Shell reference

Xila’s command-line shell implements a bounded command language for interactive use and shell -c. It is intentionally smaller than POSIX shell syntax.

Invocation

Interactive input is parsed one line at a time. When the shell executable receives already-separated arguments, those arguments are preserved literally. Use -c to explicitly parse command source, or -s PATH to execute a line-oriented VFS script:

shell -c 'echo first | wc -l'
shell -s /data/startup.xsh

EOF executes a final unterminated interactive or script line. A script supports LF/CRLF; each line has at most 4096 source bytes. Script commands share that shell invocation’s directory/environment state while reading input from the separate VFS script file. Script command failures continue by default and the final command status is returned. Syntax, read, UTF-8 and overlong-line errors identify the path/line and stop with status 2; exit stops the script. Nested noninteractive shell invocations are limited to four active levels.

Composition operators

export NAME='hello world'; echo "$NAME"
echo first > /temporary/example; echo second >> /temporary/example
cat < /temporary/example | wc -l
missing-command 2> /temporary/errors || echo recovered
long-running-command > /temporary/result & echo foreground

Syntax	Behavior
A | B	Run pipeline stages concurrently; the pipeline status is B’s status.
A && B	Run B only when A succeeds.
A || B	Run B only when A fails.
A ; B	Run sequential command lists.
A &	Start the preceding conditional list in a background task.
< FILE	Read standard input from FILE.
> FILE	Create/truncate FILE for standard output.
>> FILE	Append standard output to FILE.
2> FILE	Create/truncate FILE for standard error.
2>> FILE	Append standard error to FILE.

| binds more tightly than && and ||. The two conditional operators have equal precedence and evaluate left to right. Redirections belong to one command, are applied left to right, and override a pipeline endpoint when they target the same stream.

Background input is /devices/standard_io/null. Output and error inherit the shell’s streams unless redirected and may interleave with foreground output. The shell supports up to eight active background lists; job listing, cancellation, and foreground/background switching are not implemented.

Quoting and expansion

• Single quotes preserve contents literally.
• Double quotes allow variable expansion.
• Outside single quotes, backslash protects the following character.
• Inside double quotes, backslash protects $, ", and \.
• # at a word boundary begins a comment.
• $NAME, ${NAME}, and $? expand while a command is prepared.
• Variable names use ASCII letters, digits, and underscores, with a letter or underscore first.
• Missing variables expand to an empty string.

Expansion does not split a variable value on whitespace and does not reinterpret the value as shell operators. Quoted and unquoted fragments may form one argument.

Exit statuses

Case	Status
Success	0
General built-in failure	1
false	1
Malformed shell -c source	2
Unknown command	127
External executable	Preserved within 0–255
Pipeline	Status of the last stage
Successful background-list launch	Sets $? to 0

$? contains the preceding foreground status.

Resource limits

Resource	Limit
Source line	4096 bytes
Script buffer	4097 bytes
Tokens per line	256
Commands per line	32
Stages per pipeline	8
Expanded arguments and redirection paths per command	8192 bytes
Buffered formatted output per built-in	8192 bytes
Buffered bytes per pipe	512
Active background lists per shell	8

These limits bound shell-owned parsing and pipe storage. They do not bound allocations made by launched commands, inherited environment data, the WASM runtime, or task infrastructure.

Built-in commands

help lists registered built-ins and executable entries found in the task’s Paths locations. help COMMAND prints usage for a built-in without launching it.

The current built-in registry contains:

Area	Commands
Shell/state	true, false, exit, cd, pwd, clear, help
Environment	export, unset, printenv, which
Files/directories	ls, cat, stat, mkdir, rm, rmdir, realpath, touch, mv, cp, find
Text/streams	echo, grep, wc, head, tail, tree
Timing	sleep
Permissions	chmod
Network	web_request, dns_resolve, ping, ip

Selected recent utilities:

• grep PATTERN [FILE]... performs literal substring matching and reads standard input when no file is supplied or a file operand is -.
• touch FILE... creates missing files and updates access and modification times.
• realpath PATH... prints canonical absolute paths for existing files/directories.
• sleep DURATION accepts a nonnegative integer followed by ms, s, m, or h; no suffix means seconds.
• chmod MODE PATH accepts octal permissions from 000 through 777, optionally with a leading zero.
• tree [-L N] [PATH] streams a hierarchy without collecting sibling entries; depth is bounded to 32.
• mv SOURCE DESTINATION renames within one mounted filesystem and uses the exact destination path.
• cp SOURCE DESTINATION streams a regular file into a newly created or truncated destination with a fixed 512-byte buffer. If the destination is an existing directory, the source filename is appended. It does not copy directory trees and rejects directory sources and copies onto the same path.
• find [PATH] [-n NAME|--name NAME] [-t TYPE|--type TYPE] [-d N|--max-depth N] streams matching paths, starting at PATH or the current directory. Name matching is exact; supported types are f (file), d (directory), c (character device), b (block device), p (pipe), s (socket) and l (symlink). It includes a matching start path and hidden entries, with at most 32 levels and 1024-byte paths. Permission, missing-path and traversal errors stop the command.

Use help COMMAND for the authoritative option syntax of built-ins not detailed here.

Unsupported shell syntax

The current language supports line-oriented VFS scripts but not multiline grammar, command substitution, globbing, subshells, descriptor duplication such as 2>&1, here-documents, or shell assignment syntax. Newline characters inside explicit command source are whitespace; use ; to separate commands.

Related documentation

• Command-line shell architecture
• executables/shell/command_line/README.md .
• executables/shell/command_line/src/commands/mod.rs .