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How to Get the First N Characters of a String in Batch Script

Extracting the beginning of a string is a very common parsing task. You might need to get a prefix from a long identifier, check the first few characters of a line to determine its type, or create a short unique ID from a longer string like a timestamp. Batch scripting provides a simple, fast, and built-in syntax for this operation using variable expansion, allowing you to get the first N characters of any string with ease.

This guide will teach you the standard syntax for extracting a prefix from a string, how to store it for later use, and the critical rules you need to know, such as the zero-based indexing system and how to use this technique inside a loop.

The Core Syntax: %VariableName:~0,N%​

The ability to extract a substring is built directly into the variable expansion mechanism in cmd.exe.

The specific syntax for getting the first N characters is: %VariableName:~0,N%

  • VariableName: The name of the variable holding your source string.
  • ~: The special character that signals a modifier operation.
  • 0: The starting position. To get the beginning of the string, you always start at index 0.
  • ,N: The number of characters (the length) you want to extract.

Basic Example: Getting the First 5 Characters​

Let's take a sample string and extract the first 5 characters from it.

@ECHO OFF
SET "MyString=ABCDEFGHIJKLMNOP"

ECHO The original string is: %MyString%

REM Get the first 5 characters by starting at 0 and taking a length of 5.
ECHO The first 5 characters are: %MyString:~0,5%

Output:

The original string is: ABCDEFGHIJKLMNOP
The first 5 characters are: ABCDE

Storing the Result in a New Variable​

In most scripts, you will want to capture the extracted prefix into a new variable to use it in your logic. This is done with a standard SET command.

@ECHO OFF
SET "TransactionID=TRN-20231027-a94fe5"

REM Extract the 3-character prefix.
SET "TransactionType=%TransactionID:~0,3%"

ECHO The full ID is: %TransactionID%
ECHO The transaction type is: %TransactionType%

IF "%TransactionType%"=="TRN" (
    ECHO This is a standard transaction.
)

Output:

The full ID is: TRN-20231027-a94fe5
The transaction type is: TRN
This is a standard transaction.

Common Pitfalls and How to Solve Them​

The "Zero-Based" Indexing System​

The most common point of confusion for newcomers is that string positions in batch scripting are zero-based. This means the very first character of the string is at position 0, the second is at position 1, and so on.

For the string ABCDE:

  • Index 0 = A
  • Index 1 = B
  • Index 2 = C

Solution: Always remember this rule. To get a prefix of N characters, the syntax is always %VAR:~0,N%.

Using Substrings Inside a Loop (Delayed Expansion)​

If you are modifying a string inside a FOR loop and also trying to take a substring of it in the same loop, you must use delayed expansion. Standard percent variables (%Var%) are expanded only once, before the loop begins executing.

This script repeatedly adds a character to a string and then extracts the first 3 characters of the updated string on each iteration.

@ECHO OFF
SETLOCAL ENABLEDELAYEDEXPANSON
SET "MyVar="

FOR %%C IN (A B C D E) DO (
    REM Add the next character to the string
    SET "MyVar=!MyVar!%%C"
    
    REM Get the first 3 chars of the CURRENT string using delayed expansion
    ECHO Full string is: !MyVar!
    ECHO First 3 chars are: !MyVar:~0,3!
    ECHO.
)

Output:

Full string is: A
First 3 chars are: A

Full string is: AB
First 3 chars are: AB

Full string is: ABC
First 3 chars are: ABC

Full string is: ABCD
First 3 chars are: ABC

Full string is: ABCDE
First 3 chars are: ABC

Practical Example: Creating a Short ID from a Timestamp​

This script generates a long, sanitized timestamp and then extracts the first 12 characters to create a shorter, "unique enough" ID for a log file prefix.

@ECHO OFF
SETLOCAL

REM Create a detailed timestamp like 2023102710301550
SET "TIMESTAMP=%DATE:~10,4%%DATE:~4,2%%DATE:~7,2%%TIME::=%%TIME:.=%"
SET "TIMESTAMP=%TIMESTAMP: =0%"

ECHO Full timestamp: %TIMESTAMP%

REM Extract the first 12 characters (YYYYMMDDHHMM) for a shorter ID
SET "ShortID=%TIMESTAMP:~0,12%"

ECHO Short ID: %ShortID%

SET "LOG_FILE=AppLog_%ShortID%.log"
ECHO.
ECHO Creating log file: %LOG_FILE%
ECHO Log started > %LOG_FILE%

ENDLOCAL

Conclusion​

The substring expansion syntax in batch scripting provides a fast, native, and highly efficient way to get the first N characters of any string.

The key syntax to remember is %VAR:~0,N%.

For reliable scripting, always keep in mind:

  • The starting position for a prefix is always 0.
  • When working with a variable that is changing inside a FOR loop, you must use delayed expansion with exclamation marks: !VAR:~0,N!.

This simple and powerful tool is essential for any script that needs to parse or manipulate string data.

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