How to Replace String in Bash: Simple Syntax and Examples

In bash, you can replace a string using ${variable//search/replace} for simple replacements or use sed for more complex patterns. The ${variable//search/replace} syntax replaces all occurrences of search with replace inside a variable.

Syntax

The basic syntax for replacing strings in bash variables is:

• ${variable//search/replace}: Replaces all occurrences of search with replace in variable.
• ${variable/search/replace}: Replaces only the first occurrence.
• sed 's/search/replace/g': Command-line tool for replacing strings in text streams or files, where s means substitute and g means global (all occurrences).

bash

${variable//search/replace}

sed 's/search/replace/g' filename

Example

This example shows how to replace all occurrences of apple with orange in a bash variable and how to use sed to replace text in a file.

bash

# Using bash parameter expansion
text="I like apple and apple pie."
new_text=${text//apple/orange}
echo "$new_text"

# Using sed to replace in a file (example.txt)
# Contents of example.txt:
# I like apple and apple pie.

sed 's/apple/orange/g' example.txt

Output

I like orange and orange pie.

Common Pitfalls

Common mistakes when replacing strings in bash include:

• Not using double slashes // to replace all occurrences, which only replaces the first occurrence.
• Forgetting to quote variables, which can cause word splitting or globbing issues.
• Using sed without the g flag, which replaces only the first match per line.
• Trying to replace strings directly in files without redirecting output or using -i for in-place editing.

bash

# Wrong: replaces only first occurrence
text="apple apple apple"
echo ${text/apple/orange}

# Right: replaces all occurrences
echo ${text//apple/orange}

# Wrong: sed without global flag
sed 's/apple/orange/' example.txt

# Right: sed with global flag
sed 's/apple/orange/g' example.txt

Output

orange apple apple
orange orange orange

Quick Reference

Command	Description	Example
${variable/search/replace}	Replace first occurrence in variable	${text/apple/orange}
${variable//search/replace}	Replace all occurrences in variable	${text//apple/orange}
sed 's/search/replace/' file	Replace first occurrence per line in file	sed 's/apple/orange/' example.txt
sed 's/search/replace/g' file	Replace all occurrences per line in file	sed 's/apple/orange/g' example.txt
sed -i 's/search/replace/g' file	Replace all occurrences in file in-place	sed -i 's/apple/orange/g' example.txt

Key Takeaways

Use ${variable//search/replace} to replace all occurrences of a string in a bash variable.

Use sed with the 'g' flag to replace all occurrences in files or streams.

Always quote variables when replacing strings to avoid unexpected word splitting.

Remember that ${variable/search/replace} replaces only the first occurrence.

Use sed -i for in-place file editing but be cautious as it overwrites the original file.