Unix Talk #2
(sed)
You have learned…
 Regular expressions, grep, & egrep
 grep & egrep are tools used to search for
text in a file
 AWK -- powerful
 What about SED?
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Things in common between awk and
sed
 They are invoked using similar syntax
 Stream-oriented (hence stream editor)
 Use regular expressions for pattern
matching
 Allow the user to specify instructions in a
script
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Common syntax
 Command [options] script filename
 Script
– You tell the program what to do, which is called
instructions
– Each instruction has two parts
 Pattern
 Procedure (actions)
– (Sound familiar?)
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SED
 A tool usually designed for a short line
substitution, deletion and print
 Allows for automation of editing process
similar to those you might find in vi or ex (or
ed)
 Non-destructive
 Reads in a stream of data from a file,
performs a set of actions, & outputs the
results
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SED
 SED reads the data one line at a time, make
a copy of the input line & places it in a buffer
called the pattern space
 Modifies that copy in the pattern space
 Outputs the copy to standard output
NOTE: 1.The pattern space holds the line of
text currently being processed
2. You don’t make changes to the original file
3. If you want to capture this output, what do
you do?
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SED Syntax
 Options
–n
 only prints matches
– f scriptfile
 run commands in scriptfile
–e
 allows multiple instructions on a single line
 Sed syntax
– Sed [option] ‘instruction’ file(s)
– Sed –f scriptfile file(s)
 Must give sed instruction or scriptfile
 Can use file redirection to create a new
file
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Sed options
 -e
– Only needed when you supply more than one
instruction on the command line
– sed –e ‘script’ –e ‘script’ file
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Print Command
 cat /home/fac/pub/fruit_prices
sed –n 'p' fruit_prices
What does it do?
Try with out the –n. What happens?
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Print Command
 Can specify zero, one or two addresses to print, the address can
be a line number or the pattern for matching
– sed –n '1p' fruit_prices
 The line counter does not reset for multiple input
files
– sed –n '$p' fruit_prices #prints last line
– sed –n '6,8p' #prints lines 8-10





sed
sed
sed
sed
sed
–n
–n
–n
–n
–n
'/^$/p' fruit_prices
'1, /^$/p' fruit_prices # range
'/App*/p' fruit_prices
'/^[0-1]/p' fruit_prices
'/[^7-9]$/p' fruit_prices
 What happens if you remove the caret?
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Print Command
 sed –n '/\$1\./p' fruit_prices
 sed –n '/1./p' fruit_prices
 Need to use –n when printing,
otherwise you get multiple
copies of lines
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Read data into a variable
 Create a script:
read –p “Enter fruit name: ” fruitName
sed –n “/$fruitName/p” fruit_prices
 Always surround you patterns with “” or ‘’ to
prevent problems
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Delete Command
 sed '/^A/d' fruit_prices
 Cat the file after you have run
the command. Is the line gone?
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Delete Command




sed ‘1d’ fruit_prices
sed ‘$d’ fruit_prices
sed ‘/^$/d’ fruit_prices
sed‘1,/^$/d’fruit_prices.txt >
newfile
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Substitute
 To change one pattern to another
 Syntax
– s/pattern/replacement/flags
 Flags
– n: A number (1 to 512) indicating that a
replacement should be made for only the
nth occurrence of the pattern
– g: Make changes globally on all
occurrences in the pattern space. Normally
only the first occurrence is replaces.
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Substitute
sed ‘s/Fruit/Fruit_list/’ fruit_prices
sed ‘s/a/A/g’ fruit_prices
Try the previous command without g
If you like to change the original file, you must do
copy and redirect to update original file
 cp fruit_prices fruit_prices.old
sed ‘s/Fruit/Fruit_list/’
fruit_prices.old>fruit_prices
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Substitute
 Reuse the matched string with ‘&’
 Sed ‘s/[0-9]\.[0-9][0-9]*/\$&/’ filename
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sed – the Stream Editor.1
 sed is an editor for editing data on the fly as part of
a pipeline
 Usage:
sed -e 'command' -e 'command' -e 'command' ...
– Reads stdin and applies the commands to each line in
the order they are on the command line
– Prints each line of stdin to stdout after the commands
have been applied
– Most commands are s/ . . . / . . . / commands




Can use regexps in the 1st part
Can use parentheses, back references in the 1st part
Can use & and \1 \2 \3 . . . in the second part
Can append ‘g’ to the s/ . . . / . . . / command to change all
occurrences on a line
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sed – the Stream Editor.2
 Examples:
– Print out all usernames from /etc/passwd
sed -e 's/^\([^:]*\):.*$/\1/' </etc/passwd
– Print out ONLY the hidden files in the working directory
# delete lines that do NOT begin with a period
ls -a | sed -e '/^[^.]/d‘
OR
# print ONLY lines that DO begin with a period
# NOTE: -n option suppresses printing unless
# indicated via a p command
ls -a | sed -n -e '/^\./p'
– Print out file names followed by the time of modification
# NOTE: -r option enables extended regexps WITHOUT
# the need to escape parentheses
ls -l | sed -r -e 's/^([^ ]+ +){5}//' \
-e 's/^(.*) (.*)/\2 -> \1/'
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Append
 a \ text
 Appends text to the line following the one
matched
sed ‘/^F/a\ #here is a list of the fruit’
fruit_prices
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Insert
 i \ text
 Inserts text prior to the line that is matched
sed ‘/^Pi/i\ Orange:
$1.99’
fruit_prices.old>fruit_prices
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Scripts
 When?
 Series of sed commands
 Syntax
sed –f scriptname datafilename
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Scripts
 Cat scriptTest
s/^A/a/g
s/^P/p/
 sed –f scriptTest fruit_prices
 sed –e ‘s/^A/a/g’ –e ‘s/^P/p/’ fruit_prices
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Using sed in a Pipeline
 id
 uid=500(yxp) gid=100(fac)
 id | sed ‘s/(.*$//’
 uid=500
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Using sed in a Pipeline
 cat fruit_prices | sed ‘s/A/a/g’
 ls –l | sed –n p
 echo Hello | sed p
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