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ALX Simple Shell Team Project

This is an ALX collaboration project on Shell. We were tasked to create a simple shell that mimics the Bash shell. Our shell shall be called hsh

Project was completed using

  • C language
  • Shell
  • Betty linter

General Requirement for project

  • All files will be compiled on Ubuntu 20.04 LTS using gcc, using the options -Wall -Werror -Wextra -pedantic -std=gnu89
  • All files should end with a new line
  • A README.md file, at the root of the folder of the project is mandatory
  • Use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
  • Shell should not have any memory leaks
  • No more than 5 functions per file
  • All header files should be include guarded
  • Write a README with the description of the project

Description

hsh is a simple UNIX command language interpreter that reads commands from either a file or standard input and executes them.

How hsh works

  • Prints a prompt and waits for a command from the user
  • Creates a child process in which the command is checked
  • Checks for built-ins, aliases in the PATH, and local executable programs
  • The child process is replaced by the command, which accepts arguments
  • When the command is done, the program returns to the parent process and prints the prompt
  • The program is ready to receive a new command
  • To exit: press Ctrl-D or enter "exit" (with or without a status)
  • Works also in non interactive mode

Compilation

gcc -Wall -Werror -Wextra -pedantic -std=gnu89 *.c -o hsh

Invocation

Usage: hsh [filename]

To invoke hsh, compile all .c files in the repository and run the resulting executable.

hsh can be invoked both interactively and non-interactively. If hsh is invoked with standard input not connected to a terminal, it reads and executes received commands in order.

Example:

$ echo "echo 'hello'" | ./hsh
'hello'
$

If hsh is invoked with standard input connected to a terminal (determined by isatty(3)), an interactive shell is opened. When executing interactively, hsh displays the prompt $ when it is ready to read a command.

Example:

$./hsh
$

Alternatively, if command line arguments are supplied upon invocation, hsh treats the first argument as a file from which to read commands. The supplied file should contain one command per line. hsh runs each of the commands contained in the file in order before exiting.

Example:

$ cat test
echo 'hello'
$ ./hsh test
'hello'
$

Environment

Upon invocation, hsh 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" | ./hsh
/home/projects

PWD

The current working directory as set by the cd command.

$ echo "echo $PWD" | ./hsh
/home/projects/alx/simple_shell

OLDPWD

The previous working directory as set by the cd command.

$ echo "echo $OLDPWD" | ./hsh
/home/projects/alx/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" | ./hsh
/home/projects/.cargo/bin:/home/projects/.local/bin:/home/projects/.rbenv/plugins/ruby-build/bin:/home/projects/.rbenv/shims:/home/projects/.rbenv/bin:/home/projects/.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/projects/.cargo/bin:/home/projects/workflow:/home/projects/.local/bin

Command Execution

After receiving a command, hsh 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. hsh then proceeds with the following actions:

  1. 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.
  2. If the first character of the command is none of a slash (\), dot (.), nor builtin, hsh searches each element of the PATH environmental variable for a directory containing an executable file by that name.
  3. 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

hsh 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, hsh 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.

$ ./hsh
$ ^C
$ ^C
$

Variable Replacement

hsh interprets the $ character for variable replacement.

$ENV_VARIABLE

ENV_VARIABLE is substituted with its value.

Example:

$ echo "echo $PWD" | ./hsh
/home/projects/alx/simple_shell

$?

? is substitued with the return value of the last program executed.

Example:

$ echo "echo $?" | ./hsh
0

$$

The second $ is substitued with the current process ID.

Example:

$ echo "echo $$" | ./hsh
6494

Comments

hsh ignores all words and characters preceeded by a # character on a line.

Example:

$ echo "echo 'hello' #this will be ignored!" | ./hsh
'hello'

Operators

hsh specially interprets the following operator characters:

; - Command separator

Commands separated by a ; are executed sequentially.

Example:

$ echo "echo 'hello' ; echo 'world'" | ./hsh
'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'" | ./hsh
./hsh: 1: error!: not found
$ echo "echo 'all good' && echo 'hello'" | ./hsh
'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'" | ./hsh
./hsh: 1: error!: not found
'but still runs'

The operators && and || have equal precedence, followed by ;.

hsh 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:

$ ./hsh
$ pwd
/home/projects/alx/simple_shell
$ cd ../
$ pwd
/home/projects/alx
$ cd -
$ pwd
/home/projects/alx/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:

$ ./hsh
$ 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            hsh
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:

$ ./hsh
$ exit

env

  • Usage: env
  • Prints the current environment.

Example:

$ ./hsh
$ env
NVM_DIR=/home/projects/.nvm
...

setenv

  • Usage: setenv [VARIABLE] [VALUE]
  • Initializes a new environment variable, or modifies an existing one.
  • Upon failure, prints a message to stderr.

Example:

$ ./hsh
$ setenv NAME Poppy
$ echo $NAME
Poppy

unsetenv

  • Usage: unsetenv [VARIABLE]
  • Removes an environmental variable.
  • Upon failure, prints a message to stderr.

Example:

$ ./hsh
$ setenv NAME Poppy
$ unsetenv NAME
$ echo $NAME

$

What we learned:

  • How a shell works and finds commands
  • Creating, forking and working with processes
  • Executing a program from another program
  • Handling dynamic memory allocation in a large program
  • Pair programming and team work
  • Building a test suite to check our own code

Authors

👤 Wisdom Uche