Linux application programming questions
- Explain the difference between a static library and a dynamic library? - Static library is linked into the executable, while a dynamic library (or shared object) is loaded while the executable has started.
- How do you create a static library? - If you have a collection of object (.o) files, you can do it by running ar command. Generally a static library has a .a extension, and you can link it into an executable by providing -l libraryname to gcc.
- Where should the developed libraries be installed on the system? - GNU recommends /usr/local/bin for binaries and /usr/local/lib for libraries.
- What’s LD_LIBRARY_PATH? - It’s an environment variable that lists all the directories which should be searches for libraries before the standard directories are searched.
- How do you create a shared library? - Create the object file with -fPIC for position-independent code, then run gcc with -shared option.
- How do you install a shared library? - Run ldconfig in the standard directory that it’s installed in.
- What does ldd do? - It shows a list of installed shared libraries.
- How do you dynamically load a library in your app? - Use dlopen()
- What does nm command do? - It reports the list of symbols in a given library.
Basic shell scripting questions
- How do you find out what’s your shell? - echo $SHELL
- What’s the command to find out today’s date? - date
- What’s the command to find out users on the system? - who
- How do you find out the current directory you’re in? - pwd
- How do you remove a file? - rm
- How do you remove a - rm -rf
- How do you find out your own username? - whoami
- How do you send a mail message to somebody? - mail somebody@techinterviews.com -s ‘Your subject’ -c ‘cc@techinterviews.com‘
- How do you count words, lines and characters in a file? - wc
- How do you search for a string inside a given file? - grep string filename
- How do you search for a string inside a directory? - grep string *
- How do you search for a string in a directory with the subdirectories recursed? - grep -r string *
- What are PIDs? - They are process IDs given to processes. A PID can vary from 0 to 65535.
- How do you list currently running process? - ps
- How do you stop a process? - kill pid
- How do you find out about all running processes? - ps -ag
- How do you stop all the processes, except the shell window? - kill 0
- How do you fire a process in the background? - ./process-name &
- How do you refer to the arguments passed to a shell script? - $1, $2 and so on. $0 is your script name.
- What’s the conditional statement in shell scripting? - if {condition} then … fi
- How do you do number comparison in shell scripts? - -eq, -ne, -lt, -le, -gt, -ge
- How do you test for file properties in shell scripts? - -s filename tells you if the file is not empty, -f filename tells you whether the argument is a file, and not a directory, -d filename tests if the argument is a directory, and not a file, -w filename tests for writeability, -r filename tests for readability, -x filename tests for executability
- How do you do Boolean logic operators in shell scripting? - ! tests for logical not, -a tests for logical and, and -o tests for logical or.
- How do you find out the number of arguments passed to the shell script? - $#
- What’s a way to do multilevel if-else’s in shell scripting? - if {condition} then {statement} elif {condition} {statement} fi
- How do you write a for loop in shell? - for {variable name} in {list} do {statement} done
- How do you write a while loop in shell? - while {condition} do {statement} done
- How does a case statement look in shell scripts? - case {variable} in {possible-value-1}) {statement};; {possible-value-2}) {statement};; esac
- How do you read keyboard input in shell scripts? - read {variable-name}
- How do you define a function in a shell script? - function-name() { #some code here return }
- How does getopts command work? - The parameters to your script can be passed as -n 15 -x 20. Inside the script, you can iterate through the getopts array as while getopts n:x option, and the variable $option contains the value of the entered option.
General UNIX interview questions
- What are the main differences between Apache 1.x and 2.x?
- What does the “route” command do?
- What are the read/write/execute bits on a directory mean?
- What does iostat do?
- what does vmstat do?
- What does netstat do?
- What is the most graceful way to bring a system into single user mode?
- How do you determine disk usage?
- What is AWK?
- What is SED?
- What is the difference between binaries in /bin, and /usr/bin?
- What is a dynamically linked file?
- What is a statically linked file?
Oracle on Linux/Unix questions
How many memory layers are in the shared pool?- How do you find out from the RMAN catalog if a particular archive log has been backed-up?
- How can you tell how much space is left on a given file system and how much space each of the file system’s subdirectories take-up?
- Define the SGA and how you would configure SGA for a mid-sized OLTP environment? What is involved in tuning the SGA?
- What is the cache hit ratio, what impact does it have on performance of an Oracle database and what is involved in tuning it?
- Other than making use of the statspack utility, what would you check when you are monitoring or running a health check on an Oracle 8i or 9i database?
- How do you tell what your machine name is and what is its IP address?
- How would you go about verifying the network name that the local_listener is currently using?
- You have 4 instances running on the same UNIX box. How can you determine which shared memory and semaphores are associated with which instance?
- What view(s) do you use to associate a user’s SQLPLUS session with his o/s process?
- What is the recommended interval at which to run statspack snapshots, and why?
- What spfile/init.ora file parameter exists to force the CBO to make the execution path of a given statement use an index, even if the index scan may appear to be calculated as more costly?
- Assuming today is Monday, how would you use the DBMS_JOB package to schedule the execution of a given procedure owned by SCOTT to start Wednesday at 9AM and to run subsequently every other day at 2AM.
- How would you edit your CRONTAB to schedule the running of /test/test.sh to run every other day at 2PM?
- What do the 9i dbms_standard.sql_txt() and dbms_standard.sql_text() procedures do?
- In which dictionary table or view would you look to determine at which time a snapshot or MVIEW last successfully refreshed?
- How would you best determine why your MVIEW couldn’t FAST REFRESH?
- How does propagation differ between Advanced Replication and Snapshot Replication (read-only)?
- Which dictionary view(s) would you first look at to understand or get a high-level idea of a given Advanced Replication environment?
- How would you begin to troubleshoot an ORA-3113 error?
- Which dictionary tables and/or views would you look at to diagnose a locking issue?
- An automatic job running via DBMS_JOB has failed. Knowing only that “it’s failed”, how do you approach troubleshooting this issue?
- How would you extract DDL of a table without using a GUI tool?
- You’re getting high “busy buffer waits” - how can you find what’s causing it?
- What query tells you how much space a tablespace named “test” is taking up, and how much space is remaining?
- Database is hung. Old and new user connections alike hang on impact. What do you do? Your SYS SQLPLUS session IS able to connect.
- Database crashes. Corruption is found scattered among the file system neither of your doing nor of Oracle’s. What database recovery options are available? Database is in archive log mode.
- Illustrate how to determine the amount of physical CPUs a Unix Box possesses (LINUX and/or Solaris).
- How do you increase the OS limitation for open files (LINUX and/or Solaris)?
- Provide an example of a shell script which logs into SQLPLUS as SYS, determines the current date, changes the date format to include minutes & seconds, issues a drop table command, displays the date again, and finally exits.
- Explain how you would restore a database using RMAN to Point in Time?
- How does Oracle guarantee data integrity of data changes?
- Which environment variables are absolutely critical in order to run the OUI?
- What SQL query from v$session can you run to show how many sessions are logged in as a particular user account?
- Why does Oracle not permit the use of PCTUSED with indexes?
- What would you use to improve performance on an insert statement that places millions of rows into that table?
- What would you do with an “in-doubt” distributed transaction?
- What are the commands you’d issue to show the explain plan for “select * from dual”?
- In what script is “snap$” created? In what script is the “scott/tiger” schema created?
- If you’re unsure in which script a sys or system-owned object is created, but you know it’s in a script from a specific directory, what UNIX command from that directory structure can you run to find your answer?
- How would you configure your networking files to connect to a database by the name of DSS which resides in domain icallinc.com?
- You create a private database link and upon connection, fails with: ORA-2085: connects to . What is the problem? How would you go about resolving this error?
- I have my backup RMAN script called “backup_rman.sh”. I am on the target database. My catalog username/password is rman/rman. My catalog db is called rman. How would you run this shell script from the O/S such that it would run as a background process?
- Explain the concept of the DUAL table.
- What are the ways tablespaces can be managed and how do they differ?
- From the database level, how can you tell under which time zone a database is operating?
- What’s the benefit of “dbms_stats” over “analyze”?
- Typically, where is the conventional directory structure chosen for Oracle binaries to reside?
- You have found corruption in a tablespace that contains static tables that are part of a database that is in NOARCHIVE log mode. How would you restore the tablespace without losing new data in the other tablespaces?
- How do you recover a datafile that has not been physically been backed up since its creation and has been deleted. Provide syntax example.
SQL/XML Queries
This chapter describes the
syntax of Connect for SQL/XML queries. The following conventions are used to
document the query syntax:
- Italic type indicates variables.
- Square brackets [ ] surround optional items.
- A vertical rule | indicates an OR separator to delineate items.
- Curly brackets { } surround items that can repeat zero or more times.
The following SQL/XML query syntax shows the high-level
components of a Connect for SQL/XML query.
sqlxml_query ::= selectSQL_value_expression|XML_value_expression{,SQL_value_expression | XML_value_expression}rest_of_sql_select
The following sections define
each of the high-level components of a Connect for SQL/XML query:
SQL_value_expression
SQL_value_expression is all level 1 alternatives
for a value expression, including the optional AS clause, as defined in
the document ISO/IEC 9075-2. XML_value_expression
The syntax for
XML_value_expression is:
where:
value_expression_primary is a
primitive value expression, such as a column reference, a variable
reference, a parameter reference, and a literal. The complete
definition can be found in the document ISO/IEC
9075-2. Also, value_expression_primary can
be the SQL/XML operator XMLAGG. This operator is described in the
following section. XML_value_function is one of the following
SQL/XML operators: XMLCONCAT, XMLELEMENT, or XMLFOREST. These operators
are described next. XMLAGG
XMLAGG produces a
forest of XML elements from a collection of XML elements. XMLAGG
accepts a single XML value expression as its argument, and produces a
forest of elements by collecting the XML values that are returned from
multiple rows and concatenating the values to make one value. XMLAGG
concatenates the values returned from one column of multiple rows,
unlike XMLCONCAT, which concatenates the values returned from multiple
columns in the same row.
If a
returned value is NULL, it is ignored in the result. If all the
returned values are NULL, the result is NULL.
Subqueries in SQL/XML are allowed to return only one
row; therefore, to return more than one row of values in a SQL/XML
subquery, you must use XMLAGG.
Syntax
where:
See "XML_value_expression" for a definition of
XML_value_expression. sort_list is all alternatives
for a sort list as defined in the document ISO/IEC
9075-2.
Example 1
The following example produces
a list of all employees in the company by producing an element for each
employee, ordering the list by the last name, and concatenating the
result. The result set will have one column and one row.
SELECT XMLAGG (XMLELEMENT (Name "emp", e.LastName) ORDER BY e.LastName) AS "Employee List" FROM Employees e
Result Set:
Example 2
The
following example demonstrates using XMLAGG in a subquery.
SELECT XMLELEMENT(NAME "emp", e.LastName, XMLELEMENT(Name "projects", (SELECT XMLAGG(XMLELEMENT(NAME "project, p.ProjID)) FROM Projects p WHERE EXISTS ( SELECT * FROM Assignments a WHERE a.EmpId=e.EmpId and a.ProjId=p.ProjId)))) AS "Employees" FROM Employees e
Result Set:
Example 3
The following example uses a Group
By clause to group all tasks assigned to each employee and employee
project.
SELECT XMLELEMENT (NAME "Employee_Tasks_by_Project", XMLATTRIBUTES (a.EmpId, a.ProjId AS "Project"), XMLAGG (XMLELEMENT (NAME "Task", a.Task) ) ) AS "Employee Task List" FROM Assignments a GROUP BY a.EmpId, a.ProjId
Result Set:
XMLCONCAT
XMLCONCAT produces a forest of
elements by concatenating a list of XML values. XMLCONCAT accepts a
list of XML value expressions as its arguments, and produces a forest
of elements by concatenating the XML values that are returned from the
same row to make one value. XMLCONCAT performs a similar operation as
XMLFOREST, except that XMLCONCAT expressions must evaluate to type XML.
If a returned value is NULL, it
is ignored in the result. If all the returned values are NULL, the
result is NULL.
Syntax
See "XML_value_expression" for a
definition of
XML_value_expression.
Example 1
The following example produces an XML element for the
first and last names, concatenates the result, and creates a one-column
result set:
SELECT XMLCONCAT ( XMLELEMENT (NAME "first", e.FirstName), XMLELEMENT (NAME "last", e.LastName)) AS "Result" FROM Employees e
Result Set:
Example 2
The
following example produces an XML element for the first and last names,
concatenates the result, and creates a two-column result set:
SELECT e.EmpId, XMLCONCAT ( XMLELEMENT (NAME "first", e.FirstName), XMLELEMENT (NAME "last", e.LastName)) AS "Result" FROM Employees e
Result Set:
XMLELEMENT
XMLELEMENT is an operator that returns an XML element
given an XML element name, an optional list of attributes, and an
optional list of values as the content of the new element.
Syntax
XMLELEMENT ::= XMLELEMENT (NAMExml_element_name[XMLATTRIBUTES (value_expression[ASattribute_name] [{,value_expression[ASattribute_name]}] ) [{,SQL_value_expression|XML_value_expression}])
where:
xml_element_name is any valid
XML element name. This value must be within double quotes.
XMLATTRIBUTES creates a list of XML
attributes for the enclosing XML element.
attribute_name is any valid XML
attribute name. This value must be within double quotes. value_expression is all level 1 alternatives for
a value expression as defined in the document ISO/IEC 9075-2.
See
"SQL_value_expression" for
a definition of
SQL_value_expression.
See "XML_value_expression" for a
definition of
XML_value_expression.
Example 1
The following example creates the Employee element
with a subelement named name. All values in the LastName column of the
Employees table are returned.
SELECT XMLELEMENT(NAME "Employee", XMLELEMENT(NAME "name", e.LastName)) AS "Employee List" FROM Employees e
Result Set:
Example 2
The
following example demonstrates defining an XML attribute within an XML
element.
Result Set:
XMLFOREST
XMLFOREST produces a forest of XML elements from a given
list of arguments. XMLFOREST accepts a list of SQL value expressions as
its arguments, and produces an XML element from each value returned.
XMLFOREST uses the name of the column as the name of the XML element,
unless otherwise specified in an optional AS clause, and uses the value
of the SQL value expression as the content of the XML element.
If a value expression evaluates to
NULL, then no element is created for that expression. This is unlike
XMLELEMENT, where an empty element is created if a value expression
evaluates to NULL.
Syntax
XMLFOREST ::= XMLFOREST (value_expression[ASforest_element_name][{,value_expression[ASforest_element_name]}] )
where:
value_expression is all level 1
alternatives for a value expression as defined in the document ISO/IEC 9075-2. forest_element_name is any valid SQL
identifier as defined in the document ISO/IEC
9075-2.
Example
The following example produces four
elements (EmpID, FirstName, LastName, and Start) from the value
expressions e.EmpID, e.FirstName, e.LastName, and e.StartDate,
concatenates the elements produced for each employee, and produces one
row for each employee in the result set.
SELECT XMLFOREST ( e.EmpId, e.FirstName, e.LastName, e.StartDate AS "Start") AS "EmployeeInformation" FROM Employees e
Result Set:
rest_of_sql99_select
This section of a SQL/XML query allows you to specify
the database table from which the data is retrieved and the conditions
by which the data is retrieved. The From clause defines the database
tables and other optional SQL clauses, such as the Where clause, define
the conditions.
Syntax
These clauses are defined in
the document ISO/IEC 9075-2.
Executing SQL/XML Queries
The SQL/XML JDBC driver
processes the SQL/XML query and sends one or multiple SQL statements
through to the database to retrieve the result set.
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