IT 341: Introduction to System Administration

Using sed
• Maintaining Configuration • Inserting Text at the
Files Beginning of a File
• The sed Stream Editor • Making More Than One
• Instructions to sed Change with sed
o Replacing a String with sed • Using Character Ranges
o Adding a Line with sed
with sed
o Deleting a Line with sed
• Changing File Text Inline
with sed
 Maintaining Configuration Files
• Unix system administrators spend much of their time working
with text files
• Text files need to be configured for most major Unix services
and these files they need to be maintained
• In any working network things are always changing and the
text files that configure services must change too
• Most of these changes are straight-forward add a line delete
line or change a name
 Maintaining Configuration Files
• Changing the name of something often involves making
changes in more than one configuration file
• If you fail to make the needed changes in all affected files
some important services will stop working
• System administrators need a way to make changes quickly
and accurately in many files at the same time
• The sed stream editor helps to automate this task
 The sed Stream Editor

• sed allows you to automate the process of editing a file

• With sed you can write a line or two of code that will make
multiple changes to a file
• sed takes input from one source makes changes to the text
from that source and sends the altered text to standard output
• sed is called a stream editor because it can be used in a
pipeline to transform a stream of data
 The sed Stream Editor
• sed takes input from its second argument or standard input and
sends the transformed text to standard output
• sed can be used to modify a file by redirecting standard input to
come from that file and redirecting standard output to go to a new
file
• The general form of a sed command is
sed 'SED_INSTRUCTIONS' INPUT_FILE > OUTPUT_FILE
 Instructions to sed
• The first argument to sed is a sed command telling it
how to modify the file
• These commands take many forms and should be
contained in single quotes
• You will find a good introduction to the sed commands
at http://www.grymoire.com/Unix/Sed.html
• A glance at this page will show that sed commands can
be complex
 Instructions to sed
• But while sed has a lot of power and can do many
things most people only use a few simple commands
• sed is most frequently used to:
Substitute one string for another
Add a line
Delete a line
 Replacing a String with sed

• The most common use of sed is to replace one string with

another
• The format for the sed command to do this is
s/OLD_STRING/NEW_STRING/
• The s tells sed you want it to substitute
• The / is used to set off two patterns
• The first pattern is what needs to be replaced and the second
pattern is what it will be replaced with
 Replacing a String with sed

• If we have a text file day.txt containing

day
I am looking for daylight
night and day
day after day after day
• We could replace instances of "day" with "night" with the
following
sed 's/day/night/' day.txt
 Replacing a String with sed
• Running this, we get
$ sed 's/day/night/' day.txt
night
I am looking for nightlight
night and night
night after day after day
• The output from sed goes to standard output unless you
redirect it
• Notice the last line. Only the first instance of "day" was
replaced with "night"
 Replacing a String with sed
• Normally the s command only replaces the first string it
finds on a line
• If we want sed to replace every string on the line we must
follow the last pattern with the g option
$ sed s/day/night/g day.txt
night
I am looking for nightlight
night and night
night after night after night
• The g stands for global
 Delimiting Patterns

• A delimiter is a sequence of one or more characters used to

mark the boundary between different parts of a string
• The delimiter most often used to mark off patterns in sed is
the slash, /
• But this can cause problems in certain situations
• In Unix and Linux, / is used to separates the names of
directories in a path
/courses/it341/f13/ghoffmn
 Delimiting Patterns
• How could you use sed to replace "f13/ghoffmn" with
"s14/abird" in the path above using / as a delimiter?
• But you don't have to use /
• The s command uses the very next character as the
delimiter
• So if course_directory.txt contained the path above I could
change the text with the following sed instruction
's|f13/ghoffmn|s14/abird|'
 Delimiting Patterns

• You have to put the instructions in single quotes, '' because | is

the pipe character and has special meaning to the shell

$ cat course_directory.txt
/courses/it341/f13/ghoffmn

$ sed 's|f13/ghoffmn|s14/abird|' course_directory.txt

/courses/it341/s14/abird
 Adding a Line with sed
• sed can be used to add a line of text to a file
• The format for this sed instruction is
/STRING/a\
LINE_OF_TEXT
• The \ turns off the special meaning of the newline
which is to signal to the shell that you are done with
the command
• This command will add the line of text after every line
that contains STRING
 Adding a Line with sed
• When you write a sed instruction to add a line you
must include all of the instructions inside single quotes
• To add a line after the second line of day.txt
day
I am looking for daylight
night and day
day after day after day
 Adding a Line with sed
• …we could use sed like this:
$ sed ' /looking/a \
> and looking and looking \
> ' day.txt
day
I am looking for daylight
and looking and looking
all the live long day
night and day
day after day after day
 Adding a Line with sed
• Notice that after I hit the Return or Enter to move down to
the second line the shell prints >
• This is the secondary prompt that you get when you
continue a command onto another line
• Notice that I had to put the name of the file day.txt outside
the quote
• The second \ turns off the special meaning of newline at
the end of the added text – allowing me to add a newline
character at the end of my text
 Adding a Line with sed
• If I did not do this, the new line would be inserted on
the same line as the text that follows
$ sed ' /looking/a \
> and looking and looking' day.txt
day
I am looking for daylight
and looking and lookingall the live long day
night and day
day after day after day
 Adding a Line with sed
• sed will add text after every line that matches a pattern
$ sed ' /day/a \
> a new line of text \
> ' day.txt
day
a new line of text
I am looking for daylight
a new line of text
all the live long day
a new line of text
night and day
a new line of text
day after day after day
a new line of text
 Adding a Line with sed
• You can add more than one line in this way as long as
you put a \ at the end of each line
$ sed '/looking/a \
> first line after looking\
> second line after looking\
> third line after looking\
> ' day.txt
day
I am looking for daylight
first line after looking
second line after looking
third line after looking
night and day
day after day after day
 Deleting a Line with sed
• You can also use sed to delete a line
• The delete instruction has the following format
/STRING/d
• where STRING is some text that can only be found on the
line to be deleted
• To delete the last line of day.txt…we could use:
day $ sed /after/d day.txt
I am looking for daylight day
night and day I am looking for daylight
day after day after day
night and day
 Inserting Text at the Beginning of a File

• The a command add text after a line which contains

some string
• But what if you wanted to insert the text before a line
which contains a string?
• Then you would have to use the i command
• If we have the file fox.txt that contains
The quick brown fox
jumped over
the lazy dogs.
 Inserting Text at the Beginning of a File

• …and wanted to add a line at the beginning of the file we could do it

like this:
sed '/quick/ i\
> Here is something you have seen many times:' fox.txt
Here is something you have seen many times:
The quick brown fox
jumped over
the lazy dogs.
 Making More Than One Change with sed

• Each sed command changes one thing

• If you wanted to make several changes with a single
command line entry you can use pipes
• If we have the file colors.txt

red blue green

light red light blue light green
dark green dark red
 Making More Than One Change with sed

• …we can capitalize all the colors with the following:

sed 's/red/Red/' colors.txt | sed 's/green/Green/' | sed 's/blue/Blue/‘
Red Blue Green
light Red light Blue light Green
dark Green dark Red
 Using Character Ranges with sed
• sed let's you use ranges of characters to match strings
• Let's say we have the following in the file
letters_digits.txt
abcd 1234 5678
• If we wanted to replace every instance of the digits 1
through 4 with the single character x we could write
sed 's/[1-4]/x/g' letters_digits.txt
abcd xxxx 5678
 Using Character Ranges with sed
• Notice the g I had to add to the end of the substitution
command
• If I left this out, I would get
sed 's/[1-4]/x/' letters_digits.txt
abcd 234 5678
 Changing File Text in place with sed
• The default behavior of sed is to print the changes made to
standard output
• However, you may also use the –i option to tell sed to
make the change in place, inside the file itself
• Consider the file roses.txt:
$ cat roses.txt
Roses are red,
And violets are blue.
Sugar is sweet,
and so are you.
 Changing File Text in place with sed
• Now, execute this command:
sed -i 's/Roses/Poinsettias/' roses.txt
• You will not see the usual output, but if you look at the file
text again, it will be changed:
$ cat roses.txt
Poinsettias are red,
And violets are blue.
Sugar is sweet,
and so are you.
 Changing File Text in place with sed
• In addition, you may want to save a backup copy of the
original, which you can do by specifying a suffix when using
the –i option:
sed –i.bk '/violets/a\(In that case...\
...should you not call them blues?)' roses.txt
• First, you will now notice that there is a new file:
$ ls
roses.txt roses.txt.bk
 Changing File Text in place with sed
• The original file will reflect the changes made:
$ cat roses.txt
Poinsettias are red,
And violets are blue.
(In that case...
...should you not call them blues?)
Sugar is sweet,
and so are you.
• The backup file, however, will show the original file text
$ cat roses.txt.bk
Poinsettias are red,
And violets are blue.
Sugar is sweet,
and so are you.