Sed Lesson 1 – Introduction & Substitution

What is sed?

sed (Stream EDitor) is a non-interactive text editor that processes input one line at a time.

It is commonly used for:

• Text replacement

• Editing configuration files

• Log processing

• Shell scripting

• CI/CD automation

Unlike Vim, sed edits a text stream automatically without opening an editor.

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Basic Syntax

Read from a file:

sed 'command' file.txt

Read from standard input:

command | sed 'command'

Example:

echo "hello world" | sed 's/world/Linux/'

Output:

hello Linux

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The Substitute Command

Syntax:

s/old/new/

Meaning:

• s = substitute

• old = search pattern

• new = replacement

Example:

echo "cat dog" | sed 's/cat/lion/'

Output:

lion dog

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First Match vs All Matches

Default: replace only the first match on each line.

echo "cat cat cat" | sed 's/cat/dog/'

Output:

dog cat cat

Replace every match:

echo "cat cat cat" | sed 's/cat/dog/g'

Output:

dog dog dog

g = global (all matches on the current line).

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Using Another Delimiter

Useful for file paths.

Instead of:

sed 's/\/usr\/bin/\/opt\/bin/'

Use:

sed 's|/usr/bin|/opt/bin|'

Any character can be the delimiter as long as it is used consistently.

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Edit a File In Place

Modify the original file:

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

Create a backup first:

sed -i.bak 's/foo/bar/g' file.txt

Result:

file.txt
file.txt.bak

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Common Examples

Replace tabs with spaces:

sed 's/\t/    /g'

Remove trailing whitespace:

sed 's/[[:space:]]*$//'

Replace a version number:

sed 's/v1.2/v2.0/'

Template substitution:

VERSION="2.0"
 
sed "s/@VERSION@/$VERSION/g" config.template > config.conf

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Key Takeaways

• sed is a stream editor.

• The most common command is s/old/new/.

• By default, only the first match on each line is replaced.

• Add the g flag to replace all matches.

• -i edits files directly.

• Using delimiters such as | makes path substitutions easier to read.