Difference between revisions of "Linux-Ubuntu/C3/Text-Editing-using-sed/English"

Title Slide

• Use the g flag for global substitution.
• Execute multiple sed commands using the hyphen e option.
• Apply substitution based on context.
• Insert and remove lines from a file.
• Redirect sed output to an output file.

System Requirements

• Bash version 5 point 1 point 16;

https://EduPyramids.org

• Bash version 5 point 1 point 16
• For the prerequisite Linux tutorials please visit this website.

Code files The following code files are required to practice this tutorial.# seddemo.txt

1. sed-commands.txt

These files are provided in the Code Files link of this tutorial page.

These files are provided in the Code Files link of this tutorial page.

cat seddemo.txtpress Enter Highlight the word kumar in the lines.Point to the fourth and sixth lines

Notice the word Kumar. It appears twice in the fourth line and once in the sixth line.

sed 's/[kK]umar/Roy/' seddemo.txt

Type the following command and press Enter.

sed s [kK]umar Roy forward slashes seddemo.txt

sed is the stream editor. The letter s stands for substitution. This pattern matches bothuppercase Kumar and lowercase kumar. The word Roy is the replacement text. The forward slashes separate the pattern and the replacement. The file sed demo dot txt is the input file.

Show the output on the terminal

In the output, the word Kumar is replaced with Roy.

Show the output on the terminal and the file seddemo.txt opened next to it Highlight 4th line of both output and in the file

This happens because, by default, sed substitutes only the first match in a line. Note the substitution happens only on the terminal output. The original file sed demo dot t x t is not modified.

Global Substitution

• By default, sed replaces only the first match in each line.
• To replace all occurrences in a line, we use the g flag.
• The letter g stands for global substitution.
• When the g flag is used, sed searches the entire line for matches.
• Every matching pattern in that line is replaced.
• The g flag is written after the substitution command.

General syntax: sed 's/pattern/replacement/g' filename

• To replace all occurrences in a line, we use the g flag.
• The letter g stands for global substitution.
• When the g flag is used, sed searches the entire line for matches.
• Every matching pattern in that line is replaced.
• The g flag is written after the substitution command.

This is the general syntax.

sed 's/[kK]umar/Roy/g' seddemo.txt Press Enter

Highlight the output

Output shows, both occurrences in the fourth line are replaced.

sed 's/kumar/Roy/Ig' seddemo.txt and press Enter.Case-insensitive matching.

Instead of specifying lowercase and uppercase letters using brackets, we can use the I option. The I option makes the pattern match case insensitive. Observe that the output is the same as the previous command. Both methods can be used to produce the same result.

Multiple Substitutions using -e

• sed allows performing more than one operation in a single command.
• The -e option is used to specify multiple sed expressions.
• Each -e represents one sed command.
• Commands are executed in the order they are written.
• This is useful when multiple substitutions are required on the same input.

• sed allows performing more than one operation in a single command.
• The hyphen e option is used to specify multiple sed expressions.
• Each hyphen e represents one sed command.
• Commands are executed in the order they are written.
• This is useful when multiple substitutions are required on the same input.

sed -e 's/electronics/electrical/g' -e 's/civil/metallurgy/g' seddemo.txt

• hyphen e allows using more than one sed command in a single line.
• s slash electronics slash electrical slash g replaces all occurrences of electronics with electrical.
• s slash civil slash metallurgy slash g replaces all occurrences of civilwith metallurgy.

Highlight the replaced words in the output

Observe that words are replaced.

sed '/Anirban/s/computers/mathematics/g' seddemo.txt

Observe that computers is changed to mathematics.

In other words, select those lines which do not have an electronics stream.

sed '/electronics/d' seddemo.txt > nonElectronics.txt

Highlight /electronics/ d seddemo.txt > nonElectronics.txt

d means deletes the matched lines seddemo dot t x t is the input file. greater than means redirects output to a file non Electronics dot t x t is the output file. Press Enter.

Show the output

The file shows only those records that are not in the electronics stream. You may wonder why substitutions are not reflected in the text file. This is because all substitution results are displayed only on the terminal. The original text file is not modified unless explicitly instructed.

sed '1i Student Information' seddemo.txt

1 i Student Information

• 1 is the line number
• i is the insert command
• Student Information is the text to be inserted

Highlight the line: Student Information

Output shows the line Student Information inserted at the beginning of the file.

Student Information in the first line and 2026 in the next line.

sed '1i Student Information\n2026\nRecords' seddemo.txtpress Enter

Press Enter.

backslash n moves to a new line and prints 2026 in the next line after Student Information. Similarly after 2026, new string Records is printed in the third line.

Summary

In this tutorial, we have learnt to:

• Perform text substitution using sed command
• Use the g flag for global substitution
• Execute multiple sed commands using the -e option
• Apply substitution based on context
• Insert and remove lines from a file
• Redirect sed output to an output file

Let us summarise.

As an assignment please do the following:

• Using sed and the file seddemo.txt, replace the word computers with computer-science.
• Replace electronics with ece in all occurrences.
• Perform both substitutions in a single command using the -e option.
• Delete all lines that belong to the civil department.

Thank you