package morsmall

Shell AST.

Names in Shell are just strings with a few additional conditions.

The type word is a description of words in Shell. See POSIX, 2 Shell & Utilities, 2.3 Token Recognition

For now, a pattern is just a word.

An assignment is just a pair of a name and a word.

A file descriptor descr is an integer.

The following description does contain all the semantic subtleties of POSIX Shell. Such a description can be found in the document IEEE Std 1003.1™-2008, 2016 Edition. In the following, we will refer to it simple as POSIX.

The type command contains the definition of a Shell command in Morsmall. Compared to what can be found in the POSIX standard or in Morbig, this type is rather small. This is because a lot of syntactically distinct scripts that are semantically equivalent are identified in here. For instance, all the following scripts are equivalent and identified as such:

if t1; then c1; elif t2; then c2; fi
if { t1; }; then c1; elif { { t2; }; }; fi
if t1; then c1; else if t2; then c2; fi; fi

Simple Command

See POSIX, 2 Shell & Utilities, 2.9.1 Simple Command

Lists and Pipelines

See POSIX, 2 Shell & Utilities, 2.9.2 Pipelines and 2.9.3 Lists

• Asynchronous Lists. When encountering Async c, the Shell shall execute c asynchronously in a subshell. This means that the shell shall not wait for the command to finish before executing the next command.

• Sequential Lists. A contrario, the commands c1 and c2 in Seq (c1, c2) shall be executed sequentially.

• AND Lists. In And (c1, c2), c1 shall be executed first. If its exit status is zero, c2 shall be executed. The commands are expanded only if they are executed.

• OR Lists. In Or (c1, c2), c1 shall be executed first. If its exit status is non-zero, c2 shall be executed.

• Pipeline. In Pipe (c1, c2), the standard output of c1 shall be connected to the standard input of c2. The standard input, standard output, or both of a command shall be considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command.

• Negation. The command Not c has the same behaviour as c, except for the exit status that shall be the logical NOT of the exit status of c.

Compound Commands

See POSIX, 2 Shell & Utilities, 2.9.4 Compound Commands

• The Subshell Environment. Subshell c shall execute c a subshell environment. Variable assignments and built-in commands that affect the environment shall not remain in effect after the list finishes.

• The for Loop. For (x, l, c) shall execute a sequence of commands c for each member in a list of items. It is to be noted that l is non-mandatory and is thus an option. Besides, there is an important semantic difference between None and Some []. The former appears in a for loop where the list of words appear but is empty. In that case, the for loops through the empty list. The latter appears in a for loop where the list of words has been omitted. In that case, the for loops through the positional parameters.

• The case Conditional Construct. Case (w, [([p11;...],c1); ...]) shall execute the compound-list corresponding to the first one of several patterns that is matched by the string resulting from the expansion of the given word w. In order from the beginning to the end of the case statement, each pattern p* shall be subjected to expansion, and the result of these expansions shall be compared against the expansion of w. After the first match, no more patterns shall be expanded, and the corresponding c* shall be executed. The order of expansion and comparison of multiple patterns that label the same statement is unspecified.

• The if Conditional Construct. If (c1, c2, c3) shall execute c1 and use its exit status to determine whether to execute c2 or c3. In fact, c3 is not mandatory and is thus an option.

• The while Loop. While (c1, c2) shall continuously execute c2 as long as c1 has a zero exit status.

• The until Loop. Until (c1, c2) shall continuously execute c2 as long as c1 has a non-zero exit status.

Function Definition Command

See POSIX, 2 Shell & Utilities, 2.9.5 Function Definition Command

A function is a user-defined name that is used as a simple command to call a compound command with new positional parameters. A function is defined with a function definition command, Function (name, body).

This function definition command defines a function named name: string and with body body: command. The body shall be executed whenever name is specified as the name of a simple command.

Redirection

See POSIX, 2 Shell & Utilities, 2.7 Redirections

Type Definitions

The type command describes a command in the AST. All the command semantics are described at the top of this document.