COMP7004 - Systems Scripting

Lecture 09: SED

Dr. Vincent Emeakaroha

vincent.emeakaroha@mtu.ie

Semester 2, 2026

Sed: Stream-oriented, Non-

Interactive Text Editor

• Looks for patterns one line at a time, like grep. That is, it

operates per line contents of a file

• Non-interactive text editor

• Borrows syntax from the interactive editor ed which accepts

the same commands

• Can be used in many ways such as:

• Text substitution,

• Selective printing of text files,

• In-a-place editing of text files,

• Non-interactive editing of text files, and many more

sed Command Types and Syntax

• Syntax: sed [-n] [-e] [

‘

command

’

] [file…]

sed [-n] [-f scriptfile] [file…]

• -n - only output lines specified with the print command (or the

‘p’ flag of the substitute (‘s’) command)

• -f scriptfile - next argument is a filename containing editing

commands

• -e command - the next argument is an editing command rather

than a filename, useful if multiple commands are specified

• If the first line of a scriptfile is “#n”, sed acts as though -n had

been specified

sed Commands

• sed commands have the general form

• [address[, address]][!]command [arguments]

• sed copies each input line into a pattern space

• If the address of the command matches the line in the pattern space,

the command is applied to that line

• If the command has no address, it is applied to each line as it enters

pattern space

• If a command changes the line in pattern space, subsequent

commands operate on the modified line

• When all commands have been read, the line in pattern space is

written to standard output and a new line is read into pattern

space

sed Address

• An address can be either a line number or a pattern,

enclosed in slashes ( /pattern/ )

• A pattern is described using regular expressions

• Basic Regular Expression (BRE) as in grep

• If no pattern is specified, the command will be applied

to all lines of the input file

• To refer to the last line use: $ and the first line use: ^

sed Address

• Most commands will accept two addresses

• If only one address is given, the command operates only on

that line

• If two comma separated addresses are given, then the

command operates on a range of lines between the first and

second address, inclusively

• The ! operator can be used to negate an address, ie;

address!command causes command to be applied to all

lines that do not match address

sed Command

• command is a single letter

• Example: Deletion: d

• [address1][,address2]d

• Delete the addressed line(s) from the pattern space;

• The removed line(s) are not passed to standard output.

Address and Command Examples

• d deletes all lines

• 6d deletes line 6

• /^$/d deletes all blank lines

• 1,10d deletes lines 1 through 10

• 1,/^$/d deletes from line 1 through the first blank line

• /^$/,$d deletes from the first blank line through

the last line of the file

• /^$/,10d deletes from the first blank line through line 10

• /^ya*y/,/[0-9]$/d deletes from the first line that begins

with yay, yaay, yaaay, etc. through

the first line that ends with a digit

Examples

• sed -e 'd' /etc/services

• Outputs nothing

• sed -e '1d' /etc/services

• Delete first line and output the rest to standard output

• sed -e '1,10d' /etc/services

• Delete first 10 lines and output the rest

• sed -e '/^#/d' /etc/services

• Delete all lines starting with “#” and output the rest

sed Multiple Commands

• Braces {} can be used to apply multiple commands to an address

[/pattern/[,/pattern/]]{

command1

command2

command3

}

• Strange syntax:

• The opening brace must be the last character on a line

• The closing brace must be on a line by itself

• Make sure there are no spaces following the braces

Pattern Substitution

• Syntax: [address(es)]s/pattern/replacement/[flags]

• pattern - search pattern

• replacement - replacement string for pattern

• flags - optionally any of the following

• n a number from 1 to 512 indicating which

occurrence of pattern should be replaced

• g global, replace all occurrences of pattern in pattern space

• p print contents of pattern space

Substitute Example

• s/Puff Daddy/P. Diddy/

• Substitute P. Diddy for the first occurrence of Puff Daddy

in pattern space

• s/Tom/Dick/2

• Substitutes Dick for the second occurrence of Tom in the

pattern space

• s/wood/plastic/p

• Substitutes plastic for the first occurrence of wood and

outputs (prints) pattern space

Using !

• If an address is followed by an exclamation point (!), the

associated command is applied to all lines that don’t match

the address or address range

• Examples:

• 1,5!d would delete all lines except 1 through 5

• /black/!s/cow/horse/ would substitute “horse” for

“cow” on all lines except those that contained “black”

“The brown cow” -> “The brown horse”

“The black cow” -> “The black cow”

Quit

• Quit causes sed to stop reading new input lines and stop

sending them to standard output

• It takes at most a single line address

• Once a line matching the address is reached, the script will be

terminated

• This can be used to save time when you only want to process

some portion of the beginning of a file

• Example: to print the first 100 lines of a file (like head) use:

• sed '100q' filename

• sed will, by default, send the first 100 lines of filename to

standard output and then quit processing

Manipulating Files

• sed prints modified content of a file to the screen but does NOT

change the file

• Two ways to change this:

• Use “-i” option for GNU compatible sed

• Output redirection

• Example

• sed -e 's/coat/bag/g' fileEg01 > fileEg03

• Changes all occurrences of “coat” with “bag” in file “fileEg01”

• The modified output is saved in a new file “fileEg03”

• File “fileEg01” remains unchanged

• sed -i -e 's/coat/bag/g' fileEg01 (Changes the content of the file)

Thank You!