PowerShell Script to Find and Replace Text in a File

Use (Get-Content -Path 'file.txt') -replace 'oldText', 'newText' | Set-Content -Path 'file.txt' to find and replace text in a file with PowerShell.

Examples

Inputfile.txt content: 'Hello world'

Outputfile.txt content after replace: 'Hello PowerShell'

Inputfile.txt content: 'apple, banana, apple'

Outputfile.txt content after replace: 'orange, banana, orange'

Inputfile.txt content: 'No match here'

Outputfile.txt content after replace: 'No match here'

How to Think About It

To replace text in a file, read the file content as text, use the -replace operator to swap the old text with the new text, then write the updated content back to the file. This approach treats the file as plain text and updates all occurrences.

Algorithm

1

Read the entire content of the file into memory.

2

Replace all occurrences of the target text with the new text.

3

Write the modified content back to the same file.

Code

powershell

Get-Content -Path 'file.txt' | ForEach-Object { $_ -replace 'oldText', 'newText' } | Set-Content -Path 'file.txt'
Write-Output "Replacement complete."

Output

Replacement complete.

Dry Run

Let's trace replacing 'apple' with 'orange' in a file containing 'apple, banana, apple'.

1

Read file content

'apple, banana, apple'

2

Replace 'apple' with 'orange'

'orange, banana, orange'

3

Write updated content back to file

File now contains 'orange, banana, orange'

Step	Content
1	apple, banana, apple
2	orange, banana, orange
3	orange, banana, orange

Why This Works

Step 1: Reading the file

The Get-Content command reads the file line by line as text.

Step 2: Replacing text

The -replace operator swaps all instances of the old text with the new text in each line.

Step 3: Saving changes

The updated lines are sent to Set-Content which overwrites the original file with the new content.

Alternative Approaches

Using -Raw parameter for whole file as one string

powershell

$content = Get-Content -Path 'file.txt' -Raw; $content = $content -replace 'oldText', 'newText'; Set-Content -Path 'file.txt' -Value $content
Write-Output "Replacement complete."

This reads the entire file as one string, which is faster for large files but uses more memory.

Using StreamReader and StreamWriter for large files

powershell

$reader = [System.IO.StreamReader]::new('file.txt'); $writer = [System.IO.StreamWriter]::new('file.tmp'); while (($line = $reader.ReadLine()) -ne $null) { $writer.WriteLine($line -replace 'oldText', 'newText') } $reader.Close(); $writer.Close(); Remove-Item 'file.txt'; Rename-Item 'file.tmp' 'file.txt'; Write-Output "Replacement complete."

This method processes the file line by line without loading all content into memory, good for very large files.

Complexity: O(n) time, O(n) space

Time Complexity

The script reads and processes each line once, so time grows linearly with file size.

Space Complexity

The entire file content is stored in memory during processing, so space grows with file size.

Which Approach is Fastest?

Using -Raw is faster for medium files, but streaming line-by-line uses less memory for very large files.

Approach	Time	Space	Best For
Pipeline with Get-Content	O(n)	O(n)	Small to medium files
Get-Content -Raw	O(n)	O(n)	Medium files, faster processing
StreamReader/StreamWriter	O(n)	O(1)	Very large files with limited memory