Plan 9 rc Shell Scripting

Reference for writing shell scripts using Plan 9 rc syntax (.rc files).

File Setup

Shebang:

code

#!/usr/bin/env rc

Always use /usr/bin/env rc (not a direct path like /usr/local/plan9/bin/rc) because plan9port installation locations vary across systems.

File extension: .rc

Core Syntax Differences from Bash/sh

Variables

Assignment (NO spaces around =):

code

name=value
path=(/bin /usr/bin)

Reference (NO $ for simple cases in some contexts, but use $ for safety):

code

echo $name
for(file in *.txt) echo $file

Lists (space-separated in parentheses):

code

files=(foo.txt bar.txt baz.txt)
echo $files          # prints all elements
echo $files(1)       # prints first element (1-indexed)
echo $files(2-)      # prints from second element onward

Command Substitution

Backticks (like classic sh):

code

files=`{ls *.txt}
date=`{date}

Quoting

Single quotes for literals:

code

echo 'hello $world'   # prints: hello $world

Double quotes are NOT special in rc - use single quotes or concatenation.

Control Structures

if statement (note: condition is a command, if not is a separate statement):

code

if(test -f file.txt) {
    echo 'file exists'
}
if not {
    echo 'file does not exist'
}

# Single statement form
if(test -f file.txt)
    echo 'exists'
if not
    echo 'does not exist'

for loop:

code

for(file in *.txt) {
    echo $file
}

for(i in 1 2 3 4 5) {
    echo iteration $i
}

while loop:

code

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

switch statement:

code

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

Functions

Definition:

code

fn myfunction {
    echo 'arguments:' $*
    echo 'first arg:' $1
}

# Call it
myfunction foo bar baz

Local variables (use local keyword or just assign):

code

fn myfunction {
    local=(old $local)
    # ... function body
}

Exit Status

Check exit status:

code

if(grep -q pattern file.txt) {
    echo 'found'
}

Explicit status check:

code

command
status=$status
if(~ $status 0) {
    echo 'success'
}

Pattern Matching

The ~ operator:

code

if(~ $file *.txt) {
    echo 'text file'
}

if(~ $#list 0) {
    echo 'empty list'
}

Pipelines and Redirection

Standard pipelines:

code

cat file.txt | grep pattern | sort

Redirection:

code

echo hello >file.txt          # write
echo world >>file.txt         # append
command >[2=1]                # redirect stderr to stdout
command >[2]/dev/null         # redirect stderr to /dev/null

Important Operators

•~ - pattern match
•$#var - count elements in list
•$var(N) - Nth element of list (1-indexed)
•$var(N-) - elements from N onward
•$^var - concatenate list elements (no spaces)
•$$var - pid of command

Comments

code

# This is a comment
echo hello  # inline comment

Common Patterns

Check if variable is set

code

if(~ $#var 0) {
    echo 'var is not set'
}

Iterate over arguments

code

fn process {
    for(arg in $*) {
        echo processing $arg
    }
}

Build path list

code

path=($home/bin /usr/local/bin /usr/bin /bin)

Conditional execution

code

test -f file.txt && echo 'exists'
test -f file.txt || echo 'does not exist'

Here documents

code

cat <<EOF
line 1
line 2
EOF

Key Differences from Bash

•No $() syntax - use backticks for command substitution
•Lists are first-class - use parentheses for lists
•1-indexed arrays - $list(1) is first element
•if not instead of else - written as a separate statement, not on same line
•No [[ ]] or [ ] - use test command or ~ operator
•Pattern matching with ~ - not == or =
•$#var for count - not ${#var}
•Different redirection - >[2=1] not 2>&1
•No parameter expansion - no ${var:-default} syntax
•Functions use fn - not function keyword

Best Practices

•Always use #!/usr/bin/env rc shebang
•Use meaningful variable names
•Quote variables when word splitting matters
•Use ~ for pattern matching
•Remember lists are 1-indexed
•Write if not as a separate statement on a new line
•Prefer test for file/string comparisons
•Use $#var to check if variable is set
•Leverage list operations for cleaner code
•Keep scripts simple and readable

Common Gotchas

•NO spaces around = in assignments
•Double quotes are NOT special - use single quotes
•Arrays are 1-indexed, not 0-indexed
•if not is a separate statement - write it on a new line, not } if not {
•No ${} parameter expansion
•Different redirection syntax from bash
•test command behavior may differ from bash test

Resources

For detailed documentation, see 9 man rc or the Plan 9 documentation.