mysh: Linux Command Line Implementation

This is a custom implementation of the Linux Command Line called mysh.

Table of Contents

• Makefile Commands
• Overview
• Important Notes
• Implementation Details
  • job.c
  • builtins.c
  • mysh.c
• Tests
  • Test Interactive Mode
  • Test Batch Mode

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Makefile Commands

Command	Description
make mysh	Creates the executable file mysh. Run it using ./mysh.
make testJobMaking	Tests the job creation logic. See the Interactive Test section.
make batchmodetest	Compiles dependencies and runs test.sh in batch mode.
make removeTXTFiles	Cleans up .txt files generated during batch mode testing.
make clean	Removes all .o, mysh executable, and .dSYM files.

Note on Naming: If make fails, ensure the file is named exactly Makefile or MakeFile as per your system's case sensitivity.

Debugging: Edit the Makefile and change line 2 to DEBUG = -D DEBUG=1 to enable verbose debug information. Use the -B flag with any command to force a rebuild.

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Overview

• Interactive Mode: Used to run one command at a time via user input.
• Batch Mode: Used to run a sequence of commands from a file until the exit command or EOF (End of File) is reached.

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Important Notes

• Buffered Commands (e.g., less): On ilab, scrolling past the EOF in less may cause random characters to be printed to mysh after quitting. This occurs because the characters are sent to the tty while mysh is still in exec().
• Quotes: mysh does not handle quotes; they are treated as literal characters.
• Escaped Spaces: mysh does not support spaces in file/directory names (e.g., \ ).
• Conditionals: * In a pipe, the exit status depends on the last command in the pipe.
  • In a single command, it depends on that command's status.
• Exiting: Any call to exit (with or without arguments/pipes) will close mysh. However, the string "exit" used as a standard argument (e.g., echo exit) will not trigger a shutdown.
• Whitespace: All whitespace is ignored, including gaps between pipe symbols. |ls|echo| is treated the same as | ls | echo |.
• Constraints: Only single pipes are supported (no multi-pipe chains). Tab completion and up-arrow history are not implemented.

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Implementation Details

job.c

Represents the Job Abstract Data Type.

• execPath:
  • Builtins: The name of the builtin.
  • Non-builtins: Full path located via /usr/local/bin, /usr/bin, or /bin.
  • If the input contains a /, it is treated as a direct path.
• args: Array of strings expanded using wildcards (*). Wildcards match zero or more characters and must appear at the end of a path.
• Redirection: Stores paths for inputReDirectPath (<) and outputReDirectPath (>).

builtins.c

Handles core shell commands:

• cd: Changes working directory via chdir(). Expects exactly one argument.
• which: Locates the path of a program. Checks builtins first, then system paths.
• pwd: Prints current working directory using getcwd().

mysh.c

The main program launcher.

• Manages mysh_errno to track exit statuses for then/else logic.
• Execution Flow:
  1. Trim whitespace and split commands by pipes.
  2. Create Job objects for each sub-command.
  3. Single Job: If builtin, runs in-process with dup2 redirection. If non-builtin, uses fork(), dup2, and exec().
  4. Piped Jobs: Sets up communication using pipe() and dup2().

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Tests

Test Interactive Mode

Used to verify that mysh parses input, expansions, and redirections correctly.

Example Test Cases:

1. echo man name > txt.txt: Tests output redirection and bare name expansion.
2. cat < txt.txt: Tests input redirection.
3. ls *.c: Tests wildcard expansion for files in the current directory.
4. ls testFolder2/*.txt > txt.txt: Tests wildcards within a specific path.

Test Batch Mode

Run this using make batchmodetest. This suite covers:

• Wildcards: Including scenarios with no matching files (returns original string).
• Redirection: Input, output, and combined (e.g., ./sum < in > out).
• Pipes: Mixing system functions, builtins, and redirection within pipes.
• Conditionals: Verifying that then and else blocks execute correctly based on the success of the preceding command.