Difference between revisions of "Sed-Stream-Editor/C2/More-on-Sed-commands/English"

Learning Objectives

• e command
• -f flag, -e flag
• Transform flag y and
• Sed substitute command with
  • Escaping character and
  • Delimiter

System requirements

Prerequisites

basics of Linux.

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Code files

• The files used in this tutorial are available in the Code Files link on this tutorial page.
• Please download and extract them
• Make a copy and then use them while practising

Press the Enter key after every command.

> cd Downloads

> cat command_exec.txt

> sed ‘e’ command_exec.txt

Here I have saved 4 Unix commands in this file as shown.

We can execute all the Unix commands in this file at one go.

Type the execute command with ‘e’ option as shown.

Highlight according to narration

In this file, let us change certain lines with multiple substitution.

For example, let us change all the linux words to GNU Linux and small u to capital U in the word Unix.

Also we will change the lower case gnu to upper case GNU.

Highlight { }

Highlight s/ commands

Highlight 1,$

To perform multiple commands, enter the sed commands in a script file as shown.

Use the curly brackets to group commands as shown here.

All the required substitution commands are given here.

1,$ represents the address where the changes have to be done.

This will apply the substitute command from the first line to the end of the file.

Save the file.

Highlight mycommands.txt

Highlight linux_doc.txt

Here, mycommands.txt is the script file where the commands are saved.

linux_doc.txt is the file where the changes have to be done.

Let us execute and see the output.

And capital U in Unix word.

Also we can see the lowercase gnu changed to uppercase GNU.

> sed -n '$=' linux_doc.txt

Here each sentence is considered as a line. Blank lines will also be considered as a line.

Type the command as shown. This will display the number of lines in linux_doc.txt

There are 6 lines in this file.

We have used the hyphen e flag before each print sed command.

Linux is free and open source

Type the input as “ Linux is free and open source”

It replaces each single character with another single character given in the sed command.

That is, it converts all lowercase letters to uppercase letters, like the Unix tr command.

Press Ctrl+C to return to the command prompt.

aa-bb-cc-zz

AA-bb-cc-ZZ

Type the input as shown.

Range of characters will not work in y command.

It replaces a, hyphen and z only, in uppercase.

Press Ctrl+C to return to the command prompt.

Let us try to replace the word can’t to can.

Highlight can’t

This substitute command didn’t work and we couldn’t see any output.

We have single quotes in between can’t.

This single quote creates the problem in execution of this command.

Recall that it is a good practice to write sed commands using quotes.

There are some exceptions which need to be looked at while using quotes.

Press Ctrl+C to return to the command prompt.

Use double quotes as shown to see the expected output.

> cat sample.txt > sed “s/hundred dollars/$100/” sample.txt Highlight $

Highlight 00

> sed “s/hundred dollars/\$100/” sample.txt

Highlight \$10

Type the command as shown.

We didn’t get the expected result.

The shell meaning of the $ symbol is special inside double quotes.

Sed command will treat $1 as a shell argument and preserve the spaces.

It replaces the string “hundred dollars” to 00.

To make it functional, type the command as shown.

We can use the escape character backslash as shown here.

This will disable the special meaning of $ character of $1

Special characters are considered as a normal text in the search and replacement string.

sed 's#half#1/2#' sample.txt

Type the command as shown.

Recall that forward slash (/) is the most commonly used delimiter in the sed commands.

Here, we have used forward slash(/) inside search and replacement string.

It is a good practice to use a different symbol as a delimiter in this case.

In this command we have used hash(#) as the delimiter.

However, sed can use any character as a delimiter.

Summary

• e command
• -f flag, -e flag
• Transform flag y and
• Sed substitute command with
  • Escaping character and
  • Delimiter

Assignment:

> sed 's!half!1/2!' sample.txt

1. Change the delimiter as shown below and see the output.

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