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Strings: Quotes, Interpolation, Escaping, and Buffers

Strings are the most important data structure in shell. YSH makes them easier and safer!

This doc addresses these questions:

• How do you write different kinds of strings in YSH programs?
• How do they behave at runtime? What are the common operations?
• What are the recommended ways to use them?

Shell Features:

• Quotes (single or double quoted)
• Interpolation aka substitution (variable, command, etc.)

YSH Features:

• Escaping for safety: ${x|html}, etc.
• Buffers for efficiency and readability: ${.myproc arg1}, etc.
  • (buffers are PHP-like)

Summary

• YSH has three ways to write strings: single quoted, double quoted, and C-style (which is also QSN-style).
• Each of the three types has a multiline variant. They are Python-style triple-quoted, but they also strip leading space in an intelligent way.
• TODO: Tagged strings, like "<h2>$x</h2>"html
• TODO: For string safety, YSH adds the concept of "escapers" and interpolation with $[x] (square brackets)
• TODO: For convenience and performance, YSH adds buffers and builtin substitution: ${.myproc arg1}.

For Python/JS/C Users

• Single and double quotes are different. Double quotes allow interpolation.
• Neither style of string respects backslash escapes like \n for newline. You have to use the third form.

For Shell Users

• YSH replaces here docs with Python-style triple-quoted strings.

Preferences:

• Unquoted strings (command mode only)
• Single-quoted strings
  • when you need to express special characters
  • QSN
• Double-quoted strings
  • with $[] interpolation
  • with ${} interpolation
  • with fast command sub `${.myproc arg1}

Quick Reference

echo unquoted          # bare words are allowed in command mode

echo 'with spaces'     # single quoted string
var s = 'with spaces'

# Raw single quoted string, to emphasize literal backslashes
var s = r'C:\Program Files\'

# C-escaped single quoted string
var line = $'foo\n'

# double quoted with safe interpolation (TODO)
echo "<p>hello $[name]</p>"       # default_escaper must be set
echo "<p>hello ${name|html}</p>"  # explicit escaper

# double quoted with unsafe interpolation
echo "hello $name"
echo "hello ${name}_suffix"       # braces delimit variable name

echo $(date +%x)                  # command sub

Still TODO:

echo ${.myproc arg1}

cat <<< '''
   one
   two
   '''

cat <<< $'''
   mu = \u{3bc}
   nul = \x00
   '''

var s = """
   multiline with ${vars}
   $(date +%x)
   ${.myproc arg1}
   """

Use Unquoted Strings in Command Mode

Shell is unique! You don't have to quote strings.

• link: command vs. expression mode

and quoted strings in expression mode

Two Kinds of Single-Quoted Strings

Raw with r'C:\Program Files\'

• TODO: parse_raw_strings

C-Escaped With $'foo\n'

• Use the QSN subset

QSN For Data Interchange

TODO: explain the difference.

This is different! It's data and not code. Analogy to JSON.

• When you want represent any string on a single line (filenames)
• To make binary data readable
• To display data in a terminal (protect against terminal codes)

Use Double-Quoted Strings For Interpolation

Implicit Safe Interpolation with $[x] (TODO)

• Use $[x] for safe interpolation
  • Respects shopt --set default_escaper

Explicit Safe Interpolation With ${x|html} (TODO)

• Use ${x|html} for safe interpolation

Note you can have bugs if you use the wrong escaper!

Raw Interpolation with $x (may be unsafe)

• Use $x or ${x}
  • These are identical except for syntax
• Useful for log messages, which aren't security sensitive

Note that you should not use "${var}" in YSH code. Use $var or ${var} because of simple word evaluation.

Command Sub $(echo hi)

Fast Command Sub ${.myproc} (stdout capture)

Note that only words are allowed here; not full commands. Wrap other commands in a proc.

• Using write_to_buffer

TODO:

echo ${.myproc foo|html} # I think this should be supported

Escapers / Codecs (TODO)

For ${x|html} and ${.myproc|html}

TODO

• how to register them
• wasm plugins?

Use Triple Quoted Strings Instead of Here Docs (TODO)

TODO

Concatenate With "$str1$str2"

Or "${str1}${str2}"

• is s ++ t valid?. It isn't necessary for strings and lists
  • :| @a @b | is the same for lists
  • does this Python syntax also work? [*a, *b]
  • Dicts: {d, **e} might be better

Avoid Concatenation in a Loop

setvar s = "${s}${suffix}"

Append with Two Styles

Since there is no ++ operator, there is no ++= operator.

echo, printf, write, and ${.myproc} (write_to_buffer)

echo, printf, and write have their output captured.

proc p(arg) {
  ### A proc that has its output captured quickly.

  echo $arg
  write two

  const x = 'three'
  printf '%s\n' $x

  # newline for interactive testing, but not when captured
  if ! shopt -q write_to_buffer {
    echo  
  }
}

echo ${.p one}  # $'one\ntwo\nthree\n'

append and join

var buf = :| |
append 'one ' (buf)
append $'two\n' (buf)
echo $[join(buf)]

Appendix A: Deprecated Shell Constructs

• here docs!
  • Use tripled quoted strings.
• Backticks for command sub
  • Use $(echo hi)
• Arithmetic substitution like $((1 + 2))
  • Use YSH expressions: $[1 + 2]
• ${x%%prefix} and so forth
  • Use builtin YSH functions (TODO)
• Unused: bash $"" for localization?

Appendix B: Related Documents

• Simple Word Eval: you don't need quoting as much
• Expression Language
• QSN