utility reads the specified files, or the
standard input if no files are specified, modifying the input as specified by
a list of commands. The input is then written to the standard output.
A single command may be specified as the first argument to
. Multiple commands may be specified separated
by newlines or semicolons, or by using the -e
options. All commands are applied to the input
in the order they are specified regardless of their origin.
The options are as follows:
- The files listed as parameters for the
w function or flag are created (or truncated)
before any processing begins, by default. The
-a option causes
sed to delay opening each file until a
command containing the related w function or
flag is applied to a line of input.
- Interpret regular expressions using POSIX extended regular
expression syntax. The default behaviour is to use POSIX basic regular
- Append the editing commands specified by the
command argument to the list of
- Append the editing commands found in the file
command_file to the list of commands. The
editing commands should each be listed on a separate line.
- 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
it risks corruption or partial content in situations where disk space is
- An alias for -E, for
compatibility with GNU sed.
- By default, each line of input is echoed to the standard
output after all of the commands have been applied to it. The
-n option suppresses this behavior.
- Force output to be line buffered, printing each line as it
becomes available. By default, output is line buffered when standard
output is a terminal and block buffered otherwise. See
setvbuf(3) for a more
The form of a sed
command is as follows:
Whitespace may be inserted before the first address and the function portions of
cyclically copies a line of input,
not including its terminating newline character, into a
, (unless there is something left
after a D
function), applies all of the commands
with addresses that select that pattern space, copies the pattern space to the
standard output, appending a newline, and deletes the pattern space.
Some of the functions use a hold space
to save all
or part of the pattern space for subsequent retrieval.
An address is not required, but if specified must be a number (that counts input
lines cumulatively across input files), a dollar character
’) that addresses the last line of
input, or a context address (which consists of a regular expression preceded
and followed by a delimiter).
A command line with no addresses selects every pattern space.
A command line with one address selects all of the pattern spaces that match the
A command line with two addresses selects the inclusive range from the first
pattern space that matches the first address through the next pattern space
that matches the second. (If the second address is a number less than or equal
to the line number first selected, only that line is selected.) Starting at
the first line following the selected range, sed
starts looking again for the first address.
Editing commands can be applied to non-selected pattern spaces by use of the
exclamation character (‘
By default, sed
regular expressions are basic
regular expressions (BREs). Extended regular expressions are supported using
options. See re_format(7)
more information on regular expressions. In addition,
has the following two additions to BREs:
- In a context address, any character other than a backslash
\’) or newline character may be
used to delimit the regular expression. The opening delimiter should be
preceded by a backslash unless it is a slash. Putting a backslash
character before the delimiting character causes the character to be
treated literally. For example, in the context address \xabc\xdefx, the RE
delimiter is an ‘x’ and the second ‘x’ stands
for itself, so that the regular expression is “abcxdef”.
- The escape sequence \n matches a newline character
embedded in the pattern space. You can't, however, use a literal newline
character in an address or in the substitute command.
One special feature of sed
regular expressions is
that they can default to the last regular expression used. If a regular
expression is empty, i.e., just the delimiter characters are specified, the
last regular expression encountered is used instead. The last regular
expression is defined as the last regular expression used as part of an
address or substitute command, and at run-time, not compile-time. For example,
the command “/abc/s//XXX/” will substitute “XXX”
for the pattern “abc”.
In the following list of commands, the maximum number of permissible addresses
for each command is indicated by [0addr], [1addr], or [2addr], representing
zero, one, or two addresses.
The argument text
consists of one or more
lines. To embed a newline in the text, precede it with a backslash. Other
backslashes in text are deleted and the following character taken literally.
as well as the w
flag to the
function, take an optional
parameter, which should be separated
from the function or flag by whitespace. Files are created (or their contents
truncated) before any input processing begins.
functions all accept additional arguments. The
synopses below indicate which arguments have to be separated from the function
letters by whitespace characters.
Functions can be combined to form a function list
a list of sed
functions each followed by a
newline, as follows:
The braces can be preceded and followed by whitespace. The functions can be
preceded by whitespace as well.
Functions and function lists may be preceded by an exclamation mark, in which
case they are applied only to lines that are not
selected by the addresses.
- [2addr] function-list
- Execute function-list only
when the pattern space is selected.
- [1 addr] a\
- Write text to standard
output immediately before each attempt to read a line of input, whether by
executing the N function or by beginning a
- Branch to the : function with
the specified label. If the label is not
specified, branch to the end of the script.
- [2addr] c\
- Delete the pattern space. With 0 or 1 address or at the end
of a 2-address range, text is written to
the standard output.
- Delete the pattern space and start the next cycle.
- Delete the initial segment of the pattern space through the
first newline character and start the next cycle.
- Replace the contents of the pattern space with the contents
of the hold space.
- Append a newline character followed by the contents of the
hold space to the pattern space.
- Replace the contents of the hold space with the contents of
the pattern space.
- Append a newline character followed by the contents of the
pattern space to the hold space.
- [1addr] i\
- Write text to the standard
- (The letter ell.) Write the pattern space to the standard
output in a visually unambiguous form. This form is as follows:
Non-printable characters are written as three-digit octal numbers (with a
preceding backslash) for each byte in the character (most significant byte
first). Long lines are folded, with the point of folding indicated by
displaying a backslash followed by a newline. The end of each line is
marked with a ‘
- vertical tab
- Write the pattern space to the standard output if the
default output has not been suppressed, and replace the pattern space with
the next line of input.
- Append the next line of input to the pattern space, using
an embedded newline character to separate the appended material from the
original contents. Note that the current line number changes.
- Write the pattern space to standard output.
- Write the pattern space, up to the first newline character,
to the standard output.
- Branch to the end of the script and quit without starting a
- Copy the contents of file
to the standard output immediately before the next attempt to read a line
of input. If file cannot be read for any
reason, it is silently ignored and no error condition is set.
- Substitute the replacement
string for the first instance of the regular expression
RE in the pattern space. Any character
other than backslash or newline can be used instead of a slash to delimit
the regular expression and the replacement. Within the regular expression
and the replacement, the regular expression delimiter itself can be used
as a literal character if it is preceded by a backslash.
An ampersand (‘
&’) appearing in
the replacement is replaced by the string matching the regular expression.
The special meaning of ‘
this context can be suppressed by preceding it by a backslash. The string
#’ is a digit, is replaced by the
text matched by the corresponding backreference expression (see
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
The value of flags in the substitute
function is zero or more of the following:
- Make the substitution only for the
N'th occurrence of the regular
expression in the pattern space, where
N is a positive integer starting with
- Make the substitution for all non-overlapping matches
of the regular expression, not just the first one.
- Write the pattern space to standard output if a
replacement was made. If the replacement string is identical to that
which it replaces, it is still considered to have been a
- Append the pattern space to
file if a replacement was made. If
the replacement string is identical to that which it replaces, it is
still considered to have been a replacement.
- Branch to the : function
bearing the label if any substitutions
have been made since the most recent reading of an input line or execution
of a t function. If no label is specified,
branch to the end of the script.
- Append the pattern space to the
- Swap the contents of the pattern and hold spaces.
- Replace all occurrences of characters in
string1 in the pattern space with the
corresponding characters from string2.
Any character other than a backslash or newline can be used instead of a
slash to delimit the strings. Within
string2, a backslash followed by any
character other than a newline is that literal character, and a backslash
followed by an ‘n’ is replaced by a newline character.
- This function does nothing; it bears a
label to which the
b and t commands
- Write the line number to the standard output followed by a
- Empty lines are ignored.
- The ‘
#’ and the
remainder of the line are ignored (treated as a comment), with the single
exception that if the first two characters in the file are
#n’, the default output is
suppressed. This is the same as specifying the
-n option on the command line.
- If set to a positive integer, output from the
l function is formatted to the given width in
columns. Otherwise, sed defaults to the
terminal with, or 80 columns if the output is not a terminal.
utility exits 0 on success,
and >0 if an error occurs.
The following simulates the cat(1)
command, squeezing excess empty lines from
$ sed -n '
# Write non-empty lines.
# Write a single empty line, then look for more empty lines.
# Get the next line, discard the held <newline> (empty line),
# and look for more empty lines.
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
utility is compliant with the
IEEE Std 1003.1-2008
The flags [-aEiru
extensions to that specification.
The use of newlines to separate multiple commands on the command line is
non-portable; the use of newlines to separate multiple commands within a
command file (-f
) is portable.
command appeared in
Version 7 AT&T UNIX
The use of semicolons to separate multiple commands is not permitted for the
following commands: a
, and #