linux crash course chapter 13 3

sed syntax• Syntax: sed [options] program [filelist] sed [options] program-file [filelist] • Program is a set of commands for editing – Can either be issued on the command line or place

Chapter 13:

sed

Say what?

In this chapter …

• Basics

• Programs

• Addresses

• Instructions

• Control

• Spaces

• Examples

• GNU sed (stream editor)

• Noninteractive, batch editing

• Good for repetitive tasks

• Often used in a pipe

sed syntax

• Syntax:

sed [options] program [filelist]

sed [options] program-file [filelist]

• Program is a set of commands for editing

– Can either be issued on the command line or

placed into a file (like gawk)

• Filelist is a list files to edit

– If omitted, input taken from standard in

sed syntax con’t

• Options

in-place[=suffix]

• Instead of sending edited text to standard out, write changes back to input file

• Adding =suffix makes backup of original file

-n

• Do not send lines to output unless program explicitly says to

• sed programs contain one or more lines with the following syntax:

[address[,address]] instruction [args]

• Simple one or two line programs can be

issued at the command line

• More complex programs are usually best put

in a program file

How sed works

1 Read one line of input

2 Read first instruction in program If the

address(es) select this line, runs the

instruction on this line

3 Repeat #2 for each line in the program

4 Read next line of input and go back to step

2, until there are no more lines of input

• Select which lines are to be processed

• Can be a simple integer (line number) or a regular expression (pattern matching)

• Address $ represents last line of input

• If address omitted, all lines processed by

default

• If there is one address, only lines that match will be processed

Addresses con’t

• If two addresses are given, it selects a range

• Once the first address is matched, it and

subsequent lines are processed until the

second address is matched

• If second address is never matched,

processes remainder of lines

• If second addressed matched, sed will then try to match first address again

• d – does not write out (deletes) selected line and does not process line any further

• n – writes out current line, reads next line, and processes next program line

• a – appends lines after current line

• i – inserts lines before current line

• c – changes select line so it contains new

text

• p – print current line (override –n)

Instructions con’t

• w file – write line to a specified file

• r file – read contents of file and appends

to current line

• q – quits sed immediately

Instructions con’t

• s/pattern/replacement-str/[g][p][w file]

– Substitutes first occurrence of pattern with

replacement-str

– g replaces all occurences

– p prints changed line

– w writes changed line to file

• Use & to represent the pattern matched when replacing

– Ex s/a.*/(a.*)/ won’t work … instead use

s/a.*/(&)/

Control Structures

• ! (NOT) – causes instruction to be performed

on all lines not selected by address(es)

• { } (Instruction grouping) – causes multiple

instructions to be run on one address /

address pair; separate with semicolons

• : label – identify a location in a sed program

• b label – branch to label

• t label – conditionally branch to label if last

Substitute instruction was successful

• sed has two spaces (buffers)

• Think of them like vim’s buffers

• Lines read from input are put in pattern

space

• You can also move data back and forth from

the hold space (temporary buffer)

Spaces, cont

• g – overwrites pattern space with hold space

• G – appends hold space to pattern space

• h – overwrites hold space with pattern space

• H – appends pattern space to hold space

• x – swaps the pattern and hold spaces

• sed -n‘/line/ p’ myfile

– Prints out lines in myfile that contain ‘line’

• sed ‘2,4 d’ myfile

– Delete lines 2-4, outputs remaining

• sed in-place ‘2,4 d’ myfile

– Deletes lines 2-4 from myfile

• sed ‘s/tea/coffee/g’ myfile

– Replaces tea with coffee and prints to screen

More Examples

• sed ‘5 q’ myfile

– Prints first five lines then quits ( equiv head -5)

• sed ‘/^[0-9]/ w newfile’ myfile

– Copies lines starting in number to newfile

• sed ‘$ r newfile’ myfile

– Appends contents of newfile to end of myfile

• sed ‘G’ myfile

– What does this do?

Program File Example

1 d

s/company/Company/g

$ a\

Revised 12-1-2005\

by JMH

$ d

Another Program File

1 i \

\

Manuf\tModel\tYear\tMiles\tPrice\

===================================== s/thundbd/tbird/g

s/.*/ &/

$ a \

=====================================