Saturday, July 4, 2015

Preserving the current working directory while impersonating with runas

It's pretty common knowledge that you can use the netonly switch of runas to run a program while impersonating a domain user even if the computer you're on is not joined to the domain. E.g.:

runas /netonly /u:domain\user notepad.exe

The command will use your credentials to run the program, but the domain user's credentials when authenticating over the network. It's really handy.

Recently I tried to do this with a program that is sensitive to the current working directory. Unfortunately, runas will set the working directory to %WINDIR%\System32 when impersonating. I discovered that you can work around this issue with the following command line.

runas /netonly /u:domain\user "cmd /c \"cd \"%CD%\" ^& program"

There are few details involved in this command:

  1. The %CD% environment variable expands to the current working directory. Note that this variable expansion takes place before the entire command begins to execute, so it's expanded to the current working directory of the runas command.
  2. To escape quotes in the runas command parameter, you use \". This is only unusual in the sense that all Windows command line programs escape special characters in a different way, and runas is unique in having an ordinary way of escaping quotes.
  3. To run multiple commands in one line, you use an ampersand (i.e. command1 & command2). Since we're passing this as an argument to cmd /c, we need to escape the &. cmd uses the ^ (the caret) to escape ampersands and other special characters.

P.S. Knowing about %CD% is very useful if you use the command line in Windows a lot. For example, if you want to copy the current directory to the clipboard, you can use echo %CD% | clip.

Unrelated but also useful: if you want to copy the full path of some files in the current directory, you can use the command dir /b /s *.txt | clip.

Thursday, July 2, 2015

Removing nondigits from a column in a table using T-SQL

While T-SQL admittedly isn't the greatest language there is for text processing, you shouldn't dismiss it out of hand for simple tasks.

To remove non-digits from a column, you can use PATINDEX and STUFF in a loop. If you're careful to do this in a set-based way, T-SQL should be efficient enough in most situations.

    WITH q AS
        (SELECT table_id, PATINDEX('%[^0-9.]%', column) AS n
        FROM table)
    UPDATE table
    SET column = STUFF(column, q.n, 1, '')
    FROM q
    WHERE table_id = q.table_id AND q.n != 0;


This code should be easy enough to follow. It removes the first nondigit in column in each row of the table. Once it goes through an iteration without finding any nondigits, it exits the loop. This will iterate the maximum number of nondigits in a single field in the table, but it's still inefficient because it runs PATINDEX against each row in the table at each iteration.

To solve that problem, we can maintain a column that specifies whether the column has been scrubbed yet. In my example, "NULL" represents unknown and should be the default value for this field.

DECLARE @done bit = 0;
WHILE @done = 0 BEGIN
    WITH q AS
        (SELECT table_id, PATINDEX('%[^0-9.]%', column) AS n
        FROM table
        WHERE COALESCE(column_scrubbed, 0) = 0)
    UPDATE table
    SET column = STUFF(column, q.n, 1, N''),
        column_scrubbed = 0
    FROM q
    WHERE table_id = q.table_id AND q.n != 0;

    IF @@ROWCOUNT = 0 SET @done = 1;

    -- if column_scrubbed is still NULL, then the PATINDEX
    -- must have given 0
    UPDATE table
    SET column_scrubbed = CASE
        WHEN column_scrubbed IS NULL THEN 1
        ELSE NULLIF(column_scrubbed, 0)

It's a sloppy design to add a column to the schema just to make this operation more efficient. The best approach, in my opinion, is to store intermediate results in a table-valued variable and apply it to the actual table at the end.

-- Find nondigit character in a string
DECLARE @pattern varchar(30) = '%[^0-9.]%';

-- Maps [table_id]s to scrubbed values
    (table_id int PRIMARY KEY,
    column nvarchar(200) NOT NULL,
    scrubbed bit NOT NULL);

-- Initialize current value of [column] for each [table_id]
INSERT INTO @t (table_id, column, scrubbed)
SELECT table_id, column, 0 FROM table;

-- Iterate to remove non-digits from @t.[column], ending when [scrubbed]
-- is set for all rows.
    (SELECT table_id, PATINDEX(@pattern, column) AS n
    FROM @t
    WHERE scrubbed = 0) q
ON [@t].table_id = q.table_id
UPDATE SET column = STUFF(column, q.n, 1, N'')
UPDATE SET scrubbed = 1;

-- Apply the results to permanent table
UPDATE table
SET column =
    (SELECT [@t].column FROM @t
    WHERE [@t].table_id = table.table_id);