'(updating) sed, a stream editor'

0. abstract

sed, a stream editor, is able to perform basic text transformation on an input text stream, which is a file or input from pipeline. The text transformation pattern performed by sed is called SCRIPT here.

1. running sed

1.1 overview

Normally sed is invoked like this:

sed SCRIPT INPUT_FILE...

For instance, to replace all "hello" with "world" in the input file:

sed 's/hello/world/' input.txt > output.txt

If no INPUT_FILE is specified, or specified as -, then sed filters the contents of the standard input and stream out the transformation result as you return text to standard input.

sed 's/hello/world/' input.txt
sed 's/hello/world/' < input.txt
cat input.txt | sed 's/hello/world/'
cat input.txt | sed 's/hello/world/' - # Dylan: still not know the difference between line 3 and 4, with the same text result.

sed stream the result to standard output. With -i option specified, it edits the file in-place instead of printing to standard out. By default sed prints all processed input. Use -n to suppress output, and p to print specific line.

# print the 45th of the input file
sed -n '45p' input.txt

If -e or -f options are used to specify a SCRIPT, all non-option parameters are taken as input files.

sed 's/hello/world/' input.txt

sed -e 's/hello/world/' input.txt
sed --expression='s/hello/world/' input.txt

echo 's/hello/world/' > myscript.sed
sed -f myscript.sed input.txt
sed --file=myscript.sed input.txt

1.2 command-line options

The full format for invoking sed is:

sed OPTIONS... SCRIPT INPUTFILE...

sed may be invoked with the following command-like options:

1.3 exit status

2. sed scripts

2.1 scripts overview

A sed program consists of one or more sed commands, passed in by one or more of the -e, -f, --expression, and --file options, or the first non-option argument if zero of there options are used. This document will refer to "the" sed script; this is understood to mean the in-order concatenation of all of the SCRIPTS and SCRIPTS_FILE passed in.

sed commands follow this syntax:

[addr]X[options]

X is a single-letter sed command. [addr] is an optional line address. If [addr] is specified, the command X will be executed onl on the matched lines. [addr] can be a single line number, a regular expression, or a range of lines. Additional [options] are used for some sed commands.

For instance, the following examples deleted lines 30 to 35 in the input. 30,35is an address range. d is the delete command:

sed '30,35d' input.txt

For another instance, the following example prints all input until a line starting with 'foo' is found. If such line is found, sed will terminate with exit status 42. If such line was not found (and no other error occurred), sed will exit with status 0.

sed '/^foo/q42' input.txt

Commands with a SCRIPT or SCRIPT_FILE can be separated by semicolons or newlines(ASCII 10). Multiple scripts can be specified with -e or -f.

The following examples are all equivalent. They perform two sed operations: delete any lines matching the regular expression /^foo/, and replacing all occurrences of the string 'hello' with 'world':

sed '/^foo/d; s/hello/world/' input.txt

sed -e '/^foo/d' -e 's/hello/world/' input.txt

echo '/^foo/d' > script.sed
echo 's/hello/world/' >> script.sed
sed -f script.sed input.txt

echo 's/hello/world/' > script2.sed
sed -e '/^foo/d' -f script2.sed input.txt

Note that commands a, c, i, due to their syntax, cannot be followed by semicolons working as command separators and thus should be terminated with newlines or be placed at the end of a SCRIPT or SCRIPT_FILE. Commands can also be preceded with optional non-significant whitespace characters.

2.2 commands list

The following commands, some are standard POSIX commands while other are GNU extensions, are supported in GNU sed. Mnemonics are shown in parentheses.

2.3 the "s" command

The s command, as in substitute, is probably the most important in sed and has a lot of different options. The syntax of it is 's/REGEXP/REPLACEMENT/FLAGS'.

The REPLACEMENT can contain \n (n being a number from 1 to 0, inclusive) references, which refer to the portion of the match which is contained btween the nth ( and its matchiing ). Also, the REPLACEMENT can contain unescaped & which reference the whole matched portion of the pattern space.

As a GNU sed extension, you can include a special sequence made of a backslash and one of the letters L, l, U, u, or E. The meaning is as follows:

common commands

other commands

programming commands

extended commands

multiple commands syntax