About

Appending a consistent suffix to every line of a multi-line block is a routine task in data preparation, log formatting, and code generation. Manual editing introduces typos and costs time that scales linearly with line count - 500 lines at 3 seconds each is 25 minutes of mechanical labor. This tool processes n lines in constant time, applying a user-defined suffix S with an optional separator sep between the original content and the appended string. It handles edge cases: empty lines can be preserved or skipped, leading/trailing whitespace can be trimmed before concatenation, and the separator is configurable (space, tab, or arbitrary string). The operation is deterministic - identical input always yields identical output - so results are safe to pipe into downstream scripts or databases.

Common applications include adding file extensions to filename lists, appending domain suffixes to usernames, tagging CSV rows, and inserting line-ending characters for protocol-specific formats. Note: this tool operates on plain text only. If your lines contain structured markup, verify that the suffix does not break tag nesting. For lines exceeding 10,000 characters, browser rendering may introduce slight latency, though the concatenation logic itself remains O(n).

Formulas

The suffix operation applies a simple concatenation formula to each line independently:

L_i′ = L_i + sep + S

Where L_i is the original content of line i, sep is the chosen separator string (empty string, space U+0020, tab U+0009, or a custom string), and S is the suffix to append. When the "trim lines" option is active, L_i is first passed through a whitespace strip function:

L_i = trim(L_i)

When "skip empty lines" is enabled, the conditional logic is:

{

L_i + sep + S if len(L_i) > 0L_i otherwise (preserved empty)

Total time complexity is O(n ⋅ m) where n is the number of lines and m is the average line length, dominated by string copying during concatenation.

Reference Data

Use Case	Original Line Example	Suffix	Separator	Result
File extensions	report_2024	.csv	None	report_2024.csv
Email domains	john.doe	@company.com	None	[email protected]
CSS classes	.card	--active	None	.card--active
SQL values	('Alice', 30)	,	None	('Alice', 30),
Log tagging	Connection timeout	[ERROR]	Space	Connection timeout [ERROR]
URL paths	/api/v2/users	?limit=100	None	/api/v2/users?limit=100
Markdown lists	Item one	;	None	Item one;
HTML attributes	<div	class="box">	None	<div class="box">
Batch commands	del file	/q	None	del file /q
Unit labeling	45.7	kg	None	45.7 kg
CSV quoting	John,Doe,30	"	None	John,Doe,30"
Version tags	release	-v2.1.0	None	release-v2.1.0
Tab-separated	Name	Value	Tab	Name Value
Line numbering aid	Process data	// step 1	None	Process data // step 1
DNS records	mail	.example.org.	None	mail.example.org.

Frequently Asked Questions

By default, yes - every line receives the suffix, including blank ones. Enable the "Skip empty lines" option to leave lines with zero characters (after optional trimming) untouched. This prevents artifacts like a lonely suffix on an otherwise blank line.

Functionally, adding a leading space to your suffix string produces the same output as selecting "Space" as the separator. The separator control exists for clarity and consistency: it keeps your suffix clean (e.g., kg rather than kg) and lets you switch between tab-separated and space-separated output without editing the suffix. For tab characters, which are invisible and hard to type in an input field, the dedicated separator option is essential.

The tool normalizes all line endings internally. Whether your source uses LF (U+000A), CR (U+000D), or CRLF, each is treated as a single line break. Output uses LF line endings. If you need CRLF for a specific system, perform a find-and-replace on the output.

No hard limit is enforced. The tool processes text in-memory using JavaScript string operations. For typical use (under 50,000 lines), processing is instantaneous. Beyond 100,000 lines, you may notice a brief delay depending on browser memory allocation. There is no server-side processing - all data stays in your browser.

Not directly in this tool's scope. However, you can pre-filter your text externally (e.g., grep the lines you need), process them here, then merge back. The "skip empty lines" toggle covers the most common filtering case. For regex-based conditional suffixing, a dedicated regex replacement tool is more appropriate.

Yes, unless you enable "Trim lines." Without trimming, all leading and trailing whitespace on each line is preserved exactly as entered. Trimming removes only leading and trailing whitespace; internal spaces (e.g., between words) are never modified.