How to use sed to replace only the first occurrence in a file?

sed '0,/pattern/s/pattern/replacement/' filename

this worked for me.

example

sed '0,/<Menu>/s/<Menu>/<Menu><Menu>Sub menu<\/Menu>/' try.txt > abc.txt

^{Editor's note: both work with GNU sed only.}

A sed script that will only replace the first occurrence of "Apple" by "Banana"

Example

     Input:      Output:

     Apple       Banana
     Apple       Apple
     Orange      Orange
     Apple       Apple

This is the simple script: ^{Editor's note: works with GNU sed only.}

sed '0,/Apple/{s/Apple/Banana/}' input_filename

The first two parameters 0 and /Apple/ are the range specifier. The s/Apple/Banana/ is what is executed within that range. So in this case "within the range of the beginning (0) up to the first instance of Apple, replace Apple with Banana. Only the first Apple will be replaced.

Background: In traditional sed the range specifier is also "begin here" and "end here" (inclusive). However the lowest "begin" is the first line (line 1), and if the "end here" is a regex, then it is only attempted to match against on the next line after "begin", so the earliest possible end is line 2. So since range is inclusive, smallest possible range is "2 lines" and smallest starting range is both lines 1 and 2 (i.e. if there's an occurrence on line 1, occurrences on line 2 will also be changed, not desired in this case). GNU sed adds its own extension of allowing specifying start as the "pseudo" line 0 so that the end of the range can be line 1, allowing it a range of "only the first line" if the regex matches the first line.

Or a simplified version (an empty RE like // means to re-use the one specified before it, so this is equivalent):

sed '0,/Apple/{s//Banana/}' input_filename

And the curly braces are optional for the s command, so this is also equivalent:

sed '0,/Apple/s//Banana/' input_filename

All of these work on GNU sed only.

You can also install GNU sed on OS X using homebrew brew install gnu-sed.

An overview of the many helpful existing answers, complemented with explanations:

^{The examples here use a simplified use case: replace the word 'foo' with 'bar' in the first matching line only.
Due to use of ANSI C-quoted strings ($'...') to provide the sample input lines, bash, ksh, or zsh is assumed as the shell.}

---

GNU sed only:

Ben Hoffstein's anwswer shows us that GNU provides an extension to the POSIX specification for sed that allows the following 2-address form: 0,/re/ (re represents an arbitrary regular expression here).

0,/re/ allows the regex to match on the very first line also. In other words: such an address will create a range from the 1st line up to and including the line that matches re - whether re occurs on the 1st line or on any subsequent line.

• Contrast this with the POSIX-compliant form 1,/re/, which creates a range that matches from the 1st line up to and including the line that matches re on subsequent lines; in other words: this will not detect the first occurrence of an re match if it happens to occur on the 1st line and also prevents the use of shorthand // for reuse of the most recently used regex (see next point).¹

If you combine a 0,/re/ address with an s/.../.../ (substitution) call that uses the same regular expression, your command will effectively only perform the substitution on the first line that matches re.
sed provides a convenient shortcut for reusing the most recently applied regular expression: an empty delimiter pair, //.

$ sed '0,/foo/ s//bar/' <<<$'1st foo\nUnrelated\n2nd foo\n3rd foo' 
1st bar         # only 1st match of 'foo' replaced
Unrelated
2nd foo
3rd foo

---

A POSIX-features-only sed such as BSD (macOS) sed (will also work with GNU sed):

Since 0,/re/ cannot be used and the form 1,/re/ will not detect re if it happens to occur on the very first line (see above), special handling for the 1st line is required.

MikhailVS's answer mentions the technique, put into a concrete example here:

$ sed -e '1 s/foo/bar/; t' -e '1,// s//bar/' <<<$'1st foo\nUnrelated\n2nd foo\n3rd foo'
1st bar         # only 1st match of 'foo' replaced
Unrelated
2nd foo
3rd foo

Note:

• The empty regex // shortcut is employed twice here: once for the endpoint of the range, and once in the s call; in both cases, regex foo is implicitly reused, allowing us not to have to duplicate it, which makes both for shorter and more maintainable code.

• POSIX sed needs actual newlines after certain functions, such as after the name of a label or even its omission, as is the case with t here; strategically splitting the script into multiple -e options is an alternative to using an actual newlines: end each -e script chunk where a newline would normally need to go.

1 s/foo/bar/ replaces foo on the 1st line only, if found there. If so, t branches to the end of the script (skips remaining commands on the line). (The t function branches to a label only if the most recent s call performed an actual substitution; in the absence of a label, as is the case here, the end of the script is branched to).

When that happens, range address 1,//, which normally finds the first occurrence starting from line 2, will not match, and the range will not be processed, because the address is evaluated when the current line is already 2.

Conversely, if there's no match on the 1st line, 1,// will be entered, and will find the true first match.

The net effect is the same as with GNU sed's 0,/re/: only the first occurrence is replaced, whether it occurs on the 1st line or any other.

---

NON-range approaches

potong's answer demonstrates loop techniques that bypass the need for a range; since he uses GNU sed syntax, here are the POSIX-compliant equivalents:

Loop technique 1: On first match, perform the substitution, then enter a loop that simply prints the remaining lines as-is:

$ sed -e '/foo/ {s//bar/; ' -e ':a' -e '$!{n;ba' -e '};}' <<<$'1st foo\nUnrelated\n2nd foo\n3rd foo'
1st bar
Unrelated
2nd foo
3rd foo

Loop technique 2, for smallish files only: read the entire input into memory, then perform a single substitution on it.

$ sed -e ':a' -e '$!{N;ba' -e '}; s/foo/bar/' <<<$'1st foo\nUnrelated\n2nd foo\n3rd foo'
1st bar
Unrelated
2nd foo
3rd foo

---

^{1 1.61803 provides examples of what happens with 1,/re/, with and without a subsequent s//:}

• sed '1,/foo/ s/foo/bar/' <<<$'1foo\n2foo' yields $'1bar\n2bar'; i.e., both lines were updated, because line number 1 matches the 1st line, and regex /foo/ - the end of the range - is then only looked for starting on the next line. Therefore, both lines are selected in this case, and the s/foo/bar/ substitution is performed on both of them.
• sed '1,/foo/ s//bar/' <<<$'1foo\n2foo\n3foo' fails: with sed: first RE may not be empty (BSD/macOS) and sed: -e expression #1, char 0: no previous regular expression (GNU), because, at the time the 1st line is being processed (due to line number 1 starting the range), no regex has been applied yet, so // doesn't refer to anything.
  With the exception of GNU sed's special 0,/re/ syntax, any range that starts with a line number effectively precludes use of //.

 # sed script to change "foo" to "bar" only on the first occurrence
 1{x;s/^/first/;x;}
 1,/foo/{x;/first/s///;x;s/foo/bar/;}
 #---end of script---

or, if you prefer: ^{Editor's note: works with GNU sed only.}

sed '0,/foo/s//bar/' file 

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