README.md

alsh

A custom UNIX shell written in C

Features

• Execute commands (e.g. ls)
• Execute commands with arguments and flags (e.g. ls -la /)
• Execute commands with redirection < > >>
  • Output from a given file descriptor can be redirected or appended to a file by using n> or n>> respectively, where n is the file descriptor number
• Execute commands with pipes |
• Execute commands in the background by appending & at the end of the command
• Execute multiple commands separated by ; on the same line
  • Given the statement cmd1; cmd2, cmd1 and cmd2 are executed sequentially
• Evaluate math expressions by surrounding them with (), such as (1 + 1) and (1 + 2 * (3 + 4))
  • Supports +, -, *, /, and grouping expressions using ( and )
• Other operators
  • &&: Given the statement cmd1 && cmd2, cmd2 is executed if and only if cmd1 returns an exit status of 0, which indicates success
  • ||: Given the statement cmd1 || cmd2, cmd2 is executed if and only if cmd1 returns a non-zero exit status, which indicates failure
• View command history with history and execute previous commands with !n, where n is the command number in the history list (e.g. !3 will execute the third command in the history list)
  • To execute the previous command, use !!
  • To execute the command n lines back in the history list, use !-n (e.g. !-2 will execute the command 2 lines back in the history list)
  • To clear the history list, use history -c
  • To write the history list to a file, use history -w, which will write the list to ~/.alsh_history
• Create custom aliases for commands by using alias alias_name=command, where alias_name is the alias name and command is the command to execute when the alias is typed as a command
  • Multiple aliases can be stored at once by using alias alias_name_1=command_1 alias_name_2=command_2 alias_name_3=command_3 ...
  • To list all stored aliases for the current shell session, simply type alias without any arguments
• Create environment variables and shell variables by using the export and let keywords respectively
  • export env_var_name=env_var_value
  • let var_name=var_value
  • Multiple variables can be created at once by using <export|let> var_name_1=var_value_1 var_name_2=var_value_2 var_name_3=var_value_3 ...
  • To use the value of a variable in a command, prefix it with $, like echo $var_name
• Replace the current alsh shell's process with a new process by using exec [command]
  • Running exec without specifying a command will replace the current alsh shell's process with a new instance of another alsh shell
• repeat (n) <command> will execute the given command n times
  • Multiple repeat loops can be chained together (e.g. repeat (n) repeat (m) <command> will execute the given command m * n times)
• Execute commands from a file in the current alsh shell session by using source <fileName>
• if (<commandToTest>) <command> will only execute the given command if commandToTest returns an exit status of 0, which indicates success
  • if (<commandToTest>) <command1> else <command2> will execute the first command if commandToTest returns an exit status of 0, and the second command otherwise
  • if (<commandToTest>) <command1> else if (<commandToTest2>) <command2> else <command3> will execute the first command if commandToTest returns an exit status of 0, which indicates success, the second command if commandToTest returns a non-zero exit status and commandToTest2 returns an exit status of 0, and the third command otherwise
  • The command to test can be negated by using the - operator (e.g. if (-<commandToTest>) <command> will only execute the given command if commandToTest returns a non-zero exit status, which indicates failure)
    • Negated commands can also be negated themselves, so if (--<commandToTest>) <command> is equivalent to if (<commandToTest>) <command>
    • An odd number of - operators will negate the command, and an even number of - operators will not negate the command
• while (<commandToTest>) <command> will repeatedly execute the given command as long as commandToTest returns an exit status of 0
• If the [ command is available on the system, then it doesn't need to be surrounded with parentheses in if and while statements
  • Example: if [ 1 -eq 1 ] <command>
• Compare numerical values by using chk <num1> <cond> <num2>, where num1 and num2 are the first and second numerical values to compare respectively, and cond is the test condition to use on num1 and num2
  • Valid test conditions for cond are the following: eq, ne, lt, le, gt, ge, which stand for equals, not equals, less than, less than or equal to, greater than, and greater than or equal to respectively
  • This can be useful if the [ command is not available on the system
• If .alshrc is present in the home directory, then it will be executed at the start of any interactive alsh shell session

Installation

git clone https://github.com/AlanLuu/alsh.git
cd alsh
make
sudo make install

This will install alsh as alsh in /usr/local/bin.

To uninstall alsh, run the following command in the alsh directory:

sudo make uninstall

Testing

Test cases are defined in tests.json.

• Cases are defined in the format "test_command": "output"|null, where test_command is the command to be tested and output is the expected output of the command.
  • If null is specified in place of output, then the test command will be executed using the system shell and the output of that will be used as the expected output.

To run tests, run the following command (requires Python):

make test

License

alsh is distributed under the terms of the MIT License.