Command Format: sed [-n] [-e script] [-f sfile] [files]
Details in on-line man pages
The sed utility copies the named files (standard input default) to
the standard output, edited according to a set (script) of
commands. The -f options cause the script to be taken from file
"sfile".
The general form is:
$sed /address/instruction
Note: If no address is specified, all lines are chosen to edit.
'sed' addresses can be line numbers or regular expressions.
Example:
line numbers 2,4
2,$ ($ represents the last line)
textual address /regular-expression/
Note: Forward slashes enclose textual addresses
The sed instructions indicate what editing function to perform.
Here some useful sed instructions:
s substitute
d delete
Note: Most sed command lines contain spaces or metacharacters and
they should be quoted to protect them from the shell. There are
many more editing commands provided by sed. Here is a sample sed
command to edit the records in the database file that we are
already familiar with, namely, phone.lis:
Sample session:
$sed /s/Smith/Smythe/ phone.lis
Smythe, Joan 7-7989
Adams, Fran 2-3876
StClair, Fred 4-6122
Jones, Ted 1-3745
Stair, Rich 5-5972
Benson, Sam 4-5587
$
sed is an editor . It simply copies the standard input to the
standard output, editing the lines that match the indicated
address. The original file is not changed.
Here's another example of a sed command.
Sample session:
$sed '2,4 s/2$/3/' phone.lis
Smith, Joan 7-7989
Adams, Fran 2-3876
StClair, Fred 4-6123
Jones, Ted 1-3745
Stair, Rich 5-5972
Benson, Sam 4-5587
$
What does this sed command do? If you read command in English it
reads like this: On lines 2 through 4 substitute the 2 at the end
of the line with a 3. Notice that the phone number for
StClair,Fred changed from 4-6122 to 4-6123. The number for
Stair,Rich didn't change because it was outside the range.
The sed utility can also be use to delete parts of a line of data.
This is done by substituting nothing for the parts you want to
delete. It looks like this:
Sample session:
$sed 's/^.*, //' phone.lis
Joan 7-7989
Fran 2-3876
Fred 4-6122
Ted 1-3745
Rich 5-5972
Sam 4-5587
$
Reading this command it means:
Substitute from the beginning of the line followed by any number
of characters followed by a comma with the null string (nothing).
This has the effect of removing the text.
Here's a delete command and how it's used.
Sample session:
$sed d phone.lis
$
Why is there no output? Well, it read standard input and did the
editing function on all the selected lines. Since no lines were
specified all lines were selected to be edited. The editing was to
delete the line.
Question: Has the original file been destroyed?
Multiple commands are allowed in sed. Each instruction is applied
to each input line.
Sample session:
$sed '/Stair/d
>/Adams/d' phone.lis
Smith, Joan 7-7989
StClair, Fred 4-6122
Jones, Ted 2-1136
Benson, Sam 4-5587
$
The records for Adams and Stair have both been removed from the
database.
Note: The > character is the BourneShell secondary prompt.
The awk will scan each line of file for lines that match a set of
patterns specified by prog. With each pattern in prog there can be
an associated action to be performed when the line is found. The
set of patterns may appear literally as prog, or in a file
specified as -f file. The prog string should be enclosed in single
quotes to protect it from the Shell.
Files are read in order and if there are none specified the
standard input is read. Each line is matched against the pattern
portion of every pattern-action statement. The associated action
is performed for each matched pattern. An input line is made up
fields separated by white space. $1, $2.. define the fields. $0
refers to the whole line.
A pattern-action statement has the form:
pattern {action}
A missing action means print the line; a missing pattern always
makes a match. a statement can be one of the following:
if (conditional) statement [else statement]
while (conditional) statement
for (expression;conditional;expression) statement
break
continue
{[statement]...}
variable=expression
print [expression-list] [>expression]
printf format [,expression-list][>expression]
next # skip remaining pattern on this input line
exit # skip the rest of the input
Statements are terminated by semi-colons, new-lines (CR), or right
braces.
Let's look at the syntax for awk in a little simpler manner.
awk 'commands' [filename]
An awk program (commands) consists of a optional pattern to match
and an action to perform if a match is found on the current line.
This syntax looks like this:
awk '/pattern/{action}' [filename]
The pattern used is a regular expression enclosed in forward
slashes. If no pattern is listed, the action will be performed for
every line. An action can contain several commands. There can be
multiple patterns and actions.
awk '/pattern1/{action1}
/pattern2/{action2}' [filename}
One of awks commands is print. It puts the current line on standard
output.
Sample session:
$awk '{print}' phone.lis
Smith, Joan 7-7989
Adams, Fran 2-3876
StClair, Fred 4-6122
Jones, Ted 1-3745
Stair, Rich 5-5972
Benson, Sam 4-5587
$
The awk splits every input line at whitespace and keeps track of
the number of fields on each line and counts the number of lines
read. Each field is identified by its field number and a $.
$1 Identifies the first field
$2 Identifies the second field
.
$0 Identifies the entire line
NF Identifies the number of fields on the line
NR Identifies the number of lines that have been read
Sample session:
$awk '{print NR,$1}' phone.lis
1 Smith,
2 Adams,
3 StClair,
4 Jones,
5 Stair,
6 Benson,
$
To change the order of the names in phone.lis use awk. The comma
in the print command tells awk to separate each field with a space.
Without the comma, the output would have no spacing.
Sample session:
$awk '{print $2, $1 ""$3}' phone.lis
Joan Smith, 7-7989
Fran Adams, 2-3876
Fred StClair, 4-6122
Ted Jones, 1-3745
Rich Stair, 5-5972
Sam Benson, 4-5587
$
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