sed BSD/macOS Sed vs. GNU Sed vs. the POSIX Sed specification

To quote from @SnoringFrog's topic-creation request:

"One of the biggest gotchas using sed is scripts that fail (or succeed in an unexpected way) because they were written for one and not the other. Simple run-down of the more major differences would be good."

macOS uses the BSD version of sed^[1], which differs in many respects from the GNU sed version that comes with Linux distros.

Their common denominator is the functionality decreed by POSIX: see the POSIX sed spec.

The most portable approach is to use POSIX features only, which, however, limits functionality:

• Notably, POSIX specifies support only for basic regular expressions, which have many limitations (e.g., no support for | (alternation) at all, no direct support for + and ?) and different escaping requirements.

  • Caveat: GNU sed (without -r), does support \|, \+ and \?, which is NOT POSIX-compliant; use --posix to disable (see below).

• To use POSIX features only:

  • (both versions): use only the -n and -e options (notably, do not use -E or -r to turn on support for extended regular expressions)

  • GNU sed: add option --posix to ensure POSIX-only functionality (you don't strictly need this, but without it you could end up inadvertently using non-POSIX features without noticing; caveat: --posix itself is not POSIX-compliant)

  • Using POSIX-only features means stricter formatting requirements (forgoing many conveniences available in GNU sed):

    • Control-character sequences such as \n and \t are generally NOT supported.
    • Labels and branching commands (e.g., b) must be followed by an actual newline or continuation via a separate -e option.
    • See below for details.

However, both versions implement extensions to the POSIX standard:

• what extensions they implement differs (GNU sed implements more).
• even those extensions they both implement partially differ in syntax.

If you need to support BOTH platforms (discussion of differences):

• Incompatible features:

  • Use of the -i option without an argument (in-place updating without backup) is incompatible:

    • BSD sed: MUST use -i ''
    • GNU sed: MUST use just -i (equivalent: -i'') - using -i '' does NOT work.

  • -i sensibly turns on per-input-file line numbering in GNU sed and recent versions of BSD sed (e.g., on FreeBSD 10), but does NOT on macOS as of 10.12.
    Note that in the absence of -i all versions number lines cumulatively across input files.

  • If the last input line does not have a trailing newline (and is printed):

    • BSD sed: always appends a newline on output, even if the input line doesn't end in one.
    • GNU sed: preserves the trailing-newline status, i.e., it appends a newline only if the input line ended in one.

• Common features:

  • If you restrict your sed scripts to what BSD sed supports, they will generally work in GNU sed too - with the notable exception of using platform-specific extended regex features with -E. Obviously, you'll also forgo extensions that are specific to the GNU version. See next section.

Guidelines for cross-platform support (OS X/BSD, Linux), driven by the stricter requirements of the BSD version:

^{Note that that the shorthands macOS and Linux are occasionally used below to refer to the BSD and GNU versions of sed, respectively, because they are the stock versions on each platform. However, it is possible to install GNU sed on macOS, for instance, using Homebrew with brew install gnu-sed.}

Note: Except when the -r and -E flags are used (extended regexes), the instructions below amount to writing POSIX-compliant sed scripts.

• For POSIX compliance, you must restrict yourself to POSIX BREs (basic regular expressions), which are, unfortunately, as the name suggests, quite basic.
  Caveat: do not assume that \|, \+ and \? are supported: While GNU sed supports them (unless --posix is used), BSD sed does not - these features are not POSIX-compliant.
  While \+ and \? can be emulated in POSIX-compliant fashion :
  \{1,\} for \+,
  \{0,1\} for \?,
  \| (alternation) cannot, unfortunately.

• For more powerful regular expressions, use -E (rather than -r) to support EREs (extended regular expressions) (GNU sed doesn't document -E, but it does work there as an alias of -r; newer version of BSD sed, such as on FreeBSD 10, now also support -r, but the macOS version as of 10.12 does not).
  Caveat: Even though use of -r / -E means that your command is by definition not POSIX-compliant, you must still restrict yourself to POSIX EREs (extended regular expressions). Sadly, this means that you won't be able to use several useful constructs, notably:

  • word-boundary assertions, because they're platform-specific (e.g., \< on Linux, [[:<]] on OS X).
  • back-references inside regular expressions (as opposed to the "back-references" to capture-group matches in the replacement string of s function calls), because BSD sed doesn't support them in extended regexes (but, curiously, does so in basic ones, where they are POSIX-mandated).

• Control-character escape sequences such as \n and \t:

  • In regexes (both in patterns for line selection and the first argument to the s function), assume that only \n is recognized as an escape sequence (rarely used, since the pattern space is usually a single line (without terminating \n), but not inside a character class, so that, e.g., [^\n] doesn't work; (if your input contains no control chars. other than \t, you can emulate [^\n] with [[:print:][:blank:]]; otherwise, splice control chars. in as literals^[2]) - generally, include control characters as literals, either via spliced-in ANSI C-quoted strings (e.g., $'\t') in shells that support it (bash,ksh, zsh), or via command substitutions using printf (e.g., "$(printf '\t')").

    • Linux only:
      sed 's/\t/-/' <<<$'a\tb' # -> 'a-b'
    • OSX and Linux:
      sed 's/'$'\t''/-/' <<<$'a\tb' # ANSI C-quoted string
sed 's/'"$(printf '\t')"'/-/' <<<$'a\tb' # command subst. with printf

  • In replacement strings used with the s command, assume that NO control-character escape sequences are supported, so, again, include control chars. as literals, as above.

    • Linux only:
      sed 's/-/\t/' <<<$'a-b' # -> 'a<tab>b'
    • macOS and Linux:
      sed 's/-/'$'\t''/' <<<'a-b'
sed 's/-/'"$(printf '\t')"'/' <<<'a-b'

  • Ditto for the text arguments to the i and a functions: do not use control-character sequences - see below.

• Labels and branching: labels as well as the label-name argument to the b and t functions must be followed by either by a literal newline or a spliced-in $'\n'. Alternatively, use multiple -e options and terminate each right after the label name.

  • Linux only:
    sed -n '/a/ bLBL; d; :LBL p' <<<$'a\nb' # -> 'a'
  • macOS and Linux:
    • EITHER (actual newlines):
      sed -n '/a/ bLBL d; :LBL p' <<<$'a\nb'
    • OR (spliced-in $\n instances):
      sed -n '/a/ bLBL'$'\n''d; :LBL'$'\n''p' <<<$'a\nb'
    • OR (multiple -e options):
      sed -n -e '/a/ bLBL' -e 'd; :LBL' -e 'p' <<<$'a\nb'

• Functions i and a for inserting/appending text: follow the function name by \, followed either by a literal newline or a spliced-in $'\n' before specifying the text argument.

  • Linux only:
    sed '1 i new first line' <<<$'a\nb' # -> 'new first line<nl>a<nl>b'
  • OSX and Linux:
    sed -e '1 i\'$'\n''new first line' <<<$'a\nb'
  • Note:
    • Without -e, the text argument is inexplicably not newline-terminated on output on macOS (bug?).
    • Do not use control-character escapes such as \n and \t in the text argument, as they're only supported on Linux.
    • If the text argument therefore has actual interior newlines, \-escape them.
    • If you want to place additional commands after the text argument, you must terminate it with an (unescaped) newline (whether literal or spliced in), or continue with a separate -e option (this is a general requirement that applies to all versions).

• Inside function lists (multiple function calls enclosed in {...}), be sure to also terminate the last function, before the closing }, with ;.

  • Linux only:
    • sed -n '1 {p;q}' <<<$'a\nb' # -> 'a'
  • macOS and Linux:
    • sed -n '1 {p;q;}' <<<$'a\nb'

GNU sed-specific features missing from BSD sed altogether:

GNU features you'll miss out on if you need to support both platforms:

• Various regex-matching and substitution options (both in patterns for line selection and the first argument to the s function):

  • The I option for case-INsensitive regex matching (incredibly, BSD sed doesn't support this at all).
  • The M option for multi-line matching (where ^ / $ match the start / end of each line)
  • For additional options that are specific to the s function, see https://www.gnu.org/software/sed/manual/sed.html#The-_0022s_0022-Command

• Escape sequences

  • Substitution-related escape sequences such as \u in the replacement argument of the s/// function that allow substring manipulation, within limits; e.g., sed 's/^./\u&/' <<<'dog' # -> 'Dog' - see http://www.gnu.org/software/sed/manual/sed.html#The-_0022s_0022-Command

  • Control-character escape sequences: in addition to \n, \t, ..., codepoint-based escapes; for instance, all of the following escapes (hex., octal, decimal) represent a single quote ('): \x27, \o047, \d039 - see https://www.gnu.org/software/sed/manual/sed.html#Escapes

• Address extensions, such as first~step to match every step-th line, addr, +N to match N lines following addr, ... - see http://www.gnu.org/software/sed/manual/sed.html#Addresses

^{[1] The macOS sed version is older than the version on other BSD-like systems such as FreeBSD and PC-BSD. Unfortunately, this means that you cannot assume that features that work in FreeBSD, for instance, will work [the same] on macOS.}

^{[2] The ANSI C-quoted string $'\001\002\003\004\005\006\007\010\011\013\014\015\016\017\020\021\022\023\024\025\026\027\030\031\032\033\034\035\036\037\177' contains all ASCII control characters except \n (and NUL), so you can use it in combination with [:print:] for a pretty robust emulation of [^\n]:
'[[:print:]'$'\001\002\003\004\005\006\007\010\011\013\014\015\016\017\020\021\022\023\024\025\026\027\030\031\032\033\034\035\036\037\177'']}