Mastering Text Insertion in Linux Files Using sed Command

The i command in sed is used to insert text before a line. The general syntax for the i command is:

sed '/pattern/i\text-to-be-inserted' filename

In this tutorial, we will focus on inserting text into files using sed command.

 

 

Sed Insertion Saved us Fom Months of Manual Editing

One day, a small local ISP approached me with an intriguing problem. They had recently migrated their user database, and in the process, a crucial format change was overlooked.

They had a list of more than 100,000 user records, and each record was missing an important identifier that was needed for their new system. Each record was in the format:

username:hashed-password:account-status

But the new system needed:

username:hashed-password:UID:account-status

Here, UID stands for User Identifier, a unique alphanumeric string for each user.

Without the UID, the ISP’s entire billing and usage tracking system would malfunction, possibly leading to significant revenue loss and unhappy customers.

Manually inserting these UIDs for over 100,000 records was not only impractical but would take months with the small team they had.

I decided to use the sed command to automatically insert UIDs. I wrote a simple script that would generate a UID for each user and then use sed to insert it into the appropriate place in each record.

Here’s a simplified example of the command I used:

sed -i 's/\(username:hashed-password:\)\(account-status\)/\1UID:\2/' user-database.txt

With that single command, sed processed the file in less than 10 minutes.

Compared to manually editing the file, which could have taken a small team over 800 hours (considering 5 minutes to edit and verify each record).

 

Insert Text at a Specific Line

To insert text at a specific line number in a file using sed, you can use the following syntax:

sed 'LINE_NUMBER i\text-to-be-inserted' filename

For example, if you want to insert the text “This is line 3.” at the third line of the ‘example.txt’ file, you can use the following command:

sed '3 i\This is line 3.' example.txt

Output:

Apple
Banana
This is line 3.
Cherry

In this example, ‘3’ is the line number where the text “This is line 3.” is inserted before the line.

Please note that this command does not modify the original file. It only displays the modified content in the terminal.

If you want to save the changes to the file, you can redirect the output to a new file or use the -i option with sed.

For example, to save the changes to a new file:

sed '3 i\This is line 3.' example.txt > newfile.txt

Or, to modify the original file in place, you can use the -i option:

sed -i '3 i\This is line 3.' example.txt

 

Insert Text at the Beginning of a File

To insert text at the beginning of a file, you can use the line number ‘1’ in the sed command as follows:

sed '1 i\text-to-be-inserted' filename

For example, if you want to insert the text “This is the start of the file.” at the beginning of the ‘example.txt’ file, you would use the following command:

sed '1 i\This is the start of the file.' example.txt

Output:

This is the start of the file.
Apple
Banana
Cherry

In this example, ‘1’ is the line number which indicates the start of the file. Therefore, the text “This is the start of the file.” is inserted at the beginning of the file.

 

Insert Before Last Line

To insert text before the last line of a file, you can use the following command:

sed '$!b; i\This is the second last line.' example.txt

Output:

Apple
Banana
This is the second last line.
Cherry

In this command, $!b skips all lines except the last one, and the ‘i’ command inserts the text “This is the second last line.” before the last line.

 

Insert Text before a Specific Match

To insert text before a line containing a specific pattern, you can use the i command in sed like this:

sed '/pattern/i\text-to-be-inserted' filename

For example, if you want to insert the text “This is a fruit.” before the line containing “Cherry” in the ‘example.txt’ file, you can use the following command:

sed '/Cherry/i\This is a fruit.' example.txt

Output:

Apple
Banana
This is a fruit.
Cherry

In this command, ‘/Cherry/’ is the pattern that sed searches for in the file. Once it finds a line with the pattern, it inserts the text “This is a fruit.” before that line.

 

Insert Text between Two Specific Lines

You can combine the i and a commands in sed to insert text between two specific lines.

For example, if you want to insert text between the second and third lines of the example.txt file, you can use the following command:

sed '2 a\This is between second and third line' example.txt

Output:

Apple
Banana
This is between second and third line
Cherry

In this example, ‘2’ is the line number after which the text “This is between second and third line” is inserted.

 

Conditional Insertions Based on Patterns

You can insert text before or after a line only if that line matches a specific pattern.

For example, if you want to insert text before a line that contains “Banana” in the ‘example.txt’ file, but only if that line also contains “Apple”, you can use the following command:

sed '/Banana.*Apple/ i\This is a fruit.' example.txt

Let’s assume that the ‘example.txt’ file to contain a line with both “Apple” and “Banana”:

Apple
Banana Apple
Cherry

Output:

Apple
This is a fruit.
Banana Apple
Cherry

In this example, the pattern ‘/Apple.*Banana/’ matches the line “Banana Apple”, so the text “This is a fruit.” is inserted before that line.

 

Insert Escape Characters

Escape characters are characters that are used to represent certain special characters like newline, tab, etc. In sed, escape characters are preceded by a backslash ().

For example, if you want to insert a line containing a tab, a newline, and a backslash at the beginning of the ‘example.txt’ file, you can use the following command:

sed '1 i\\\t\\n\\' example.txt

Output:

\t\n\
Apple
Banana
Cherry

In this command, ‘1’ is the line number where the text “\t\n” is inserted.

Since the backslash is used to escape characters in sed, you need to use two backslashes () to represent a single backslash.

 

Insert Variable Value and Command Substitution

You can also insert the value of variables or the output of commands using sed.

For example, if you want to insert the current date and time at the beginning of the ‘example.txt’ file, you can use the following command:

sed "1 i\\$(date)" example.txt

Output:

Sun Sep  5 08:21:10 UTC 2023
Apple
Banana
Cherry

In this command, 1 is the line number where the output of the date command is inserted.

 

Insert Multiple Lines around a Specific Pattern

If you want to insert two lines “This is the first line.” and “This is the second line.” before and after each line containing “Banana” in the ‘example.txt’ file, you can use the following command:

sed -e '/Banana/ i\This is the first line.' -e '/Banana/ a\This is the second line.' example.txt

Output:

Apple
This is the first line.
Banana
This is the second line.
Cherry

In this command, ‘/Banana/’ is the pattern that matches lines containing “Banana”.

In the above command, the first -e option inserts “This is the first line.” before each line containing “Banana”, and the second -e option appends “This is the second line.” after each line containing “Banana”.

 

Handling Multiple Files

sed can also handle multiple files at once, which allows you to insert text into multiple files or insert different text into multiple files with a single command.

For example, if you want to insert the line “This is a fruit.” at the beginning of two files, ‘example1.txt’ and ‘example2.txt’, you can use the following command:

sed -i '1 i\This is a fruit.' example1.txt example2.txt

In this command, ‘1’ is the line number where the text “This is a fruit.” is inserted in both files. The -i option modifies the files in place.