how to take back up of all sql server databases?

You can take a look on this article's 3rd option to take back up of full database using SQL server

Another solution to take back up of all databases using SQL server is

• Create a stored procedure
  

  Create PROCEDURE [dbo].[AllDataBasesBackUp]
  AS
  
  BEGIN
  SET NOCOUNT ON;
  
  DECLARE @name VARCHAR(50) -- database name
  DECLARE @path VARCHAR(256) -- path for backup files
  DECLARE @fileName VARCHAR(256) -- filename for backup
  DECLARE @fileDate VARCHAR(20) -- used for file name
  SET @path = 'D:\All_databases_Backup\'
  SELECT @fileDate = CONVERT(VARCHAR(20),GETDATE(),104)
  DECLARE db_cursor CURSOR FOR
  SELECT name
  FROM MASTER.dbo.sysdatabases
  WHERE name NOT IN ('master','model','msdb','tempdb','ReportServer','ReportServerTempDB')
  OPEN db_cursor
  FETCH NEXT FROM db_cursor INTO @name
  WHILE @@FETCH_STATUS = 0
  BEGIN
  SET @fileName = @path + @name + '_' + @fileDate + '.BAK'
  BACKUP DATABASE @name TO DISK = @fileName
  FETCH NEXT FROM db_cursor INTO @name
  END
  CLOSE db_cursor
  DEALLOCATE db_cursor
  END

• Execute the above procedure.
  
  

  EXEC [AllDataBasesBackUp]?

Now, you can check D:\All_databases_Backup\ (Location, all database back up file is created there using above stroed procedures

You can also use powershell to take backup of all sql server database

Get-SqlDatabase -ServerInstance localhost |
Where { $_.Name -ne 'tempdb' } | foreach{$_ |
Backup-SqlDatabase -CompressionOption On -BackupFile "$($_.NAME)_db_$(Get-Date -UFormat %Y%m%d%H%M).bak"};

Source: How to back up SQL Server database ? (Various Ways explained)

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