Shell quoting 101

gistfile1.txt

avih's POSIX shell quoting primer
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  LWTS: Literal White-space, Tilde, or shell-Special (semicolon,
        parenthesis, and few more). Literal is important, i.e. at
        the script source text. E.g. $x containing WTS is not LWTS.

  EXP (expansions): $param, ${...}, $((...)), $(...)  (avoid `...`).

  IFS (variable): Can only (maybe) split direct result of EXP.

  GLOB (pattern): has * or ? or [ and ] (same syntax as PATTERN).
                  Can only (maybe) [further] split result of above.

- [sub-]WORDs without LWTS, EXP, GLOB don't need quotes.
  E.g. echo, yes, no, x86, /x/y, x-y, x.y, x:y, @, = .

- LWTS need quotes to preserve their literal value (tilde not always,
  but use quote if not sure. $'...' is already quoted, but avoid it).

- In assignments there's _no_ IFS-split or glob-expansion, so EXP
  don't need quotes, even with white space inside EXP.

- Between 'case' and 'in' there is _no_ IFS/glob (like assignment).

- In commands and arguments there _is_ IFS-split/glob-expansion, so
  quotes can prevent those (literal tilde too, as mentioned). If
  we want the tilde/split/glob then don't quotes those parts. If we
  _know_ EXP result is without IFS/glob chars, quotes are optional.

- Assignment-arguments, like  export a="$b"  had IFS/glob before
  POSIX 2024. Now it doesn't, but do use quotes for portability.

Nested quotes:

- Inside command substitution $(...) it's the normal rules above
  regardless of outer quotes. Safe to recurse as much as needed.

- Substring expansion like ${x#PAT} can have quotes in PAT like the
  patterns in case...esac, regardless of outer quotes. However, try
  to avoid the outer quotes by using an assignment, as some shells
  mis-handle nested substring quotes, espesially single-quotes.

- In maybe-other-value ${x:-"$y"} or "${a+$b}" etc, there _can't_ be
  both outer and inner quotes. Outer quotes make it single argument,
  inner quotes make it zero or more. E.g. ${x+"$a" "$b"} is 0 or 2.

Examples:
  echo hello world             # no LWTS/EXP/GLOB -> no quotes
  a= b=foo c=$x$y* d=$(...)    # assignments, no LWTS -> no quotes
  e=$(( a + b )) f=${w%"$z"* } # even with literal white-sp in EXP
  g="x $y" h="(" i=";"         # LWTS always need quotes
  a="; \$" ; b=$a              # quote LWTS in a=..., no LWTS in b=$a
  case $x in ... esac          # no IFS/glob between case and in
  test "$x" = hello            # no LWTS, only $x is EXP -> quote
  "$cmd" "$(...)" foo "$x"     # cmd/args: only foo is not EXP
  a=$x b=$y mycmd x "$x" "$y"  # only cmd/args have IFS/GLOB -> quote
  for x in $values; do ...     # IFS-split. but also glob! careful
  for x in "$mypath"/*; do...  # glob expansion in arbitrary path
  export CFLAGS="$MY_FLAGS"    # use quotes for portability