There are two main components on a shell:
- Analyzer
- Evaluator
---
title: Analyzer
---
flowchart LR
input["INPUT: Raw CMD"]
analyzer["ANALYZER"]
output["OUTPUT: Command AST"]
input --> analyzer
analyzer --> output
---
title: Evaluator
---
flowchart LR
input["INPUT: Command AST"]
evaluator["EVALUATOR"]
output["OUTPUT: CMD execution"]
input --> evaluator
evaluator --> output
---
title: Command Abstract Syntax Tree (AST)
---
%%{ init: { 'flowchart': { 'curve': 'stepAfter' } } }%%
graph TD
A((Pipe)):::pipe --> B((Redir)):::redir
A --> C((Pipe)):::pipe
B --> D((Filename)):::arg
B --> E((Redir)):::redir
E --> EL((Filename)):::arg
E --> ER((Redir)):::redir
ER --> EER((Filename)):::arg
ER --> EEL((CMD)):::cmd
C --> F((Redir)):::redir
F --> FL((Filename)):::arg
F --> FR((Redir)):::redir
FR --> FFR((Filename)):::arg
FR --> FFL(... Redirs ...):::redir
FFL --> FFFL((Redir)):::redir
FFFL --> FFFR((Filename)):::arg
FFFL --> FFFFL((CMD)):::cmd
C --> G(... Pipes ...):::pipe
G --> H((Pipe)):::pipe
H --> HL((Redir)):::redir
HL --> HR((Filename)):::arg
H --> I((Redir)):::redir
I --> IR((Filename)):::arg
I --> IL(... Redirs ...):::redir
IL --> IIL((CMD)):::cmd
HL --> HHL(... Redirs ...):::redir
HHL --> HHHL((CMD)):::cmd
classDef pipe stroke:#f00
classDef arg stroke:#0f0
classDef cmd stroke:#ff0
classDef redir stroke:#0ff
---
title: Simplified Abstract Syntax Tree (AST)
---
%%{ init: { 'flowchart': { 'curve': 'stepAfter' } } }%%
graph TD
A((Pipe)):::pipe --> B((Redir\nFilename)):::redir
A --> C((Pipe)):::pipe
B --> E((Redir\nFilename)):::redir
E --> ER((Redir\nFilename)):::redir
ER --> EEL((CMD)):::cmd
C --> F((Redir\nFilename)):::redir
F --> FR((Redir\nFilename)):::redir
FR --> FFL(... Redirs ...):::redir
FFL --> FFFL((Redir\nFilename)):::redir
FFFL --> FFFFL((CMD)):::cmd
C --> G(... Pipes ...):::pipe
G --> H((Pipe)):::pipe
H --> HL((Redir\nFilename)):::redir
H --> I((Redir\nFilename)):::redir
I --> IL(... Redirs ...):::redir
IL --> IIL((CMD)):::cmd
HL --> HHL(... Redirs ...):::redir
HHL --> HHHL((CMD)):::cmd
classDef pipe stroke:#f00
classDef arg stroke:#0f0
classDef cmd stroke:#ff0
classDef redir stroke:#0ff
We can reduce the AST into a linked list if we include the redirections' linked list as an attribute of a command
---
title: Reduced AST
---
%%{ init: { 'flowchart': { 'curve': 'stepAfter' } } }%%
graph LR
a((CMD)):::cmd --> b((CMD)):::cmd --> c((...)):::cmd --> d((CMD)):::cmd
classDef cmd stroke:#ff0
%%{ init: { 'flowchart': { 'curve': 'stepAfter' } } }%%
graph TB
subgraph CMD node
direction TB
aa((CMD)):::cmd --> bb((Redir)):::redir -->bbb((Filename)):::arg
cc-->cccc((...)):::arg
bb-->cc((Redir)):::redir --> ccc((...)):::redir
aa-->aaa((arg)):::arg-->arg2((arg)):::arg-->arg3((...)):::arg
aa-->next((next)):::cmd-->next2((next)):::cmd-->next3((...)):::cmd
classDef arg stroke:#0f0
classDef cmd stroke:#ff0
classDef redir stroke:#0ff
end
typedef struct s_ast
{
char *bin;
int ac;
char **av;
t_redir *redir;
struct s_ast *next;
} t_ast;typedef struct s_redir
{
t_byte id;
char *file;
} t_redir;In this minishell, the interface is very small and is mostly a tool for the Analyzer since it doesn't handle any pointer interaction. However it does handle:
- A prompt to input a command
- History
- Handles user signals:
Key Combination Signal Name ctrl-CSIGINT ctrl-DEOF (End of File) ctrl-\SIGQUIT
The analyzer takes the command, analyze the lexic, syntax and semantic, and process the data into an Command AST, that the Evaluator will take and use it to evaluate the command, redirection, pipe, etc. It handles:
- Lexic Analysis
- Syntax Analysis
- Semantic Analysis
- Quotes
Quotes Behavior Single quotes Prevent interpreting anything inside. Double quotes Prevent interpreting anything except $inside.- Not interpret unclosed quotes
- Translations
- meta-characters
- Relative path from command to absolute path
- Env variables
- $? which the evaluator has to save
The evaluator takes the Command AST, launch the appropriate executable on a child process, handling pipes, signal inheritance, clean process management, etc.
- Search and launch executables
- Based on the
PATHenv variable. - Based on an aboslute path
- Based on builtins commands
- Based on the
- Interpret redirections and pipes
Redirection Symbol Description <Redirects input. >Redirects output. <<Reads input until a line containing the delimiter is seen. >>Redirects output in append mode. - Builtins
Command Option echo-ncdOnly a relative or absolute path pwdNo options exportNo options unsetNo options envNo options or arguments exitNo options
---
title: Shell Command Processing Flow
---
erDiagram
Shell_Command }|--|{ Analyzer : "Analyze"
Shell_Command }|--|{ Command_AST : "Creates"
Analyzer }o--o{ Command_AST : "Creates"
Command_AST }o--|| Evaluator : "Evaluate"