- Michael Savage Student ID: 17313526 MYSHELL -
||NAME||
Myshell - A simple command line interpreter that is
compatible with Python3 and newer..
||SYNOPSIS||
The shell has many uses. It is started by entering
python3 myshell.py
It is capable of
• Processing the internal commands (1-8 inclusive),
• Taking command line input from a batchfile* (9),
• Supporting i/o-redirection* on output (10),
• Executing programs in the background (11), and
• Executing child processes* (12)
All other command line input is interpreted as
program invocation*. The 'subprocess' module creates
and execs a child process* with the call() method.
This is only possible in python3 +.
To find out more about each command, press the space
bar to read more. Each command description includes
an example of usage.
There are supportive definitions at the end of the
help file too for some keywords I feel are important.
Definitions are available for words where you see
[keyword]*.
-
INTERNAL SHELL COMMANDS -
-
cd [directory]*
-
clr
-
dir [directory]*
-
environ
-
echo [argument]
-
help (or ?)
-
pause
-
quit (or 'q')
-
OTHER SHELL COMMANDS -
-
python3 myshell.py [batchfile]*
-
[command] arg1 arg2 (> OR >>) outputfile
-
[command] arg1 arg2 &
-
python3 test.py
||DESCRIPTION||
-
INTERNAL SHELL COMMAND DESCRIPTIONS -
====================
-
cd [directory]* ====================
• change the current working directory* to [directory]. • if no argument is given, then the CWD is used. • if [directory] = "..", then the CWD is changed to the parent directory*.
- Examples: -
-
cd .. - -
cd - -
cd /users/case2 -
====================
-
clr ====================
• clear the shell screen by printing "\033". This is the ASCII escape character. • additional arguments are ignored.
- Examples: -
-
clr - -
clr 1234552xagz -
====================
-
dir [directory]* ====================
• list the contents of directory* [directory]. • if no argument is given, then current directory objects are listed. • directory* contents can be redirected to an output file.
- Examples: -
-
dir - -
dir /users/case2/savagem7 - -
dir > gg - -
dir /users >> a.txt -
====================
-
environ ====================
• list all the environment strings as keys and values. • additional arguments are ignored. • the shell pathname is modified as "/myshell" is appended to the end of the name and it is conveniently bold. • environment strings can be redirected to an output file.
- Examples: -
-
environ - -
environ dw23rw - -
environ > hello - -
environ >> hello -
====================
-
echo [argument] ====================
• displays [arguments] on the display using print. • if no arguments are given, a message is displayed saying this. • [arguments] can be redirected* to an output file. • for convenience, if no arguments are given when being redirected* then an empty string is printed.
- Examples: -
-
echo hello world - -
echo - -
echo 1 + 2 + 3 + 4 + 5 - -
echo python! > gg -
====================
-
help ('?') ====================
• display the user manual using the more filter. • additional arguments are ignored. • help file can be redirected* to an output file.
- Examples: -
-
help - -
? - -
help ada3445 - -
? > helpfile -
====================
-
pause ====================
• pauses operation in the shell until 'Enter' is pressed. • additional arguments are ignored.
- Examples: -
-
pause - -
pause unpause -
====================
-
quit ('q') ====================
• quits the shell with a thank you message. • additional arguments are ignored.
- Examples: -
-
quit - -
q - -
quit 1234 -
-
OTHER SHELL COMMAND DESCRIPTIONS -
===============================
-
python3 myshell.py [batchfile]* ===============================
• as said in the synopsis, starting the shell is done by python3 myshell.py • the shell can take an additional argument at launch that will be a batchfile*. • the [batchfile]* is assumed to contain a set of command lines that the shell can process. • the line number and arguments are printed in bold, followed by execution of that line. • when the end-of-file is reached, the shell exits. • "file input error" is printed when the [batchfile]* is invalid or does not exist.
- in this example, the batchfile* is i.txt which -
- contains 2 lines of commands. -
- Examples: -
-
python3 myshell.py i.txt - -
- -
Line 1 contains: echo hello -
-
hello -
-
Line 2 contains: pause -
-
Shell paused.. Press 'Enter' to continue -
========================================
-
[command] arg1 arg2 (> OR >>) outputfile ========================================
• in this shell, i/o redirection* is possible on output. • redirection* means capturing output from a source such as a file, program or script and then sending it as input to another source. • in other words, the stdout that would be displayed on the terminal is now put into outputfile. • output redirection* is possible for the commands; - dir, - environ, - echo, and - help • output can be truncated* by using '>' or it can be appended with '>>'. • if the file does not exist, it will be created.
- in this example gg is created and environ is -
- appended to the same file. abc is created by -
- environ but then it is truncated* by echo. -
- the help command appends to newfile. -
- Examples: -
-
dir > gg - -
dir /users > newfile - -
- -
environ > abc - -
environ >> gg - -
- -
echo I am truncating abc > abc - -
echo > newfile - -
- -
help >> newfile - -
-
=====================
-
[command] arg1 arg2 & =====================
• the ampersand (&) at the end of the line indicates that background execution will take place. • subprocess.Popen() is used. New commands can be entered while the child process* executes.
- Examples: -
-
echo hello & - -
dir & -
==================
-
Program Invocation* ==================
• Any input that does not correspond to the previous (11) commands will be interpreted as an executable child process*. • subprocess.call() is used even though subprocess.run() is the newer, safer version. This is because of limitations.
- for the following example, I have made a python file -
- called test.py which includes the code: -
-
- -
i = 0 - -
while i < 4: - -
print("hello person {}".format(i)) - -
i += 1 -
- Examples: -
-
python3 test.py - -
- -
hello person 0 - -
hello person 1 - -
hello person 2 - -
hello person 3 -
-
Bringing All The Shell Commands Together -
-
python3 myshell.py - -
echo import time > x.py - -
echo time.sleep(10) >> x.py - -
python3 x.py & - -
echo hello - -
dir - -
echo environ > x.py - -
echo pause >> x.py - -
q - -
python3 myshell.py x.py - -
python3 myshell.py - -
help > x.py - -
cd .. - -
dir - -
clr - -
quit -
||DEFINITIONS||
-
batchfile • a computer file containing a list of instructions to be carried out in turn.
-
child process • created by a parent process. It is a subprocess that is a clone of the process that made it. It terminates when it has ended.
-
directory • a logical section of a file system used to hold files. Directories may also contain other directories.
-
invocation • means the execution of a program or function. Child process is executed by using the subprocess module. Subprocess.call() works in python3 or newer.
-
redirection • pointing data to a new output source. Myshell uses the terminal to display stdout. • echo 'hello' displays 'hello' on the terminal. • echo 'hello' > gg displays 'hello' in the file gg.
-
truncate • replace or overwrite data that is present in a file. This is possible with '>'. If gg had echo hello on the first line and we entered pause > gg, then echo hello is deleted and pause replaces it.