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INFO1112 - Assignment 1 – mysh
Your task is to implement your own, Unix shell!
Ultimately, your shell should support the following features:
a custom initialisation file, .myshrc , which is formatted as JSON.
run and execute any program on the system's PATH
define your own custom commands using JSON files
support additional quality-of-life features to mimic other Unix shells
Without further ado, let's break down the main components on how the shell works!
Starting and Running the Shell
Starting the Shell
Starting the shell is simply done by running:
python3 mysh.py
The shell expects no additional arguments, and additional arguments are silently ignored.
Running the Shell
When the shell is running, you will see the prompt that has been set from the PROMPT environment variable, and the user will be able to type in commands to be executed:
>>
Effectively, your shell will run infinitely (in an infinite loop), until it is terminated, either by EOF (e.g. Ctrl + D), or if the exit built-in command is run (see here for more details on this). You can think of the shell as an "event-driven" program, where it is waiting for a command input (which will be the event), which it then reacts to.
While the prompt is being displayed, there are a couple of extra things to keep in mind:
If the user enters Ctrl + C, but no command is running (i.e. the prompt is being displayed), Ctrl + C should be silently ignored, and a new prompt should be displayed.
If the user inputs a blank command (an empty line, with or without whitespace), the shell should simply display a new prompt.
Note
If your terminal is terminated by EOF, make sure to print an extra newline character! (Thank you to the students who helped raise this to be compatible with Ed test cases, as this was initially overlooked!)
Syntax and Execution
Splitting a Line Into Arguments
Like other Unix shells, mysh is a line-based command parser. A user inputs one line at a time, and the line is executed as a command.
When we input one line of input, the line is split by whitespace, spaces, tabs, etc. (with an exception, more on this below), with each "word" forming an "argument". For example, the line:
hello there how are you
can be split into a list of arguments (using Python syntax) as:
["hello", "there", "how", "are", "you"]
There is one exception to this: quoted strings (both single and double quotes)! The outer level of quotes are stripped from the quoted string. If a string is inside some quotes, and it includes whitespace, the whitespace is preserved.
For example, the below line:
hello there "how are you" 'going today'
will be split as:
["hello", "there", "how are you", "going today"]
while the below line:
hello there 'my name is "Sarah"'
will be split as:
["hello", "there", 'my name is "Sarah"']
Quotes can also be escaped by typing a backslash (\) before the quote character. This means that if you have quoted argument of single or double quotes, and type in the same type of quote character prepended with a backslash (e.g. \" or \' ), it won't be interpreted as a closing quote, but instead, as a quote character – similar to the behaviour of Python.
For example, the below line:
my "\"string\"" which 'is \'quoted\''
will be split as:
["my", '"string"', "which", "is 'quoted'"]
Tip
The shlex module will be incredibly useful to help parse these strings, and will meet the edge cases below! Please use it!
Edge cases
It is valid to have double quotes inside single quotes (as in the above example), and single quotes inside double quotes.
It is valid for quotes to be spliced inside of arguments, and can even be next to each other, as long as the outer layer of quotes is stripped out! For example, the line below:
hello there "quotes are""next to 'each'"-other
should be split as:
["hello", "there", "quotes arenext to 'each'-other"]
Error cases
An unterminated quoted argument should be considered a syntax error by your program and should output an error message to stderr , formatted below. For example, for the line:
line with "unterminated string
the program should output:
mysh: syntax error: unterminated quote
to stderr .
Note that unterminated quote characters are valid if they are inside quotes of a different type. For example, the line:
this is "Tom's PC"
can be validly split as:
["this", "is", "Tom's PC"]
Executing Commands
When a line is split, the first "word" is always defined as the name of the command to execute. For example, for the line:
sort input.txt -o output.txt
will be parsed as:
["sort", "input.txt", "-o", "output.txt"]
with "sort" being the name of the command (or program) we wish to execute, and ["input.txt", "-o", "output.txt"] being the arguments passed to the "sort" program.
The command name will be attempted to be matched to a built-in command (see Built-in Commands for all built-in commands), otherwise, it will execute an executable on the system with this name, either by checking in the system's PATH for a matching executable name, or by executing the executable given by an absolute or relative path. For instance:
./my_compiled_prog
would execute an executable my_compiled_prog in the current directory of the user.
If a valid command is successfully entered, the shell will then execute the command, and wait for it to complete, before displaying the prompt again, and asking the user to input another command.
Tip
The info you learn about fork and exec in Week 3 will be very useful for helping to implement the ability to run programs in your shell! You can also find implementations of these system calls as functions in the Python os module.
Important
If a command is run in a separate process, it is important to set the process group of the child process to a brand new one, and set the new foreground terminal process group to the new group the child process is in. This is so functionality, such as pressing Ctrl + C, only affects the child, rather than the parent process. In short, the logic should look something like this in the parent process:
1. After forking, create a new process group for the child process with os.setpgid(child_pid, child_pid) in the parent process.
2. Get the child's new process group ID with os.getpgid(child_pid) .
3. Open the current terminal device, located in /dev/tty to get its file descriptor.
4. Set the terminal foreground process group with os.tcsetpgrp(, child_pgid) .
5. Wait for the child process to complete.
6. Restore the terminal foreground process group back to the parent's process group with os.tcsetpgrp(, parent_pgid) .
7. Make sure the /dev/tty file descriptor is closed as well!
Useful functions:
os.setpgid
os.getpgid
os.tcsetpgrp
os.getpgrp
Error cases
If the first argument does not contain a slash and the argument does not refer to a built-in command or executable on the PATH , the shell should output:
mysh: command not found:
to stderr .
If the first argument contains a slash anywhere within it (indicating it is a relative or absolute path), and it does not refer to a valid file or directory, the shell should output:
mysh: no such file or directory:
to stderr .
If the first argument contains a slash and is a valid path, but cannot be executed due to it being a directory, the shell should output:
mysh: is a directory:
to stderr .
If the first argument contains a slash and is a valid path, but the user cannot execute the file due to lacking appropriate executable permissions, the shell should output:
mysh: permission denied:
to stderr .
Pipes
Like other Unix shells, one of the features that mysh will support is piping the stdout result of one command as stdin input for another command!
In POSIX shells, such as Bash, commands are piped from one to another using the pipe operator ( | ). For instance, in the line below:
echo "Hello!" | cat
both echo and cat are immediately started by the shell. echo prints Hello! to what normally would be stdout , however, instead, the shell captures this output. Meanwhile, cat is also waiting for input to come from what normally would be stdin , however, the shell will instead feed it the output of echo . The result is that after echo prints Hello! , the shell captures this output, and passes it along to cat as if it was from stdin , which it then reads, and thus prints Hello! to the terminal screen.
In mysh, pipes operate in the same way. The syntax for pipes is below:
| | ... |
Each pipe operator ( | ) is placed between 2 commands with arguments, with the stdout output of the previous command being captured and being fed as stdin to the next command. All commands as part of a sequence of pipes (also known as a pipeline) start simultaneously, and each command waits for input to read (as if it was from stdin ) from the previous command.
For example, in the line:
a b c | d e|f
the command a is run with arguments b c , where its stdout output is passed to stdin to command d (run with arguments e ), and in turn, d 's stdout output is passed as stdin to command f .
However, if the pipe operator is in single or double quotes, it is not interpreted as a shell pipe, but rather as a literal '|' character instead.
For instance, in the line:
a | b 'c | d'
we interpret this as passing the stdout of command a to the stdin of command b , which was invoked with the single argument c | d (just a string).
Tip
The scaffold for the assignment contains a module, parsing , which contains a function, split_by_pipe_op which you can use to split strings by an unquoted pipe operator!
Tip
The os.pipe , as well as the os.dup2 / os.dup functions will be incredibly useful here!
Important
Each process which is executed as part of a pipeline must be part of the same process group while executing, with this process group being set as the foreground terminal process group, so that if the user wants to terminate execution of the pipeline early (e.g. by pressing Ctrl + C), all processes in the pipeline will terminate. See the Executing commands section for more details about setting this.
Error cases
If a line contains a trailing pipe operator without a command after it, for example:
a | b |
the shell should output:
mysh: syntax error: expected command after pipe
to stderr .
If a line contains multiple pipes, but there is no command between 2 pipes, for example:
a | b | | c
this should output the same syntax error to stderr as above.
Using Shell Variables
Just like other Unix shells, mysh supports defining your own variables, which are saved into the process's environment variables, and can be used as part of the shell prompt.
Defining your own variables can be done via the var command, and usage of this command can be found its corresponding section, so let's talk about how we can use defined shell variables first.
Shell variables may be used as part of some command line input with the ${} syntax. If an argument contains this syntax, it should be replaced with the value of the environment variable matching
${} .
For example, if we have an environment variable greeting with the value Hello, world! , and the user enters the line:
echo ${greeting}
${greeting} should be substituted with its value, with the line being interpreted as:
echo Hello, world!
However, the user may choose to "escape" this syntax with a backslash ('\'), as they may not want to have this interpreted as a string. To "escape" the variable usage syntax, the user writes a backslash before the $ symbol, i.e. \${greeting} . This means that for the line:
echo \${greeting}
the line should instead be interpreted as:
echo ${greeting}
i.e. calling the echo command, with the literal string ${greeting} as a single argument.
If the variable is used inside a quoted argument (either single or double quotes), substitution should occur as normal. For example, if the name environment variable had the value Rosanna , the line:
echo "My name is ${name}"
would be interpreted as:
echo "My name is Rosanna"
The same goes for if an argument is inside single quotes single quotes - for instance, the line:
echo 'My name is ${name}'
would be interpreted as:
echo 'My name is Rosanna'
Error cases
If contains characters which are invalid for a variable name (see var ), a syntax error should be printed to stderr in the form:
mysh: syntax error: invalid characters for variable
The command which contained the invalid variable name should also not be executed.
If there are multiple variables with invalid characters, mysh will only raise an error on the first invalid variable.
If there is no environment variable named , the string ${} will be substituted (if applicable) by an empty string. For instance, if no variable name exists, the following line:
echo "My name is ${name}"
will be parsed as:
echo "My name is "
Built-in Commands
As part of mysh, some commands are defined as "built-in" commands, that is to say, that the commands do not necessarily run a separate executable, but instead, is executed as part of some built-in functionality for the shell.
var
The var built-in command is used to set the value of environment variables within mysh. The syntax for defining this command is:
var [-s]
where is the name of the shell variable, which can be:
any alphabetical letters A-Z, capital or lowercase
any digits 0-9
underscores (_)
and represent the value of the shell variable.
The -s flag is optional – see the Setting the result of a command subsection for more details. In this case, more than 1 argument may be provided.
Important
All shell variables are interpreted as strings, as these must be stored in the process's environment, which only supports string environment variables. See os.environ for more info.
In basic usage, if the user were to input:
var count 0
the shell would create a new environment variable count , with its value set to 0 .
If the user were to input:
var greeting "Hello, world!"
the shell would create a new environment variable greeting , with its value set to Hello, world! .
If var is successfully called, there should be no additional output created.
Setting the result of a command
The var command also accepts one possible optional flag – -s . This flag allows you to set a variable to a value that is the stdout result of a command, rather than a string value.
If the -s flag is specified, the syntax for var will look like the below:
var -s
is a single argument which represents the execution of a command as if it was to be executed by the shell. This may be just a command name (e.g. ls ), a command with arguments (e.g. "ls -l ~" ), or a pipeline (e.g. 'ls -1 ~ | wc -l' ) – the latter 2 options being entered as a quoted argument to be passed as a single argument.
For example, if the user was to input:
var -s add_res "expr 2 - 5"
the shell should store the result of executing the command expr 2 - 5 into the environment variable add_res .
Similarly, if the user was to input:
var -s usyd_info_units 'sort usyd-scs-units.csv | grep "INFO"'
the shell should store the result of executing sort usyd-scs-units | grep "INFO" into the environment variable usyd_info_units .
Edge cases
Flag arguments ( -s , as well as any invalid flags) are only interpreted as flags if they are the first argument passed to var . If they are in any other position, they are interpreted as a regular argument to var (following the syntax requirements above).
Error cases
If the user enters any flag arguments which are not -s , the shell should output the following to stderr :
var: invalid option: -
For instance, if the user enters var -t test_var test_value , the shell should output:
var: invalid option: -t
If multiple options are specified, such as var -tu , the first invalid option found should be outputted.
If the user does not use the -s flag, and inputs more than 2 arguments, the shell should print an error to stderr , saying:
var: expected 2 arguments, got
If the the user enters does NOT contain valid characters (ones other than those specified), the shell should print an error to stderr , saying:
var: invalid characters for variable
pwd
Like the pwd command found in most Unix shells, the pwd built-in command should print the current working directory as an absolute path that the shell is located in to stdout .
The user may optionally pass one flag: -P , which, if passed, will resolve additionally resolve all symbolic links in the shell's current working directory to their real path before printing this as an absolute path to stdout .
Example usage
If the user was in their home directory (e.g. /home/bob ), the following:
pwd
should output
/home/bob
If the user was in the directory /home/bob/current-year , where /home/bob/current-year was a symbolic link to /home/bob/2024 , the following:
pwd
should output
/home/bob/current-year
and the following
pwd -P
should output
/home/bob/2024
Error cases
If the user enters any additional non-flag arguments (e.g. arguments which do not start with a - ), the shell should output the following to stderr :
pwd: not expecting any arguments
If the user enters any flag arguments which are not -P , the shell should output the following to stderr :
pwd: invalid option: -
For instance, if the user enters pwd -f , the shell should output:
pwd: invalid option: -f
If multiple options are specified, such as with pwd -f -p or pwd -fp , the first invalid option found should be outputted.
cd
The cd command will change the current working directory to the path specified by a single argument, and modify the contents of the environment variable PWD to reflect the current absolute path after the directory has been changed (i.e. what the output of running pwd by itself would be).
The syntax to execute it is:
cd
may either be an absolute or relative path.
There is a special case where the used can execute cd with no arguments. In this case, the shell should change the current working directory (and update PWD accordingly) to the home directory of the user.
Error cases
If the user inputs more than one argument to cd , the shell should output the following error to stderr :
cd: too many arguments
If the the provided as an argument is invalid (does not exist), the shell should output the following error to stderr :
cd: no such file or directory:
If the provided is not a directory, the shell should output the following error to stderr :
cd: not a directory:
If the user does not have the correct permissions to change directory to , the shell should output the following error to stderr :
cd: permission denied:
which
The which command identifies whether a command refers to a built-in command in the shell, or otherwise provides the absolute path of the first executable matching the command's name in the system's PATH variable.
Important
If the PATH environment variable is unset for any reason, which falls back to the path set in os.defpath .
The syntax defining it is:
which
where
If an argument is a built-in command, which should output (to stdout ):
: shell built-in command
Otherwise, if the command is on the system PATH , and it is a file which has executable permissions, which will output the first matching absolute path to this command. For example, which echo might output (to stdout ):
/bin/echo
If no matching built-in command or executable is found for the argument, which should output the following to stdout :
not found
If
cd: shell built-in command
/bin/ls
foo-bar not found
Warning
Since which is a built-in command in our shell, your program should not invoke the system-installed which command (e.g. /usr/bin/which ) when it is executed! This will be double-checked with automatic test cases and manual marking.
Error cases
If no arguments are entered, the shell should output the following to stderr :
usage: which command ...
exit
A nice, simple built-in command. When executed, the shell exits.
Its syntax is:
exit []
Exit optionally takes one additional additional argument, , which is the integer exit code the shell should exit with.
Error cases
If there is more than 1 argument provided to exit , the shell should output the following to stderr :
exit: too many arguments
and not terminate the shell.
If a non-integer argument is passed to exit , the shell should output the following to stderr :
exit: non-integer exit code provided:
and not terminate the shell.
Configuration and Initialisation
Default Environment Variables
Many Unix shells will use environment variables to modify how they operate. For instance, one common environment variable used by shells is PS1 , which dictates the shell prompt.
Tip
You can test the above in your shell! Try running PS1='My Prompt >> ' , and watch your prompt change in real time!
These environment variables are often inherited by a child process from its parent when its starts, and are set per process, and all of the process's children.
Just like other Unix shells, mysh also has some environment variables which it uses to affect how its process runs. If these are undefined when mysh is launched, they are set to the default values defined below:
Important
These should be set in the actual process environment when your program is launched. Refer to os.environ for more details.
PROMPT : >>
MYSH_VERSION : 1.0
Initialisation File
Unix shells often use an initialisation file which can be used to customise some environment variables that change how the shell, or other programs, run. The name of this initialisation file, and the syntax it uses depends on the shell being used – for example, Bash will use .bashrc , and Zsh .zshrc .
Just like other Unix shells, our shell, mysh, also has an initialisation file – called .myshrc , structured as a JSON object. By default, this file lives in the home directory of the user – ~/.myshrc , unless the MYSHDOTDIR environment variable is set prior to mysh being started, which will cause mysh to look for the file in ${MYSHDOTDIR}/.myshrc . The user can use this file to set or override any previously-set environment variables for the current process, with the key of the JSON object being the environment variable's name, and the value being the environment variable's value.
Each environment variable's corresponding value may also reference other environment variables, using the same syntax defined in the Using Shell Variables section. Variables are initialised from top to bottom in the file, meaning that if a variable in .myshrc references a variable which is defined later in the file, it should be interpreted as if the variable did not exist.
Warning
Remember that strings are the only valid data type for environment variable values.
Below is a sample of a .myshrc file:
{
"PROMPT": "${USER}'s cool shell! > ",
"UNI_FILES": "~/Documents/Uni",
"CURRENT_SEM_FILES": "${UNI_FILES}/2024/S2"
}
Tip
The json Python standard library module will be incredibly helpful to help parse these files!
Error cases
An invalidly-formatted .myshrc file (that cannot be parsed by the json module), should result in the following error message being printed to stderr upon startup:
mysh: invalid JSON format for .myshrc
and all environment variable values in .myshrc ignored.
For every environment variable value in .myshrc which contains invalid characters (refer to var ), the following message should be printed to stderr :
mysh: .myshrc: : invalid characters for variable name
and its value not set in the process's environment variables. Other valid environment variables, however, should still be set.
For every environment variable value in .myshrc which is not a string, the following message should be printed to stderr :
mysh: .myshrc: : not a string
and its value not set in the process's environment variables. Other valid environment variables, however, should still be set.
Testing
As part of your submission, you are expected to create your own test cases which you should use to test your program with continually while you are in development. All files related to testing should reside in the tests/subfolder of your submission. Files/folders used for testing which are outside of tests/ may not be considered for testing marks.
The type of tests that should be created are input/output (I/O) tests, or end-to-end tests, which are designed to test the entire execution of your program given a set of inputs passed as stdin to your program, and comparing your program's stdout / stderr to some expected file. Generally, this is done by having a corresponding .in file containing only the exact lines of input you would enter into your program, and a .expected or .out file containing the expected output of your program. You don't strictly have to follow this exact structure, however - the most important thing is that end-to-end testing is performed.
You should also submit a testing script, tests/run_tests.sh , which will run all tests developed for your program automatically, without user intervention. The testing script should output whether your program passes/fails one of your corresponding test cases. As part of marking your testing submission, staff will run tests/run_tests.sh .
README/Report
As part of your submission, you are required to create a README.md file that acts as a report for your program that is 750 words or less in length. Your README file is required to answer the following questions, containing references source code lines or functions as appropriate to support your answers:
How does your shell translate a line of input that a user enters into a command which is executed in the command line?
What is the logic that your shell performs to find and substitute environment variables in user input? How does it handle the user escaping shell variables with a backslash (\) so that they are interpreted as literal strings?
How does your shell handle pipelines as part of its execution? What logic in your program allows one command to read another command's stdout output as stdin ?
Your README file should also contain a brief description on the categories/types of tests you've created, and how your test files are structured in the tests/ directory.
Allowed Modules
The following Python standard library modules are permitted to be used in your submission:
abc
collections.abc
dataclasses
json
os
os.path
pathlib
re
readline (note: not assessable, but useful for optional quality-of-life features!)
shlex
signal
sys
typing
Any other modules which are not on this list may not be used in your assignment, and will result in a mark deduction (20% of A1 max grade, or 4 marks) if they are used.
This list, however, may be expanded in future. If you would like to use a certain Python module, please ask on Ed, and staff will be happy to review your request, and if appropriate, update the list of allowed modules. A pinned post will be created which will track any updates to the list of allowed modules.
Restricted Functions
In addition, some functions are restricted, as they break the spirit of the assignment of making your own shell. Using these functions will also result in a mark deduction of 40% of A1 max grade (8 marks).
os.system – as this just passes a command string to the shell - we want to implement the shell ourselves!
os.popen – similar to above.
Any other function which spawns a sub-shell to execute a command. If you are unsure, please search/ask on Ed.
__import__ function to import a module – please use the standard import to import modules and/or functions.
Whilst falling under restricted modules, if the following modules are used, these will be subject to the 40% penalty mentioned above, as they also break the spirit of the assignment:
subprocess
multiprocess
cmd
any other module which interacts with processes/sub-shells to execute a program/command. If you are unsure, please search/ask on Ed.
Marking Breakdown
The mark breakdown for this assessment is as follows:
Automated test cases (13 marks) - Marks from automated test cases from Ed, which include public, hidden, and private test cases.
Different test cases will have different weightings (points) attached to them, visible on Ed.
Private test cases may be run after the due date.
Own test cases (2 marks)
Code style (2 marks) - broken down:
Automated code style checking with Pylint (1 marks) - feel free to run pylint on your code to check your code style yourself
The marks for this will follow the following scheme:
score ≥ 7.5/10 = 1/1 marks
5.5/10 ≤ score < 7.5/10 = 0.75/1 marks
2.5/10 ≤ score < 5.5/10 = 0.5/1 marks
1/10 ≤ score < 2.5/10 = 0.25/1 marks
score < 1 = 0/1 marks
Manual code style checking (1 marks)
README/Report (3 marks)