sed not giving me correct substitute operation for newline with Mac - differences between GNU sed and BSD / OSX sed
With BSD/macOS sed
, to use a newline in the replacement string of an s
function call, you must use an \
-escaped actual newline - escape sequence \n
is not supported there (unlike in the regex part of the call).
Either: simply insert an actual newline:
sed -i '' 's/\\n/\ /g' test1.txt
Or: use an ANSI C-quoted string (
$'...'
) to splice in the newline ($'\n'
; works inbash
,ksh
, orzsh
):sed -i '' 's/\\n/\'$'\n''/g' test1.txt
GNU sed
, by contrast, does recognize \n
in replacement strings; read on for a comprehensive overview of the differences between these two implementations.
Differences between GNU sed
(Linux) and BSD/macOS sed
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).
- Caveat: GNU
- 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.
- Control-character sequences such as
- (both versions): use only the
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.
- BSD
-i
sensibly turns on per-input-file line numbering in GNUsed
and recent versions of BSDsed
(e.g., on FreeBSD 10), but does NOT on macOS as of 10.15.
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.
- BSD
- Use of the
- Common features:
- If you restrict your
sed
scripts to what BSDsed
supports, they will generally work in GNUsed
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.
- If you restrict your
Guidelines for cross-platform support (macOS/BSD, Linux), driven by the stricter requirements of the BSD version:
Note that I'm using the shorthands macOS and Linux for 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 for 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 GNUsed
supports them (unless--posix
is used), BSDsed
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) (GNUsed
doesn't document-E
, but it does work there as an alias of-r
; newer version of BSDsed
, such as on FreeBSD 10, now also support-r
, but the macOS version as of 10.10 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 BSDsed
doesn't support them in extended regexes (but, curiously, does so in basic ones, where they are POSIX-mandated).
- word-boundary assertions, because they're platform-specific (e.g.,
Control-character escape sequences such as
\n
and\t
:- In regexes (both in patterns for 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 usingprintf
(e.g.,"$(printf '\t')"
).- Linux only:
sed 's/\t/-/' <<<$'a\tb' # -> 'a-b'
- macOS and Linux:
sed 's/'$'\t''/-/' <<<$'a\tb' # ANSI C-quoted string
sed 's/'"$(printf '\t')"'/-/' <<<$'a\tb' # command subst. with printf
- Linux only:
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/' <<<$'a-b' # -> 'a<tab>b'
sed 's/-/\n/' <<<$'a-b' # -> 'a<newline>b'
- macOS and Linux:
sed 's/-/'$'\t''/' <<<'a-b'
sed 's/-/'"$(printf '\t')"'/' <<<'a-b'
sed 's/-/\'$'\n''/' <<<'a-b'
Note that newlines need to be backslash-escaped so that they are properly interpreted as part of the replacement string and not as the end of the command, and that usingprintf
does not work for newlines since trailing newlines are removed by command substitutions ($(...)
).
- Linux only:
Ditto for the text arguments to the
i
anda
functions: do not use control-character sequences - see below.
- In regexes (both in patterns for selection and the first argument to the
- Labels and branching: labels as well as the label-name argument to the
b
andt
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' <<<$'a\nb'
- EITHER (actual newlines):
- Linux only:
- Functions
i
anda
for inserting/appending text: follow the function name by\
, followed either by a literal newline or a spliced-in$'\n'
before specifying the text argument.- Linux only:
sed '1 i new first line' <<<$'a\nb' # -> 'new first line<nl>a<nl>b'
- macOS and Linux:
sed -e '1 i\'$'\n''new first line' <<<$'a\nb'
- Note:
- Without
-e
, the text argument is inexplicably not newline-terminated on output in macOS (bug?). - Do not use control-character escapes such as
\n
and\t
in the text argument, as they're only supported on Linux. - If the text argument therefore has actual interior newlines,
\
-escape them. - If you want to place additional commands after the text argument, you must terminate it with an (unescaped) newline (whether literal or spliced in), or continue with a separate
-e
option (this is a general requirement that applies to all versions).
- Without
- Linux only:
Inside function lists (multiple function calls enclosed in
{...}
), be sure to also terminate the last function, before the closing}
, with;
.- Linux only:
sed -n '1 {p;q}' <<<$'a\nb' # -> 'a'
- macOS and Linux:
sed -n '1 {p;q;}' <<<$'a\nb'
With the
-f
option (to read commands from a file), only GNUsed
supports-
as a placeholder for stdin; use-f /dev/stdin
to portably read commands from stdin, including from here-documents (assuming your platform supports/dev/stdin
, which is typically the case nowadays).
GNU sed
-specific features missing from BSD sed
altogether:
GNU features you'll miss out on if you need to support both platforms:
Various regex-matching and substitution options (both in patterns for line selection and the first argument to the
s
function):- The
I
option for case-INsensitive regex matching (incredibly, BSDsed
doesn't support this at all). - The
M
option for multi-line matching (where^
/$
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
- The
Escape sequences
Substitution-related escape sequences such as
\u
in the replacement argument of thes///
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-CommandControl-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 followingaddr
, ... - 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'']
This might seem a little odd, but try:
sed -i '' 's/\\n/\
/g' test1.txt
I.e., use an actual newline instead of \n
.
The explanation is that you have a weird sed
!
For details, see the mac sed manual: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/sed.1.html
In the description of the s
command there, it says:
A line can be split by substituting a newline character into it. To specify
a newline character in the replacement string, precede it with a backslash.
Also, in the description of the -i
option, it says that the extension is not optional, and that if you don't want one you must specify an empty argument. So everything makes sense in the end!