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Program 2 • Function pointers, a shell 

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CMPS-109 Program 2 • Function pointers, a shell

1. Overview
You will maintain a tree structure with a simple hierarchy, maintained by a map of
functions. Programming will be done C++ style, not C style, as shown in the following table.
Do not use : Instead, use :
char* strings <string>
C arrays <vector>
<stdio.h>, <cstdio> <iostream>, <iomanip>
pointers <shared_ptr>
union inheritance
<header.h> <cheader>
Header files : Include only C++11/14/17 header files and facilities where feasable,
and use namespace std. Include <cheader> files only when C++-style files are
unavailable. Include <header.h> files from C only when an appropriate <cheader>
files is unavailable. Use the script cpplint.py.perl (a wrapper for cpplint.py) to
check style.
2. Program Specification
The program specification is given in terms of a Unix man(1) page.
NAME
yshell — in memory simulated tree shell
SYNOPSIS
yshell [-@ flags]
DESCRIPTION
This shell reads commands from stdin and write output to stdout, with errors
being set to cerr. Each line read by the shell is parsed into words by splitting
using space characters, with any number of spaces between words. There may
also be leading and trailing spaces. The first word on any line is a command to
be simulated, and the rest are operands to that command. If either stdin or
stdout is not a tty, each line from stdin is echoed to stdout.
The commands modify an inode tree, where each inode is either a file or a
directory. Files contain data and directories contain inodes. An inode is specified by means of a pathname. A pathname consists of a sequence of characters
separated by slash (/) characters.
The inode tree has a root, which is a special node, and also a current inode as
well. Whenever a pathname is decoded, if the first character is a slash (/),
decoding begins at the root, otherwise it begins with the current directory.
Whenever a pathname component is a dot (.), it refers to the current directory.
If a component is a double dot (..) it refers to the parent of the current directory. Every directory has both of these entries, with the root being its own parent. Multiple adjacent slashes are treated as a single slash. Trailing slashes
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 2 of 7
are permitted only on directories.
Every inode has three attributes : an inode number, which is uniquely
assigned, starting from 1 for the root; contents, which is a map from filenames to inodes for a directory, and text for a file ; and a size, which is the
byte count for text, and the number of sub-inodes for a directory.
OPERANDS
None. All input comes from stdin.
OPTIONS
The -@ option is followed by a sequence of flags to enable debug output, written
to cerr.
COMMANDS
The following commands are interpreted. Error messages are printed and
nothing is done in the case of invalid operands.
# string
If the first non-space character on a line is a hash, the line is a comment
and is ignored.
cat pathname . . .
The contents of each file is copied to stdout. An error is reported if no
files are specified, a file does not exist, or is a directory.
cd [pathname]
The current directory is set the the pathname given. If no pathname is
specified, the root directory (/) is used. It is an error if the pathname
does not exist or is a plain file, or if more than one operand is given.
echo [words . . .]
The string, which may be empty, is echoed to stdout on a line by itself.
exit [status]
Exit the program with the given status. If the status is missing, exit
with status 0. If a non-numeric argument is given, exit with status 127.
ls [pathname . . .]
A description of the files or directories are printed to stdout. It is an
error if any of the file or directory does not exist. If no pathname is specified, the current working directory is used. If a pathname specified is a
directory, then the contents of the directory are listed. A directory listed
within a directory is shown by a terminating slash. Elements of a directory are listed lexicographically.
For each file listed, output consists of the inode number, then the size,
then the filename. Output is lined up into columns and each column is
separated from the next by two spaces. The numeric fields are exactly 6
characters wide and the units position in a column must be aligned.
lsr [pathname . . .]
As for ls, but a recursive depth-first preorder traversal is done for subdirectories.
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 3 of 7
make pathname [words . . .]
The file specified is created and the rest of the words are put in that file.
If the file already exists,anew one is not created, but its contents are
replaced. It is an error to specify a directory. If there are no words, the
file is empty.
mkdir pathname
A new directory is created. It is an error if a file or directory of the same
name already exists, or if the complete pathname to the parent of this
new directory does not already exist. Two entries are added to the directory, namely dot (.) and dotdot (..). Directory entries are always kept in
sorted lexicographic order.
prompt string
Set the prompt to the words specified on the command line. Each word is
separated from the next by one space and the prompt itself is terminated
by an extra space. The default prompt is a single percent sign and a
space (% ).
pwd
Prints the current working directory.
rm pathname
The specified file or directory is deleted (removed from its parent’s list of
files and subdirectories). It is an error for the pathname not to exist. If
the pathname is a directory, it must be empty.
rmr pathname
A recursive removal is done, using a depth-first postorder traversal.
EXIT STATUS
0 No errors were detected.
1 Error messages were printed to cerr.
3. A Sample Run
The following table shows a sample run. Each interaction with the shell is listed in
a separate box with shell output in Courier Roman and user input in Courier Bold
typeface. A commentary about what is happening is opposite in the right column.
% pwd
/
Initially the cwd is the root directory.
% ls
/:
1 2 .
1 2 ..
The absence of an operand to ls means
that dot is used as its operand, which
is currently the root. Directories
always contain at least two items,
namely dot and dotdot. The inode
number of the root is always inode #1.
The parent of dotdot is itself.
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 4 of 7
% make foo this is a test Make a file called foo which contains
the string ‘‘this is a test’’, which is
14 characters. An inode is allocated,
namely inode #2.
% make bar test a is this Another file, similarly created, with
inode #3.
% ls
/:
1 4 .
1 4 ..
3 14 bar
2 14 foo
Same as the previous output of ls,
except with two more files. Note that
files are kept in lexicographic order, so
bar is listed before foo.
% cat food
cat: food: No such file or directory
An error message is printed, causing
the return code from the shell eventually to be 1 rather than 0. Note the
error format: command followed by
object causing the problem followed by
the reason for the failure.
% cat foo
this is a test
Files can consist of only one line,
namely a string.
% echo O for a muse of fire
O for a muse of fire
Arguments to echo are simply written
to the standard output.
% prompt => The prompt is changed to the characters ‘‘=>’’ followed by a space. Multiple
words would have been permitted.
=> rm bar The file bar is deleted and the size of
the root directory is reduced by 1.
=> make baz foo bar baz A new file is created with inode #4.
=> mkdir test Inode #5 is created as a directory
called test. This directory is a child of
the root and contains the two usual
entries, dot and dotdot.
=> prompt % The prompt is changed back to a % followed by a space.
% ls /
/:
1 5 .
1 5 ..
4 11 baz
2 14 foo
5 2 test/
Just checking the contents of the root.
% cd test The cwd is now test.
% pwd
/test
Yes, it is.
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 5 of 7
% cd Without arguments cd goes back to the
root directory.
% pwd
/
OK.
% cd test Go to a directory called test which is a
subdirectory of the cwd, whose alias
name is always dot.
% pwd
/test
% cd .. Dotdot is always an alias for the parent of the cwd.
% pwd
/
% cd test
% make me me me me
This would have errored out if test
were not a directory or did not exist.
The next available inode is #6.
% cat me
me me me
% cd ..
% cd test
% cat me
me me me
% cd
% lsr /
/:
1 5 .
1 5 ..
4 11 baz
2 14 foo
5 3 test/
/test:
5 3 .
1 5 ..
6 8 me
Recursive directory listing. This is
done using a preorder traversal. Withing a given level, lexicographic ordering is used. Recursion will go through
all subdirectories at all levels.
% cd test
% mkdir foo
% cd foo
% mkdir bar
% cd bar
% mkdir baz
% cd baz
Note that foo uses inode #7, bar uses
inode #8, and baz uses inode #9.
% ls .
.:
9 2 .
8 3 ..
At this point dot is baz and dotdot is
bar.
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 6 of 7
% cd /
% lsr test
/test:
5 4 .
1 5 ..
7 3 foo/
6 8 me
/test/foo:
7 3 .
5 4 ..
8 3 bar/
/test/foo/bar:
8 3 .
7 3 ..
9 2 baz/
/test/foo/bar/baz:
9 2 .
8 3 ..
A rather large test showing inode numbers, file and directory sizes, and filenames. Note that directory names are
indicated in the listing with a trailing
slash. Again, the size of a file is the
number of characters in it and the size
of a directory is the sum of the number
of files The subdirectory count is not
recursive.
% ^D End of file or Control/D causes the
shell to exit.
4. A Tour of the Code
Begin by studying the code provided in the code/ subdirectory. There are four modules arranged into a header (.h) file and an implementation (.cpp) file, the main program in main.cpp, and, of course, a Makefile. Notice that comments are in the
header for when specifying general functionality, and only in the implementation as
a way of explaining how something works.
Makefile Study the various targets all, ${EXECBIN}, %.o, ci, lis, clean,
spotless, submit, verify, and deps, which perform their usual
functions.
debug.{h,cpp} The debug module is already written for you. It is useful in
tracing through your code. Other parts of the code may want
to have more DEBUGF and DEBUGS calls added. Note that you
should also use gdb to track down bugs. Use valgrind to check
for invalid memory references and memory leak.
util.{h,cpp} The util module is just a collection of independent functions
which are herded together. It is a module without any cohesion, but a useful place to park various random functions.
main.cpp The main program is mostly complete. It scans options, then
loops reading and executing commands.
commands.{h,cpp} Note that the functions are provided but do not do more than
print a trace. Each function as a inode_state argument,
passed by reference, which it might update, and a wordvec arlgument. words[0] is the name of the command itself, so the
first argument is words[1]. This will take the most work, but
commands can be added one at a time, by addition to the
CMPS-109 • Winter 2019 • Program 2 • Function pointers, a shell 7 of 7
organization that is already there. You may add private functions is you need to. This is the major execution engine.
file_sys.{h,cpp} The inode, or file system, module is the main data structure
that you are working on. As you implement commands, also
implement the functions of this module as well.
5. What to Submit
Submit the files Makefile, README, and all C++ header and source files. All header
files must end with .h, and source files must end with .cpp as the suffix.
Run gmake to verify that the build is possible. And when you run submit, do so from
the Makefile target of that name. That way you won’t forget to submit a file. If you
forget to submit a file, or submit the wrong version, you will lose at least 50% of
your program’s value. Make sure that you do not get any warnings from g++ and
that checksource and cpplint.py.perl dot not complain about anything. Be sure
that valgrind shows no uninitialized variables and no memory leak. Do not submit
any file that is built by the Makefile.
If you are doing pair programming, follow the additional instructions in :
/afs/cats.ucsc.edu/courses/cmps109-wm/Syllabus/pair-programming
Note that points will be deducted for an improperly formatted PARTNER file.
The code must compile and run using g++ on unix.ucsc.edu, regardless of whether it
runs elsewhere. When this document was formatted (January 18, 2019) that was :
bash-1$ which g++
/opt/rh/devtoolset-7/root/usr/bin/g++
bash-2$ g++ –version | head -1
g++ (GCC) 7.3.1 20180303 (Red Hat 7.3.1-5)
bash-3$ uname -srp
Linux 3.10.0-957.1.3.el7.x86_64 x86_64
bash-4$ hostname
unix2.lt.ucsc.edu

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