A simple UNIX command interpreter written as part of the low-level programming and algorithm track at Holberton School.
Shellby is a simple UNIX command language interpreter that reads commands from either a file or standard input and executes them.
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.
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.
Alternatively, if command line arguments are supplied upon invocation, shellby treats the first argument as a file from which to read commands. The supplied file should contain one command per line. Shellby runs each of the commands contained in the file in order before exiting.
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.
-
PWD The current working directory as set by the cd command.
-
OLDPWD The previous working directory as set by the cd command.
-
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.
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.
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).
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.
Shellby
7024
interprets the $ character for variable replacement.
-
$ENV_VARIABLEENV_VARIABLEis substituted with its value.
-
$??is substitued with the return value of the last program executed.
-
$$- The second
$is substitued with the current process ID.
- The second
Shellby ignores all words and characters preceeded by a # character on a line.
Shellby specially interprets the following operator characters:
-
;- Command separator- Commands separated by a
;are executed sequentially.
- Commands separated by a
-
&&- AND logical operatorcommand1 && command2:command2is executed if, and only if,command1returns an exit status of zero.
-
||- OR logical operatorcommand1 || command2:command2is executed if, and only if,command1returns a non-zero exit status.
The operators && and || have equal precedence, followed by ;.
-
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 ascd $OLDPWDand the pathname of the new working directory is printed to standad output. - If the argument,
--is given, the command is interpreted ascd $OLDPWDbut the pathname of the new working directory is not printed. - The environment variables
PWDandOLDPWDare updated after a change of directory.
- Usage:
-
alias
- Usage:
alias [NAME[='VALUE'] ...] - Handles aliases.
alias: Prints a list of all aliases, one per line, in the formNAME='VALUE'.alias NAME [NAME2 ...]: Prints the aliasesNAME,NAME2, etc. one per line, in the formNAME='VALUE'.alias NAME='VALUE' [...]: Defines an alias for eachNAMEwhoseVALUEis given. Ifnameis already an alias, its value is replaced withVALUE.
- Usage:
-
exit
- Usage:
exit [STATUS] - Exits the shell.
- The
STATUSargument is the integer used to exit the shell. - If no argument is given, the command is interpreted as
exit 0.
- Usage:
-
env
- Usage:
env - Prints the current environment.
- Usage:
-
setenv
- Usage:
setenv [VARIABLE] [VALUE] - Initializes a new environment variable, or modifies an existing one.
- Upon failure, prints a message to
stderr.
- Usage:
-
unsetenv
- Usage:
unsetenv [VARIABLE] - Removes an environmental variable.
- Upon failure, prints a message to
stderr.
- Usage:
*Beka bekelebekatadesse5@gmail.com- Holberton School student
Shellby emulates basic functionality of the sh shell. This README borrows form the Linux man pages sh(1) and dash(1).