anvillm/skills/rc-shell/SKILL.md

name	intent	description	user-invocable
rc-shell	scripting, shell	Plan 9 rc shell scripting syntax and conventions. Use when writing .rc shell scripts or working with Plan 9 rc syntax.	false

Plan 9 rc Shell Syntax

Shebang: #!/usr/bin/env rc Extension: .rc

Variables

name=value                   # NO spaces around =
files=(a b c)                # lists in parens
$name $files(1) $files(2-)   # substitute, 1st element, from 2nd onward
$files(2-4)                  # range
$#files                      # count
$"files                      # join with spaces
$^files                      # concat (no spaces)

Command Substitution

result=`{command}            # backticks, NOT $()
result=`split{command}       # split using 'split' instead of $ifs

Control Flow

if(test -f file) {
    echo exists
}
if not {                     # separate statement, new line
    echo missing
}

for(i in 1 2 3) { echo $i }
for(i) { echo $i }           # iterates over $* if no list

while(test $n -lt 10) { n=`{echo $n + 1 | bc} }

switch($file) {
case *.txt
    echo text
case *
    echo other
}

Functions & Pattern Matching

fn name { echo $* $1 }       # define (all args, first arg)
fn name                      # remove definition
~ $file *.txt                # pattern match (sets $status)
if(~ $#list 0) { echo empty }

Redirection & Pipes

>out >>out <in <<EOF         # stdout, append, stdin, here doc
>[2]err >[2=1] >[2=]         # fd 2 to file, fd 2 to fd 1, close fd 2
<[0=3]                       # fd 0 from fd 3
<{cmd} >{cmd} <>{cmd}        # process substitution
cmd1 | cmd2                  # pipe
|[2] |[1=2]                  # pipe fd 2, pipe fd 2 to fd 1

Operators

; & && || ! @ - sequence, async, and, or, invert, subshell ^ - concatenate (auto-inserted without whitespace)

Special Variables

$* $status $apid $pid $ifs $path $home

Built-ins

. file cd eval exec exit shift wait whatis

Key Differences from Bash

• Backticks {cmd}, not $(cmd)
• Lists (a b c), 1-indexed
• if not on new line
• test or ~, not [[ ]]
• $#var not ${#var}, $"var not "$*"
• >[2=1] not 2>&1
• No ${var:-default}
• fn name not function name
• Single quotes only (double quotes not special)
• Free carets: $x.c → $x^.c