interview tips

while () {
    # process the line
}
__DATA__
# your data goes here

while (<:data>) {
    # process the line
}
__END__
# your data goes here

use POSIX qw(strftime);

$raw_time = (stat(DATA))[9];
$size     = -s DATA;
$kilosize = int($size / 1024) . 'k';

print "
Script. size is $kilosize\n";
print strftime("
Last script. update: %c (%Z)\n", localtime($raw_time));

__DATA__
DO NOT REMOVE THE PRECEDING LINE.






Everything else in this file will be ignored.

#!/usr/bin/perl

print "content-type: text/html \n\n";

# DEFINED HASH
%coins = ( "Quarter" , .25,
           "HalfDollar" ,    .50,
           "Penny" ,    .01,
           "Dime" ,    .10,
           "Nickel",   .05 );

# PRINT OLD HASH
while (($key, $value) = each(%coins)){
     print $key.", ".$value."
";
}

# DELETE THE ELEMENT PAIRS
delete($coins{Penny});
delete($coins{HalfDollar});

# PRINT THE NEW HASH
print "
";
while (($key, $value) = each(%coins)){
     print $key.", ".$value."
";
}

          keys  values
        +------+------+
        |  a   |  3   |
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+

        $ary{'a'}                       is true
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is true
        exists $ary{'a'}                is true (perl5 only)
        grep ($_ eq 'a', keys %ary)     is true

        undef $ary{'a'}

          keys  values
        +------+------+
        |  a   | undef|
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+

        $ary{'a'}                       is FALSE
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is FALSE
        exists $ary{'a'}                is true (perl5 only)
        grep ($_ eq 'a', keys %ary)     is true

        delete $ary{'a'}

          keys  values
        +------+------+
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+

        $ary{'a'}                       is false
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is false
        exists $ary{'a'}                is FALSE (perl5 only)
        grep ($_ eq 'a', keys %ary)     is FALSE

Chop and Chomp are two very similar functions. Both of them 
are used to delete symbols from the tail of the given 
string. While both work with string and list parameters, 
Chop deletes any ending symbol Chomp deletes only specified 
substring from the end. If you pass list to any of these 
two, all list elements will get processed and the return 
value would be the result of last operation.

chop() removes the last character from a scalar value.

chomp() checks whether the last characters of a string or 
list of strings match the input line separator defined by 
the $/ system variable. If they do, chomp removes them.

=for comment
This comment
    can be indented
        as much as I want
            and even more than I want
=cut

    $num_keys = scalar keys %hash;

perl -wE'say~~(()=<>)' yourfile

perl -pe '}{$_=$.' filename

perl -le 'open FILE, "/etc/passwd"; @_=; print $.'

perl -lne 'END{print $.}' /etc/passwd


open (FILE, $ARGV[0]) or die "Can't open '$ARGV[0]': $!";
$lines++ while ();
close FILE;
print "$lines\n";

if(“Hello there, neigbor”=~ /\S(\w+),/){

print “That actually matched ‘$&’.\n”;

}

匹配的部分是“there,”（空格，單詞，和一個逗號）。變數$1 中的值為there，而$&為整個被匹配的部分。

$_ Perl uses its favorite default variable. This is (mostly) like any other scalar variable, except for its unusual name
@_ you have to store this list somewhere, so Perl automatically stores the parameter list (another name for the argument list) in the special array variable named @_ for the duration of the subroutine
what happened to the program's own name (that's found in Perl's special variable $0, not @ARGV)
The exit status is then available in the $? variable (reminiscent of the same variable in the Bourne Shell) and is the same kind of number as the value returned by the system function: zero for success and nonzero for failure.

61.    How to read a complete file into a String ?

while(<>){
chomp;
$a.=$_
}
print $a;

62.    What is the difference between $@ and $! ?

When a normally fatal error happens during the execution of an eval block, the block is done running, but the program doesn't crash. Right after an eval finishes, you'll be wanting to know if it exited normally or caught a fatal error for you. The answer is in the special $@ variable. If the eval caught a fatal error, $@ will hold what would have been the program's dying words, perhaps something like: Illegal division by zero at my_program line 12. If there was no error, $@ will be empty. That means that $@ is a useful Boolean (true/false) value (true if there was an error), so you'll sometimes see code like this after an eval block:

    print "An error occurred: $@" if $@;

$!
 in the message? That's the human-readable complaint from the system. In general, when the system refuses to do something we've requested (like opening a file), $! will give you a reason (perhaps "permission denied" or "file not found," in this case).

---

Unix & Shell Scripting

1) How you can search multiple pattern using "grep" command?

grep 'pattern1\|pattern2' filename
grep -E 'pattern1|pattern2' filename
egrep 'pattern1|pattern2' filename
grep -e pattern1 -e pattern2 filename
grep -E 'pattern1.*pattern2|pattern2.*pattern1' filename
grep -E 'pattern1.*pattern2' filename-----include order
$ grep Manager employee.txt | grep Sales

2) What are different option in "grep: command?

grep-ipattern files：不區分大小寫地搜尋。預設情況區分大小寫，
grep-lpattern files ：只列出匹配的檔名，
grep-Lpattern files ：列出不匹配的檔名，
grep-wpattern files：只匹配整個單詞，而不是字串的一部分（如匹配‘magic’，而不是‘magical’）

Grep NOT using grep -v

3) How you can select a field from a particular row?

 cat grade.txt | awk 'NR==3 {print $2}'

4) How u can replace a blank space from a file?

cat grade.txt | sed 's/  *//g'|sed 's/\t//g' -----remove blank space and tab

there are two spaces in the search block,
*

5) What is the command to list the all process are running in that terminal?

ps -t

6) What is "kill" command? what is the meaning of using "9" with :kill: command?

kill -9 ------force kill process

強制踢人命令格式：pkill -kill -t pts
踢出tty1使用者的命令為： pkill -kill -t tty1

7) Any idea about scheduling the task?

Cron table

8) Do you know about the cron job?
9) Why we are using nohup?

All processes, except the at and batch requests, are killed when you log out. If you want a background process to continue running after you log out, you must use the nohup command to submit that background command.

1.       How do I remove a file whose name begins with a "-" ?

A.   rm ./-filename   OR rm -- -filename  OR rm - -filename

 

2.       How do I set the permissions on a symbolic link?

A.      Permissions on a symbolic link don't really mean anything. The only permissions that count are the permissions on the file that the link points to.

  ln -s 2.ksh link.ksh

Property/Action		Symbolic link	Hard Link	Junction
When symbolic link/ hard link/ junction is deleted ...		No change on target	Reference counter is decremented; when it reaches 0, the target is deleted	Target is deleted (except when using special tools)
When target is moved		Symbolic link becomes invalid	Hard link remains valid	Junction becomes invalid
Relative path		allowed	(unused)	not allowed (on save becomes an absolute path)
Drive requirement		Any drive allowed	Only same drive (link stored in descriptor)	Any drive allowed
Read of target		allowed		allowed
Windows	for files	from Vista / Windows 2008; administrator rights needed	yes	no
	for folders		no	yes
Unix	for files	yes	yes	(unused)
	for folders	yes	yes(root rights needed, although not for all filesystems)	(unused)

4.       How do I "include" one shell script. from within another shell script?

A.      All of the shells from the Bourne shell category (including rc) use the "." command. All of the shells from the C shell category use "source".

 eg . $HOME/.profile    ------------------

5.       What does fork do?

A.      The fork() function is used to create a new process from an existing process

 copy-on-write

9.       What is a zombie?

A.      When a program forks and the child finishes before the parent, the kernel still keeps some of its information

about the child in case the parent might need it -- for example, the parent may need to check the child's exit status. To be able to get this information, the parent calls wait(); when this happens, the kernel can discard the information.

In the interval between the child terminating and the parent calling wait(), the child is said to be a `zombie'. (If you do `ps', the child will have a `Z' in its status field to indicate this.) Even though it's not running, it's still taking up an entry in the process table.

 

Alternate definition of Zombie Process

A Unix process that has terminated (either because it has been killed by a signal or

because it has called exit()) and whose parent process has not yet received notification of its termination by executing wait() system call.

 

10.   How do you prevent Zombie from occuring?

A.      You need to ensure that your parent process calls wait() (or waitpid(), wait3(), etc.) for every child

process that terminates; or, on some systems, you can instruct the system that you are uninterested in child exit states.

 

Another approach is to fork() twice, and have the immediate child process exit straight away. This causes the grandchild process to be orphaned, so the init process is responsible for cleaning it up.

 

The other technique is to catch the SIGCHLD signal, and have the signal handler call waitpid() or wait3().

 

11.   How do I remove the marker(^M) at the end of each lines?

A.      On the Unix systems, this command will filter out the end-of-line markers:

 

       tr  -d '\015'   < file_with_eol_marker  >  file_without_eol_marker

      dos2unix

      unix2dos

       If you are on Omega, a simpler command is available:

       rmcr  file_with_eol_marker  >  file_without_eol_marker

 

12.   How do you find CPU limit on your account?

A.      You can determine your currently CPU limit by using the command:

 

       limit cputime

 

13.   How can you change the CPU limit for your account?

A.      The `limit' command may be used to change your CPU limit. For example, if you want to raise your cpu time limit up to 11 hours, use this command:

 

      limit cputime 11h

 

      Note that your limits can never exceed the "hard limits". You can determine your "hard limits" by   

      running the command:

 

      limit -h

 

14.   What is an Orphan process?

A.      A Unix process whose original parent has terminated and which has become a child of "init(1)".

 

15.   What is difference between soft link and hard link?

A.      Soft Link can be created on a directory where as hard link necessarily has to be on a file.

      Soft link can reside on same File System where as hard link can reside on other File system as well.

 

 

17.   How do you find all the processes that belong to you?

ps –ef | grep myname

-e to display all the processes.

-f to display full format listing.

18.   How do you kill a process?    kill -9 PID

Number	Name	Description	Used for
0	SIGNULL	Null	Check access to pid
1	SIGHUP	Hangup	Terminate; can be trapped
2	SIGINT	Interrupt	Terminate; can be trapped
3	SIGQUIT	Quit	Terminate with core dump; can be
9	SIGKILL	Kill	Forced termination; cannot be trapped
15	SIGTERM	Terminate	Terminate; can be trapped
24	SIGSTOP	Stop	Pause the process; cannot be trapped
25	SIGTSTP	Terminal	stop Pause the process; can be
26	SIGCONT	Continue	Run a stopped process

 

19.   How do you find the version of OS that you are using?   uname -a

20.   What is difference in using single and double quotes in shell script?

Use single quote when you want to literally print everything inside the single quote.

Use double quotes when you want to display the real meaning of special variables.

echo $HOME

>/home/Emma

echo '$HOME'

>$HOME

echo "$HOME"

>/home/Emma

 

21.   What is the use of eval in shell scripting?

eval is useful when cmd contains something which needs to be evaluated by the shell.

vnix$ set -- one two three  # sets $1 $2 $3
vnix$ echo $1
one
vnix$ n=1
vnix$ echo ${$n}  # attempt to echo $1
bash: ${$n}: bad substitution
vnix$ eval echo \${$n}
one

22.   What is use of 'tee' command?

Tee commands are quite useful particularly if you want to view output and keep it for other use.

cat 1.txt | tee 2.txt

output 1.txt and save it to 2.txt

23.   What is the use of 'nohup'?

All processes, except the at and batch requests, are killed when you log out. If you want a background process to continue running after you log out, you must use the nohup command to submit that background command.

24.   How do you run a process in background?

& makes the command run in the background

9. If you have submitted a job to run in foreground, how could you move it to background?

Correct Answer:

First suspend the job with -z, and then use the bg command to move it to the background.

---

Databases

1.     How you can transfer the data from database to a file? how u can list the only uniq value of some field?

 

export to itftacttest.dat of del select curr_mo from ITFT.ACTUALS_DETAIL_2011, ITFT.ACTUALS_CUR_DATE where LEDGER_YEAR_NUM = CURR_YEAR and LEDGER_MONTH_NUM = CURR_MO fetch first 1 row only

 

Export to c:\org.txt of del select * from org

例二：改變del格式檔案的格式控制符
export to c:\staff.txt of del modified by coldel$ chardel'' decplusblank select * from staff
在 該例中，modified子句用於控制各種符號，coldel表示欄位之間的間隔符，預設情況為逗號，現在改為$號；chardel表示字串欄位用什麼 符號引用，預設情況下為一對雙引號括起來，現在改為用一對單引號括起來；decplusblank表示對於十進位制資料型別，用空格代替最前面的加號，因為 預設情況下會在十進位制資料前面加上正負號的。

 

2.     What is normalization?

(1NF):Eliminate duplicative columns from the same table and data must be broken up into smallest units

(2NF):Remove subsets of data that apply to multiple rows of a table and place them in separate tables.

     

Create relationships between these new tables and their predecessors through the use of foreign keys

(3NF):Remove columns that are not dependent upon the primary key.such as total=price*Qty

3.     Tell me about the Join? What are the different Joins?

Nested loops join: 確定一個驅動表(outer table)，另一個表為inner table，驅動表中的每一行與inner表中的相應記錄JOIN。類似一個巢狀的迴圈。適用於驅動表的記錄集比較小（<10000）而且inner表需要有有效的訪問方法（Index）。需要注意的是：JOIN的順序很重要，驅動表的記錄集一定要小，返回結果集的響應時間是最快的。

Hash join: 將兩個表中較小的一個在記憶體中構造一個HASH表（對JOIN KEY），掃描另一個表，同樣對JOIN KEY進行HASH後探測是否可以JOIN。適用於記錄集比較大的情況。需要注意的是：如果HASH表太大，無法一次構造在記憶體中，則分成若干個partition，寫入磁碟的temporary segment，則會多一個寫的代價，會降低效率。

Merge join:將兩個表排序，然後將兩個表合併。通常情況下，只有在以下情況發生時，才會使用此種JOIN方式：

1.RBO模式

2.不等價關聯(>,=,<=,<>

3.HASH_JOIN_ENABLED=false

4.資料來源已排序

4.     What is inner Join , outer join, self join?

Inner Join: combine columns value of two tables base opon the join-predicate.

Full outer join:return all matched value from two tables, null in case of no matching.

Left outer join: return all values from left table plus matched value from right table.null in case of no matching

Right outer join:return all values from right table plus matched value from left table.null in case of no matching

Self join: self join is just like other join, except two instance of the same table will be joined in the query

5.     You have table Employee. List all the employee with the manger name? fields are empId, F_name, L_name, ManId.

select a.*,b.* from T1 a, T1 b where a.id=b.managerid

 

6.     What is Index? Why we are using Index? Is it fine if we can define the index with every column?

Index is a data structure that improve the speed of data retrieval operations on a table at the cost of lower writes and increase storage space. 

If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same time, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used

7.     What is cursor? Describe all stages?

Cursor allow row-by-row processing of the resultset.

DECLARE vend_cursor CURSOR
    FOR SELECT * FROM Purchasing.Vendor
OPEN vend_cursor
FETCH NEXT FROM vend_cursor

WHILE @@FETCH_STATUS = 0

BEGIN
----- 
FETCH NEXT FROM vend_cursor
END 
CLOSE vend_cursor;
DEALLOCATE vend_cursor

8.     What is procedure? why we are using the procedure?

  a set of SQL statements with an assigned name that's stored in the database in compiled form. so that it can be shared by a number of programs. 

First, stored procedures allow a lot more flexibility offering capabilities such as conditional logic.

Second, because stored procedures are stored within the DBMS, a single stored procedure can execute a complex set of SQL statements.Productivity.

Third, SQL Server pre-compiles stored procedures such that they execute optimally.

Fourth, client developers are abstracted from complex designs. They would simply need to know the stored procedure's name and the type of data it returns.

CREATE PROCEDURE HumanResources.uspGetEmployeesTest2 
    @LastName nvarchar(50), 
    @FirstName nvarchar(50) 
AS 

BEGIN

    SET NOCOUNT ON;
    SELECT FirstName, LastName, Department
    FROM HumanResources.vEmployeeDepartmentHistory
    WHERE FirstName = @FirstName AND LastName = @LastName
    AND EndDate IS NULL;

END

9.     What is trigger? How many trigger define on a table?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.

CREATETRIGGER[trigger_delete]

ONdbo.TableA

afterDELETE,insert,update

AS

 

 

BEGIN

    INSERTINTO dbo.TBVALUES(4,'Keeking')

    print'inserted表'

    select*frominserted

    print'deleted表'

    select*fromdeleted

END

10.  Is it possible that trigger not getting fired even some transaction is happing with data?

GRANT/REVOKE isn't allowed in stored procedures or triggers

DISABLE TRIGGER Person.uAddress ON Person.Address;

11.  What is truncate and delete?

12.  What is bcp? option of bcp?

No bcp in DB2. bcp used in MS SQLserver and Sybase.

BCP AdventureWorks.sales.currency out c:\currency1.txt -c -U"sa" -P"password"

注：BCP除了可以在控制檯執行外，還可以通過呼叫SQL Server的一個系統儲存過程xp_cmdshell以SQL語句的方式執行BCP。如上述第一條命令可改寫為

EXEC master..xp_cmdshell 'BCP AdventureWorks.sales.currency out c:\currency1.txt -c -U"sa" -P"password"'

bcp [database_name.] schema.{table_name | view_name | "query" {in data_file | out data_file | queryout data_file | format nul}

   [-a packet_size]
   [-b batch_size]
   [-c]
   [-C { ACP | OEM | RAW | code_page } ]
   [-d database_name]
   [-e err_file]
   [-E]
   [-f format_file]
   [-F first_row]
   [-h"hint [,...n]"] 
   [-i input_file]
   [-k]
   [-K application_intent]
   [-L last_row]
   [-m max_errors]
   [-n]
   [-N]
   [-o output_file]
   [-P password]
   [-q]
   [-r row_term]
   [-R]
   [-S [server_name[\instance_name]]
   [-t field_term]
   [-T]
   [-U login_id]
   [-v]
   [-V (80 | 90 | 100 )]
   [-w]
   [-x]
   /?

13.  Have you done performance tuning?

 No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables.

1.Avoid tablespace scan for large tables

2.Avoid sorting if possible.(index for order by and group by,union all vs union,distinct)

3.Consider lock_isolation to avoid lock

4.Do not forget to run status.

runstats on table tablename and indexes all

5.Use buffer pool

6.Right Data type

14.  What are the scripts you have written in your previous project?

SQL on join and aggregate function?