Bash Guide for Beginners Chapter 5. The GNU sed stream editor

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At the end of this chapter you will know about the following topics:

  • What is sed?
  • Interactive use of sed
  • Regular expressions and stream editing
  • Using sed commands in scripts
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TIP: This is an introduction

These explanations are far from complete and certainly not meant to be used as the definite user manual for sed. This chapter is only included in order to show some more interesting topics in the next chapters, and because every power user should have a basic knowledge of things that can be done with this editor.

For detailed information, refer to the sed info and man pages.

Introduction

What is sed?

Stream EDitor is used to perform basic transformations on text read from a file or a pipe. The result is sent to standard output. The syntax for the sed command has no output file specification, but results can be saved to a file using output redirection. The editor does not modify the original input.

What distinguishes sed from other editors, such as vi and ed, is its ability to filter text that it gets from a pipeline feed. You do not need to interact with the editor while it is running; that is why sed is sometimes called a batch editor. This feature allows use of editing commands in scripts, greatly easing repetitive editing tasks. When facing replacement of text in a large number of files, sed is a great help.

sed commands

The sed program can perform text pattern substitutions and deletions using regular expressions, like the ones used with the grep command; see Examples using grep.

The editing commands are similar to the ones used in the vi editor:

Table 5-1. Sed editing commands
Command Result
a\ Append text below current line.
c\ Change text in the current line with new text.
d Delete text.
i\ Insert text above current line.
p Print text.
r Read a file.
s Search and replace text.
w Write to a file.

Apart from editing commands, you can give options to sed. An overview is in the table below:

Table 5-2. Sed options
Option Effect
-e SCRIPT Add the commands in SCRIPT to the set of commands to be run while processing the input.
-f Add the commands contained in the file SCRIPT-FILE to the set of commands to be run while processing the input.
-n Silent mode.
-V Print version information and exit.

The sed info pages contain more information; we only list the most frequently used commands and options here.

Interactive editing

Printing lines containing a pattern

This is something you can do with grep, of course, but you can't do a "find and replace" using that command. This is just to get you started.

This is our example text file:

sandy ~> cat -n example
     1  This is the first line of an example text.
     2  It is a text with erors.
     3  Lots of erors.
     4  So much erors, all these erors are making me sick.
     5  This is a line not containing any errors.
     6  This is the last line.

sandy ~>

We want sed to find all the lines containing our search pattern, in this case "erors". We use the p to obtain the result:

sandy ~> sed '/erors/p' example
This is the first line of an example text.
It is a text with erors.
It is a text with erors.
Lots of erors.
Lots of erors.
So much erors, all these erors are making me sick.
So much erors, all these erors are making me sick.
This is a line not containing any errors.
This is the last line.
sandy ~>

As you notice, sed prints the entire file, but the lines containing the search string are printed twice. This is not what we want. In order to only print those lines matching our pattern, use the -n option:

sandy ~> sed -n '/erors/p' example
It is a text with erors.
Lots of erors.
So much erors, all these erors are making me sick.

sandy ~>

Deleting lines of input containing a pattern

We use the same example text file. Now we only want to see the lines not containing the search string:

sandy ~> sed '/erors/d' example
This is the first line of an example text.
This is a line not containing any errors.
This is the last line.

sandy ~>

The d command results in excluding lines from being displayed.

Matching lines starting with a given pattern and ending in a second pattern are showed like this:

sandy ~> sed -n '/^This.*errors.$/p' example
This is a line not containing any errors.

sandy ~>

Note that the last dot needs to be escaped in order to actually match. In our example the expression just matches any character, including the last dot.

Ranges of lines

This time we want to take out the lines containing the errors. In the example these are lines 2 to 4. Specify this range to address, together with the d command:

sandy ~> sed '2,4d' example
This is the first line of an example text.
This is a line not containing any errors.
This is the last line.
sandy ~>

To print the file starting from a certain line until the end of the file, use a command similar to this:

sandy ~> sed '3,$d' example
This is the first line of an example text.
It is a text with erors.
sandy ~>

This only prints the first two lines of the example file.

The following command prints the first line containing the pattern "a text", up to and including the next line containing the pattern "a line":

sandy ~> sed -n '/a text/,/This/p' example
It is a text with erors.
Lots of erors.
So much erors, all these erors are making me sick.
This is a line not containing any errors.
sandy ~>

Find and replace with sed

In the example file, we will now search and replace the errors instead of only (de)selecting the lines containing the search string.

sandy ~> sed 's/erors/errors/' example
This is the first line of an example text.
It is a text with errors.
Lots of errors.
So much errors, all these erors are making me sick.
This is a line not containing any errors.
This is the last line.
sandy ~>

As you can see, this is not exactly the desired effect: in line 4, only the first occurrence of the search string has been replaced, and there is still an 'eror' left. Use the g command to indicate to sed that it should examine the entire line instead of stopping at the first occurrence of your string:

sandy ~> sed 's/erors/errors/g' example
This is the first line of an example text.
It is a text with errors.
Lots of errors.
So much errors, all these errors are making me sick.
This is a line not containing any errors.
This is the last line.
sandy ~>

To insert a string at the beginning of each line of a file, for instance for quoting:

sandy ~> sed 's/^/> /' example
> This is the first line of an example text.
> It is a text with erors.
> Lots of erors.
> So much erors, all these erors are making me sick.
> This is a line not containing any errors.
> This is the last line.
sandy ~>

Insert some string at the end of each line:

sandy ~> sed 's/$/EOL/' example
This is the first line of an example text.EOL
It is a text with erors.EOL
Lots of erors.EOL
So much erors, all these erors are making me sick.EOL
This is a line not containing any errors.EOL
This is the last line.EOL
sandy ~>

Multiple find and replace commands are separated with individual -e options:

sandy ~> sed -e 's/erors/errors/g' -e 's/last/final/g' example
This is the first line of an example text.
It is a text with errors.
Lots of errors.
So much errors, all these errors are making me sick.
This is a line not containing any errors.
This is the final line.
sandy ~>

Keep in mind that by default sed prints its results to the standard output, most likely your terminal window. If you want to save the output to a file, redirect it:

sed option 'some/expression' file_to_process > sed_output_in_a_file

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More examples

Plenty of sed examples can be found in the startup scripts for your machine, which are usually in /etc/init.d or /etc/rc.d/init.d. Change into the directory containing the initscripts on your system and issue the following command:

grep sed *

Non-interactive editing

Reading sed commands from a file

Multiple sed commands can be put in a file and executed using the -f option. When creating such a file, make sure that:

  • No trailing white spaces exist at the end of lines.
  • No quotes are used.
  • When entering text to add or replace, all except the last line end in a backslash.

Writing output files

Writing output is done using the output redirection operator >. This is an example script used to create very simple HTML files from plain text files.

sandy ~> cat script.sed
1i\
<html>\
<head><title>sed generated html</title></head>\
<body bgcolor="#ffffff">\
<pre>
$a\
</pre>\
</body>\
</html>
sandy ~> cat txt2html.sh
#!/bin/bash

# This is a simple script that you can use for converting text into HTML.
# First we take out all newline characters, so that the appending only happens
# once, then we replace the newlines.

echo "converting $1..."

SCRIPT="/home/sandy/scripts/script.sed"
NAME="$1"
TEMPFILE="/var/tmp/sed.$PID.tmp"
sed "s/\n/^M/" $1 | sed -f $SCRIPT | sed "s/^M/\n/" > $TEMPFILE
mv $TEMPFILE $NAME

echo "done."
sandy ~>

$1 holds the first argument to a given command, in this case the name of the file to convert:

sandy ~> cat test
line1
line2
line3

More on positional parameters in Chapter 7. Conditional statements.

sandy ~> txt2html.sh test
converting test...
done.

sandy ~> cat test <html> <head><title>sed generated html</title></head> <body bgcolor="#ffffff"> <pre> line1 line2 line3 </pre> </body>

</html> sandy ~>

This is not really how it is done; this example just demonstrates sed capabilities. See Chapter 6. The GNU awk programming language, Gawk variables for a more decent solution to this problem, using awk BEGIN and END constructs.

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TIP: Easy sed

Advanced editors, supporting syntax highlighting, can recognize sed syntax. This can be a great help if you tend to forget backslashes and such.

Summary

The sed stream editor is a powerful command line tool, which can handle streams of data: it can take input lines from a pipe. This makes it fit for non-interactive use. The sed editor uses vi-like commands and accepts regular expressions.

The sed tool can read commands from the command line or from a script. It is often used to perform find-and-replace actions on lines containing a pattern.

Exercises

These exercises are meant to further demonstrate what sed can do.

  1. Print a list of files in your scripts directory, ending in ".sh". Mind that you might have to unalias ls. Put the result in a temporary file.
  2. Make a list of files in /usr/bin that have the letter "a" as the second character. Put the result in a temporary file.
  3. Delete the first 3 lines of each temporary file.
  4. Print to standard output only the lines containing the pattern "an".
  5. Create a file holding sed commands to perform the previous two tasks. Add an extra command to this file that adds a string like "*** This might have something to do with man and man pages ***" in the line preceding every occurence of the string "man". Check the results.
  6. A long listing of the root directory, /, is used for input. Create a file holding sed commands that check for symbolic links and plain files. If a file is a symbolic link, precede it with a line like "--This is a symlink--". If the file is a plain file, add a string on the same line, adding a comment like "<--- this is a plain file".
  7. Create a script that shows lines containing trailing white spaces from a file. This script should use a sed script and show sensible information to the user.