - terminate myshell by calling the function exit(). When an additional argument is provided, it should be interpreted as the numeric exit-status. When requested without any arguments, the exit-status of the recently executed command should be used.
2. cd - change from the current working (default) directory to the directory specified as the first argument to the cd command. If the command cd is given without arguments, then the variable HOME should be used as the new directory.
   As with the role of PATH for command execution, the variable CDPATH points to a character string which is interpreted as a colon separated list of directory names. If the user enters a directory-name which does not contain commence with a '/' character, then the members of CDPATH are considered as directories to locate the required directory.
3. time - the 'following' command's execution time should be reported in milliseconds (e.g. 84msec) to myshell's stderr stream.

Helpful C99 and POSIX functions: exit(), chdir(), gettimeofday().

Step 4. Sequential execution

There are three forms of sequential execution, and thus three subparts to this step. The three forms differ in the action they take when a command fails. By convention, a command which fails will return a non-zero exit status.

1. A sequence of commands separated by the  ";"  token requires myshell to execute each command sequentially, waiting until each command has finished before beginning the next. In this form, myshell should continue to the next command regardless of the result of earlier commands. The sequence   ls ; date monday ; ps   would execute ls, then on its completion execute date monday (which will fail), then execute ps (even though the previous command failed). The exit value of sequential execution is the exit status of the command sequence to the right of the ";" token.
2. If the commands are separated by the token  "&&"  then myshell should continue with further commands only if the command sequence to the left of the token returns an exit value indicating success. For the command sequence a && b, myshell should wait for command a to terminate, test its return value and execute b if and only if a's return value indicates success. The exit value of an "&&" command sequence is the exit status of the last command executed.
3. If the token  "||"  is used to separate commands then myshell should continue to the next command only if the first command sequence returns an exit value indicating failure. This is the converse of part 2. For the command sequence a || b, myshell should wait for command a to terminate, test its return value and execute b if and only if a's return value indicates failure. The exit value of a "||" command sequence is the exit status of the last command executed.

Step 5. Subshell execution

The sequence   ( commands )   should cause commands to be executed in a subshell. A subshell is created by forking a copy of myshell to execute the commands. The exit value for a subshell is the exit value of the command sequence inside the parentheses.

Step 6. stdin and stdout file redirection

Commands may redirect their standard input and standard output from and to files, respectively, before the command is executed.

1. The sequence   command < infile   requests that the command use infile as its standard input.
2. The sequence   command > outfile   requests that the command use outfile as its standard output. If the file outfile does not exist then it is created. If it does exist it is truncated to zero then rewritten.
3. The sequence   command >> outfile   requests that the command appends its standard output to the file outfile. If outfile does not exist it is created.

NOTE: in all of the above cases command could be a subshell. It is valid to have both input and output redirection for the same command. Thus the sequence   ( sort ; ps ) < junk1 >> junk2   would take input from the file junk1 and append output to the file junk2.

Helpful C99 and POSIX functions: open(), close(), dup2().

Step 7. Pipelines

The sequence   command1 | command2   requests that the stdout of command1 be presented as stdin to command2. By default, the stderr output of command1 is not redirected and appears at its default location (typically the terminal). With reference to this example, the myshell parser will not permit the stdout of command1 to be redirected to a file (with >), nor the stdin of command2 to be received from a file (with <). Note that the sequence command1 | command2 | command3   requests that a pipeline of two different pipes be established.

Helpful C99 and POSIX functions: pipe(), dup2().

Step 8. Shell scripts

Once an executable file name has been found (possibly using the search path) the standard function execv() can be called to try to execute it. If execv() fails, you should assume that the file is a shell script (a text file) containing more myshell commands. In this case you should execute another copy of myshell to read its input from the shell script. No specific filename extension is required.

Helpful C99 and POSIX functions: access(), fopen(), fclose().

Step 9. 🌶 Background execution

The token  "&"  causes the preceding command to be executed without myshell waiting for it to finish (asynchronously). Thus the sequence ls ; ps & date should start the command sequence ls, once this has completed start ps and immediately proceed to the command date. The exit value of background execution is success unless the fork call fails, in which case the exit value should indicate failure.

Commands placed into the background may take a long time to execute and, thus, we wish to know when they have completed. The parent is informed of the termination of child processes using asynchronous signal-passing. When so informed, myshell should report which background process has terminated.

Similarly, when myshell exits, it should first terminate all of its still-running background processes, and wait for them to finish.

Helpful C99 and POSIX functions: signal() and kill().

Starting files

• myshell.h - provides the definition of myshell's user-defined datatypes and the declaration of global variables.
• myshell.c - provides the main() function, and calls the parse_shellcmd() function.
• globals.c - defines global variables, and the helpful print_shellcmd0() function.
• execute.c - where you define your execute_shellcmd() function.
• parser.c - defines the parse_shellcmd() and free_shellcmd() functions, which should be used without modification (and need not be understood).