Ubuntu ships with GNUsed, where the suffix for the-i option is optional. OS X ships with BSDsed, where the suffix is mandatory. Trysed -i ''

To complementmicrotherion's helpful, to-the-point answer:

• with a portable solution
• with background information

tl;dr:

The equivalent of thisGNUsed (standard on mostLinux distros) command:

sed -i    's/foo/bar/' file

is thisBSD/macOSsed command:

sed -i '' 's/foo/bar/' file  # Note the '' as a *separate argument*

WithBSD/macOSsed, the following commandsdonot work at all or not as intended:

sed -i    's/foo/bar/' file  # Breaks; script is misinterpreted as backup-file suffixsed -i''  's/foo/bar/' file  # Dittosed -i -e 's/foo/bar/' file  # -e is misinterpreted as backup-file suffix

For a discussion ofall the differences between GNUsed and BSD/macOSsed, seethis answer of mine.

Portable approach:

^{Note:Portable here means that the command works with both implementations discussed. It is not portable in aPOSIX sense, becausethe-i option is not POSIX-compliant.}

# Works with both GNU and BSD/macOS Sed, due to a *non-empty* option-argument:# Create a backup file *temporarily* and remove it on success.sed -i.bak 's/foo/bar/' file && rm file.bak

For an explanation, see below; for alternative solutions, including a POSIX-compliant one, seethis related answer of mine.

Background information

InGNUsed (standard on most Linux distros) andBSD/macOSsed,the-i option, which performsin-place updating^[1] of its input files, accepts anoption-argument that specifies whatsuffix (filename extension) to use for thebackup file of the file being updated.

E.g., inboth implementations, the following keeps the original file,file, as backup filefile.bak:

sed -i.bak 's/foo/bar/' file  # Keep original as 'file.bak'; NO SPACE between -i and .bak

Even thoughwithGNUsed the suffix argument isoptional, whereas withBSD/macOSsed it ismandatory, the above syntax works withboth implementations, becausedirectly adjoining the option-argument (.bak) to the option (-i) --i.bak, as opposed to-i .bak - works both as anoptional and amandatory option-argument:

• Syntax-i.bak is theonly form that works for anoptional option-argument.
• Syntax-i.bakalso works as amandatory option-argument, as analternative to-i .bak, i.e., specifying the option and its argumentseparately.

Not specifying a suffix - which is often the case - means thatno backup file should be kept, and that is wherethe incompatibility arises:

• WithGNUsed, not specifying a suffix means simply using-iby itself.

• WithBSD/macOSsed, not specifying a suffix means specifyingthe empty string as the - mandatory - suffix, andfortechnical reasons, the empty string can only be passed as aseparate argument: that is,-i ''not-i''.

-i'' doesn't work, because tosed it is indistinguishable from just-i, because theshell effectivelyremoves the empty quotes (it concatenates-i and'' and removes quotes with syntactical function), and passes just-i inboth cases.

With (effectively) just-i specified, it is thenext argument that is interpreted as the option-argument:

sed -i 's/foo/bar/' file # BREAKS with BSD/macOS Sed

's/foo/bar/' - intended to the the Sedscript (command) - is now interpreted as thesuffix, and the wordfile is interpreted as the script.
Interpreting such a word as script then leads to obscure error message such as
sed: 1: "file": invalid command code f,
because thef is interpreted as a Sed command (function).

Similarly, with:

sed -i -e 's/foo/bar/' file # CREATES BACKUP FILE 'file-e'

-e is interpreted as thesuffix argument, and NOT as Sed's-e option (which can be used to specifymultiple commands, if needed).
As as result,instead of keeping NO backup, you get a backup file with suffix-e.

That this command doesn't work as intended is less obvious, because the in-place updating does succeed, given that the syntax requirement of the suffix argument is satisfied by the-e argument.

That the accidental creation of these backup files easily goes unnoticed is the likeliest explanation forCrt's incorrect answer andthis incorrect answer to a similar question having received so many up-votes (as of this writing).

^{[1] Strictly speaking, a temporary file is created behind the scenes that thenreplaces the original file; this approach can be problematic: see the bottom half ofthis answer of mine.}

man is your friend.

OS X

 -i extension         Edit files in-place, saving backups with the specified extension.         If a zero-length extension is given, no backup will be saved.  It         is not recommended to give a zero-length extension when in-place         editing files, as you risk corruption or partial content in situ-         ations where disk space is exhausted, etc.

In OS X, you can use the GNU version of sed:gsed.

# if using brewbrew install gnu-sed#if using portssudo port install gsed

Then, if your script should be portable, depending on your OS you can define which command to use.

SED=sedunamestr=`uname`if [[ "$unamestr" == "Darwin" ]] ; then    SED=gsed    type $SED >/dev/null 2>&1 || {        echo >&2 "$SED it's not installed. Try: brew install gnu-sed" ;        exit 1;    }fi# here your sed command, e.g.:$SED -i "/ $domain .*#drupalpro/d" /etc/hosts

• linux
• macos
• sed
• bsd
• inplace-editing