Shellby - Simple Shell 🐚 A simple UNIX command interpreter written as part of the low-level programming and algorithm track at Holberton School.
Description 💬 Shellby is a simple UNIX command language interpreter that reads commands from either a file or standard input and executes them.
Invocation 🏃 Usage: shellby [filename]
To invoke shellby, compile all .c files in the repository and run the resulting executable:
gcc *.c -o shellby ./shellby Shellby can be invoked both interactively and non-interactively. If shellby is invoked with standard input not connected to a terminal, it reads and executes received commands in order.
Example:
$ echo "echo 'hello'" | ./shellby 'hello' $ If shellby is invoked with standard input connected to a terminal (determined by isatty(3)), an interactive shell is opened. When executing interactively, shellby displays the prompt $ when it is ready to read a command.
Example:
Example:
$ cat test echo 'hello' $ ./shellby test 'hello' $ Environment 🌳 Upon invocation, shellby receives and copies the environment of the parent process in which it was executed. This environment is an array of name-value strings describing variables in the format NAME=VALUE. A few key environmental variables are:
HOME The home directory of the current user and the default directory argument for the cd builtin command.
$ echo "echo $HOME" | ./shellby /home/vagrant PWD The current working directory as set by the cd command.
$ echo "echo $PWD" | ./shellby /home/vagrant/holberton/simple_shell OLDPWD The previous working directory as set by the cd command.
$ echo "echo $OLDPWD" | ./shellby /home/vagrant/holberton/printf PATH A colon-separated list of directories in which the shell looks for commands. A null directory name in the path (represented by any of two adjacent colons, an initial colon, or a trailing colon) indicates the current directory.
$ echo "echo $PATH" | ./shellby /home/vagrant/.cargo/bin:/home/vagrant/.local/bin:/home/vagrant/.rbenv/plugins/ruby-build/bin:/home/vagrant/.rbenv/shims:/home/vagrant/.rbenv/bin:/home/vagrant/.nvm/versions/node/v10.15.3/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/vagrant/.cargo/bin:/home/vagrant/workflow:/home/vagrant/.local/bin Command Execution 🔪 After receiving a command, shellby tokenizes it into words using " " as a delimiter. The first word is considered the command and all remaining words are considered arguments to that command. Shellby then proceeds with the following actions:
If the first character of the command is neither a slash () nor dot (.), the shell searches for it in the list of shell builtins. If there exists a builtin by that name, the builtin is invoked. If the first character of the command is none of a slash (), dot (.), nor builtin, shellby searches each element of the PATH environmental variable for a directory containing an executable file by that name. If the first character of the command is a slash () or dot (.) or either of the above searches was successful, the shell executes the named program with any remaining given arguments in a separate execution environment. Exit Status 👋 Shellby returns the exit status of the last command executed, with zero indicating success and non-zero indicating failure.
If a command is not found, the return status is 127; if a command is found but is not executable, the return status is 126.
All builtins return zero on success and one or two on incorrect usage (indicated by a corresponding error message).
Signals ❗ While running in interactive mode, shellby ignores the keyboard input Ctrl+c. Alternatively, an input of end-of-file (Ctrl+d) will exit the program.
User hits Ctrl+d in the third line.
$ ./shellby $ ^C $ ^C $ Variable Replacement 💲 Shellby interprets the $ character for variable replacement.
$ENV_VARIABLE ENV_VARIABLE is substituted with its value.
Example:
$ echo "echo
Example:
$ echo "echo
Example:
$ echo "echo $$" | ./shellby 6494 Comments #️⃣ Shellby ignores all words and characters preceeded by a # character on a line.
Example:
$ echo "echo 'hello' #this will be ignored!" | ./shellby 'hello' Operators 🎸 Shellby specially interprets the following operator characters:
; - Command separator Commands separated by a ; are executed sequentially.
Example:
$ echo "echo 'hello' ; echo 'world'" | ./shellby 'hello' 'world' && - AND logical operator command1 && command2: command2 is executed if, and only if, command1 returns an exit status of zero.
Example:
$ echo "error! && echo 'hello'" | ./shellby ./shellby: 1: error!: not found $ echo "echo 'all good' && echo 'hello'" | ./shellby 'all good' 'hello' || - OR logical operator command1 || command2: command2 is executed if, and only if, command1 returns a non-zero exit status.
Example:
$ echo "error! || echo 'but still runs'" | ./shellby ./shellby: 1: error!: not found 'but still runs' The operators && and || have equal precedence, followed by ;.
Shellby Builtin Commands 🔩 cd Usage: cd [DIRECTORY] Changes the current directory of the process to DIRECTORY. If no argument is given, the command is interpreted as cd $HOME. If the argument - is given, the command is interpreted as cd $OLDPWD and the pathname of the new working directory is printed to standad output. If the argument, -- is given, the command is interpreted as cd $OLDPWD but the pathname of the new working directory is not printed. The environment variables PWD and OLDPWD are updated after a change of directory. Example:
$ ./shellby $ pwd /home/vagrant/holberton/simple_shell $ cd ../ $ pwd /home/vagrant/holberton $ cd - $ pwd /home/vagrant/holberton/simple_shell alias Usage: alias [NAME[='VALUE'] ...] Handles aliases. alias: Prints a list of all aliases, one per line, in the form NAME='VALUE'. alias NAME [NAME2 ...]: Prints the aliases NAME, NAME2, etc. one per line, in the form NAME='VALUE'. alias NAME='VALUE' [...]: Defines an alias for each NAME whose VALUE is given. If name is already an alias, its value is replaced with VALUE. Example:
$ ./shellby $ alias show=ls $ show AUTHORS builtins_help_2.c errors.c linkedlist.c shell.h test README.md env_builtins.c getline.c locate.c shellby alias_builtins.c environ.c helper.c main.c split.c builtin.c err_msgs1.c helpers_2.c man_1_simple_shell str_funcs1.c builtins_help_1.c err_msgs2.c input_helpers.c proc_file_comm.c str_funcs2.c exit Usage: exit [STATUS] Exits the shell. The STATUS argument is the integer used to exit the shell. If no argument is given, the command is interpreted as exit 0. Example:
$ ./shellby $ exit env Usage: env Prints the current environment. Example:
$ ./shellby $ env NVM_DIR=/home/vagrant/.nvm ... setenv Usage: setenv [VARIABLE] [VALUE] Initializes a new environment variable, or modifies an existing one. Upon failure, prints a message to stderr. Example:
$ ./shellby $ setenv NAME Poppy $ echo $NAME Poppy unsetenv Usage: unsetenv [VARIABLE] Removes an environmental variable. Upon failure, prints a message to stderr. Example:
$ ./shellby $ setenv NAME Poppy $ unsetenv NAME $ echo $NAME
$ Authors ✒️ Alex Yu Brennan D Baraban License 🔒 This project is licensed under the MIT License - see the LICENSE file for details.
Acknowledgements 🙏 Shellby emulates basic functionality of the sh shell. This README borrows form the Linux man pages sh(1) and dash(1).
This project was written as part of the curriculum for Holberton School. Holberton School is a campus-based full-stack software engineering program that prepares students for careers in the tech industry using project-based peer learning. For more information, visit this link.