16 KiB
Notes relatives au projet
Comparative testing with bash should be done with bash --norc --posix.
Ideas for testing
- use prysk or shellspec with shell=./minishell
Shell Operation
cf. 3.1.1 Shell Operation
Breaks the input into words and operators, obeying the Quoting Rules. These tokens are delimited by metacharacters.
Parses the tokens into simple and compound commands (see Shell Commands)
Performs the various Shell Expansions, breaking the expanded tokens into lists of filenames and commands and arguments.
Performs any necessary redirections and removes the redirection operators and their operands from the argument list.
Executes the command (see Command Execution);
TODO: add missing operations
Quoting Rules
cf. 3.1.2 Quoting
Quoting escapes metacharacters.
The quoting mechanisms we have to implement are:
cf. Subject
- Single quotes, which prevent metacharacters interpretation.
- Double quotes, which prevent metacharacters interpretation except for '$' (See Shell Parameter Expansion).
In the Bash Reference Manual, these are defined as follows (keeping only the parts we have to implement):
cf. 3.1.2.2 Single Quotes
Preserves the literal value of each character within the quotes.
cf. 3.1.2.3 Double Quotes
Preserves the literal value of all characters within the quotes, with the exception of '$'.
Per the subject: minishell should not interpret unclosed quotes
Shell Commands
cf. 3.2 Shell Commands
A Shell Command may be either a Simple Command, a Pipeline, a List of Commands (composed of one or more Pipelines), or a Grouped Command (composed of one or more List of Commands).
Simple Commands
cf. 3.2.2 Simple Commands
It’s just a sequence of words separated by blanks, terminated by one of the shell’s control operators. The first word specifies a command to be executed, with the rest of the words being that command’s arguments.
The return status (see Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.
Pipelines
cf. 3.2.3 Pipelines
A pipeline is a sequence of one or more commands separated by the control operator '|'.
The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command’s output. This connection is performed before any redirections specified by the first command.
The shell waits for all commands in the pipeline to complete before reading the next command.
Each command in a multi-command pipeline, where pipes are created, is executed in its own subshell, which is a separate process.
e.g.
export TT=1 | echo $TT
prints an empty string, because TT is unset in the second subshell.
The exit status of a pipeline is the exit status of the last command in the pipeline.
The shell waits for all commands in the pipeline to terminate before returning a value.
Lists of Commands
cf. 3.2.4 Lists of Commands
A list is a sequence of one or more pipelines separated by one of the operators ‘&&’, or ‘||’, and optionally terminated by a newline.
AND and OR lists are sequences of one or more pipelines separated by the control operators ‘&&’ and ‘||’, respectively. AND and OR lists are executed with left associativity.
e.g.
A && B && C
is the same as
(A && B) && C
An AND list has the form
A && B
B is execute if and only if A has an exit status of 0 (succes).
An OR list has the form
A || B
B is execute if and only if A has a non-zero exit status (failure).
The return status of AND and OR lists is the exit status of the last command executed in the list.
Group of Commands
cf. 3.2.5 Compound Commands
Each group begins with the control operator '(' and ends with the control operator ')'.
Any redirections associated with a group of commands apply to all commands within that group of commands unless explicitly overridden.
cf. 3.2.5.3 Grouping Commands
When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
( LIST )
The parentheses are operators, and are recognized as separate tokens by the shell even if they are not separated from the LIST by whitespace.
Placing a list of commands between parentheses forces the shell to create a _subshell, and each of the commands in LIST is executed in that subshell environment. Since the LIST is executed in a subshell, variable assignments do not remain in effect after the subshell completes.
The exit status of this construct is the exit status of LIST.
Shell Expansion
cf. 3.5 Shell Expansions
Expansion is performed on the command line after it has been split into token's. There are seven kinds of expansion performed. in the following order:
- brace expansion
- tilde expansion
- parameter and variable expansion
- arithmetic expansion
- command substitution (left to right)
- word splitting
- filename expansion
We only have to implement the following kinds:
- parameter expansion
- word splitting
- filename expansion
After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves ("quote removal").
Only brace expansion, word splitting, and filename expansion can increase the number of words of the expansion; other expansions expand a single word to a single word.
Shell Parameter Expansion
cf. 3.5.3 Shell Parameter Expansion
The '$' character introduces parameter expansion, command substitution, or arithmetic expansion.
The form is $VAR, where VAR may only contain the following characters:
- a-z
- A-Z
- _
- 0-9 (not in the first character)
Just noticed an interesting case:
bash-5.2$ VAR=hello # we set a shell variable (NOT environment variable)
bash-5.2$ VAR=hi env | grep VAR=; env | grep VAR= # here VAR is an environment variable, which is only valid for the next command (the second env returns nothing, confirming that it is not valid for that command)
VAR=hi
bash-5.2$ env | grep VAR= # var is not an environment variable
bash-5.2$ echo $VAR # but it is a shell variable
hello
Luckily for us, we don't have to handle shell variables, nor do we have to handle VAR=value or VAR=value cmd.
Word Splitting
cf. 3.5.7 Word Splitting
The shell scans the results of parameter expansion that did not occur within double quotes for word splitting.
The shell splits the results of the other expansions into words.
The shell treats the following characters as a delimiter:
- (space)
- (tab)
- (newline)
Explicit null arguments ('""' or '''') are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained and passed to a command as an empty string. When a quoted null argument appears as part of a word whose expansion is non-null, the null argument is removed. That is, the word '-d''' becomes '-d' after word splitting and null argument removal.
Note that if no expansion occurs, no splitting is performed.
Filename Expansion
cf. 3.5.8 Filename Expansion
Bash scans each word for the character '*'.
If one of these characters appears, and is not quoted, then the word is regarded as a PATTERN, and replaced with an alphabetically sorted list of filenames matching the pattern (see: Pattern Matching). If no matching filenames are found, the word is left unchanged.
When a pattern is used for filename expansion, the character '.' at the start of a filename or immediately following a slash must be matched explicitly. In order to match the filenames '.' and '..', the pattern must begin with '.'
When matching a filename, the slash character must always be matched explicitly by a slash in the pattern.
Pattern Matching
cf. 3.5.8.1 Pattern Matching
- Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern.
The special pattern characters have the following meanings: '*' Matches any string, including the null string.
The special pattern characters must be quoted if they are to be matched literally.
e.g. this is the required behaviour
bash-5.1$ ls *there
'hello*there' 'hi*there' noonethere
bash-5.1$ ls *'*'there
'hello*there' 'hi*there'
Quote Removal
cf. 3.5.9 Quote Removal
After the preceding expansions, all unquoted occurrences of the characters ''' and '"' that did not result from one of the above expansions are removed.
Redirection
cf. 3.6 Redirections
Before a command is executed, its input and output may be "redirected" using a special notation interpreted by the shell. "Redirection" allows commands' file handles to be made to refer to different files, and can change the files the command reads from and writes to.
The redirection operators may precede or appear anywhere within a simple command or may follow a command.
e.g. this is the correct behaviour
bash-5.1$ ls > hello.txt *here
bash-5.1$ cat hello.txt
hello*there
hi*there
noonethere
Redirections are processed in the order they appear, from left to right.
e.g. this is the correct behaviour
bash-5.1$ ls > hello.txt share > here.txt *.txt
bash-5.1$ ls -l hello.txt here.txt
-rw-r--r-- 1 kcolin 2024_le-havre 0 Feb 7 15:54 hello.txt
-rw-r--r-- 1 kcolin 2024_le-havre 68 Feb 7 15:54 here.txt
bash-5.1$ cat here.txt
hello.txt
here.txt
log.txt
newlog-strict.txt
newlog.txt
share:
man
'<' refers to the standard input (fd 0, STDIN_FILENO)
'>' refers to the standard output (fd 1, STDOUT_FILENO)
TODO: check unless otherwise noted The word following the redirection operator, unless otherwise noted, is subjected to parameter expansion, filename expansion, word splitting, and quote removal.
If it expands to more than one word, Bash reports an error, except when in posix mode. TODO: decide if we follow posix default or bash default
Note: This behaviour change is, as far as I can tell, not documented in the Bash Reference Manual.
# bash behaviour
bash-5.1$ var="file1 file2"
bash-5.1$ echo "hello world" > $var
bash: $var: ambiguous redirect
# posix behaviour
bash-5.1$ var="file1 file2"
bash-5.1$ echo "hello world" > $var
bash-5.1$ cat "$var"
hello world
In bash mode and in posix mode, if the variable is not defined, bash prints the following error:
bash-5.1$ echo "hello world" > $nonexist
bash: $nonexist: ambiguous redirect
Executing Commands
cf. 3.7 Executing Commands
Simple Command Execution
cf. 3.7.1 Simple Command Expansion
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right, in the following order.
-
The words that the parser has marked as redirections are saved for later processing.
-
The words that are not redirections are expanded (see Shell Expansions). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.
-
Redirections are performed as described above (see Redirections).
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits with a status of zero.
Command Search and Execution
cf. 3.7.2 Command Search and Execution
After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.
-
The shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked.
-
If the name is not a builtin, and contains no slashes, Bash searches each element of '$PATH' for a directory containing an executable file by that name. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127.
-
If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any.
-
If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a "shell script" and the shell executes it as described in Shell Scripts. TODO: check if we need to implement the Shell Scripts behaviour
-
The shell waits for the command to complete and collects its exit status.
Subshell
cf. 3.7.3 Command Execution Environment
The shell has an execution environment, which consists of the following:
open files inherited by the shell at invocation, as modified by redirections
the current working directory as set by cd or inherited by the shell at invocation
shell variables, passed in the environment
A command invoked in this separate environment cannot affect the shell’s execution environment.
A subshell is a copy of the shell process.
Environment
cf. 3.7.4 Environment
When a program is invoked it is given an array of strings called the "environment". This is a list of name-value pairs, of the form 'name=value'.
Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for 'export' to child processes. Executed commands inherit the environment. The 'export' and 'unset' builtins allow parameters to be added to and deleted from the environment. If the value of a parameter in the environment is modified using the 'export' builtin, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the 'unset' builtin, plus any additions via the 'export' command.
Here Documents
cf. Bash Reference Manual 3.6.6 Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.
TODO: The following paragraph may not apply fully to our project, check it again!
No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline is ignored, and ‘\’ must be used to quote the characters ‘\’, ‘$’, and ‘`’.
Definitions
cf. Bash Reference Manual cf. 2 Definitions
token A sequence of characters considered a single unit by the shell. It is either a word or an operator
word A sequence of characters treated as a unit by the shell. Words may not include unquoted metacharacters.
operator A control operator or a redirection operator. Operators contain at least one unquoted metacharacter.
control operator A token that performs a control function.
It is a newline or one of the following: '|', ‘||’, ‘&&’, ‘(’, or ‘)’.
redirection operator For our project:
'<' redirects input
'>' redirects output
'<<' is here_doc with delimiter. delimiter is a word. Does not have to update history
'>>' redirects output in append mode
blank A space or tab character