bash-sed

Last updated Sep 20, 2023 Edit Source

sed stands for “stream editor”.

It can perform text search and replace, insertion and deletion.
sed is efficient in that it can make changes to the file without creating a temp file to redirect output to.
Similar in function to bash-awk, but it is not as robust and lacks logic to it’s operations.
- Though, the syntax is often easier and in one line.

# Syntax:

# Basic syntax:

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sed [options] command [input-file]

With piping: Piping allows manipulation of output from other commands, or even chains of commands.

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cat report.txt | sed 's/Nick/John/g'

# Quick Cheatsheet:

Description	Command Syntax
Replace all occurrences of a pattern with a replacement string	`s/pattern/replacement/g`
Delete all lines that match a pattern	`/pattern/d`
Print only the lines that match a pattern	`/pattern/p`
Insert a line before or after the line that matches a pattern	`i/pattern/text` or `a/pattern/text`
Append a line to the end of the file	`a text`
Flag to have changes written to the file	`-i`
Delete characters that match pattern, for all lines	sed ’s/[pattern]//g'
To have OR/AND operations, best to use `-e` flag to chain operations	sed -e ‘…’ -e ‘…’ file

# Deleting lines:

This deletes lines from a file, without even needing to open it.

Delete with pattern matching:

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sed '/pattern/d' filename.txt

To delete the first or last line:

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#Delete first line: 
sed -i '1d' filename


#Delete last line:
sed '$d' filename.txt

To delete line from range x to y:

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sed 'x,yd' filename.txt

To delete from nth to last line:

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sed 'nth,$d' filename.txt

#e.g: 

sed '12,$d' filename.txt

# Generic snippets:

# To find and replace text:

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sed 's/foo/bar/g' file.txt

This command will find all instances of the word “foo” in the file file.txt and replace them with the word “bar”.
The g flag tells sed to replace all instances of the pattern, not just the first one.

# Replacing string on a specific line number:

You can restrict the sed command to replace the string on a specific line number. An example is:

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sed '3 s/unix/linux/' geekfile.txt

# Replacing from nth occurrence, to all subsequent occurrences in a line:

Use the combination of /1, /2 etc and /g to replace all the patterns from the nth occurrence of a pattern in a line. The following sed command replaces the third, fourth, fifth… “unix” word with “linux” word in a line.

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sed 's/unix/linux/3g' geekfile.txt

# Printing only the replaced lines:

Use the -n option along with the /p print flag to display only the replaced lines. Here the -n option suppresses the duplicate rows generated by the /p flag and prints the replaced lines only one time.

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sed -n 's/unix/linux/p' geekfile.txt

# Replacing strings only from a range of lines :

You can specify a range of line numbers to the sed command for replacing a string.

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sed '1,3 s/unix/linux/' geekfile.txt

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