Unix Programming Interview Questions

1) Explain about the case statement.
The case statement compares word to the patterns from top to bottom, and performs the commands associated with the first, and only the first, pattern that matches. The patterns are written using the shells pattern matching rules, slightly generalized.
 
2) Explain the basic forms of each loop?
There are three loops; for, while and until. For loop is by far the most commonly used form of loop. Basically like other programs it executes a given set of commands and instructions. While and until forms of loop use the exit status from a command based system. They control the execution of the commands in the body of the loop.
 
3) Describe about awk and sed?
The awk program processes this to report the changes in an easier to understand format. Sed output is always behind its input by one line; there is always a line of input that has been processed but not printed, and this would introduce an unwanted delay.
 
4) Explain about signal argument?
The sequence of commands is a single argument, so it must almost always be quoted. The signal numbers are small integers that identify the signal. For example, 2 is the signal generated by pressing the DEL key, and 1 is generated by hanging up the phone. Unless a program has taken explicit action to deal with signals, the signal will terminate it.
 
5) Explain about exec?
The exec is just for efficiency, the command would run just as well without it. Exec is a shell built-in that replaces the process running this shell by the named program, thereby saving one process- the shell that would normally wait for the program to complete. Exec could be used at the end of the enhanced cal program when it invokes /usr/bin/cal.
 
6) Explain about trap command
The trap command sequence must explicitly invoke exit, or the shell program will continue to execute after the interrupt. The command sequence will be read twice: once when the trap is set and once when it is invoked. Trap is used sometimes interactively, most often to prevent a program from being killed by the hangup signal.
 
7) Explain about sort command?
The sort command has an option –o to overwrite a file:
$ sort file1 -0 file2
Is equivalent to
$ sort file1 > file2
If file 1 and file 2 are the same file, redirection with > will truncate the input file before it is sorted. The –o option works correctly because the input is sorted and saved in a temporary file before the output file is created. Many other commands could also use a –o option.
 
8) Explain about the command overwrite?
Overwrite is committed to changing the original file. If the program providing input to overwrite gets an error, its output will be empty and overwrite will dutifully and reliably destroy the argument file. Overwrite could ask for conformation before replacing the file, but making overwrite interactive would negate its efficiency. Overwrite could check that its input is empty.
 
9) Explain about kill command?
The kill command only terminates processes specified by process-id when a specific background process needs to be killed, you must usually run ps to find the process-id and then re type it as an argument to kill. Killing process is dangerous and care must be taken to kill the right processes.
 
10) Explain about the shell variable IFS?
The shell variable IFS (internal field separator) is a string of characters that separate words in argument lists such as back quotes and for statements. Normally IFS contains a blank, a tab, and a new line, but we can change it to anything useful, such as just a newline.
 
11) Explain about the rules used in overwrite to preserve the arguments to the users command?
Some of the rules are
• $* and $@ expand into the arguments and are rescanned; blanks in arguments will result in multiple arguments.
• “$*” is a single word composed of all the arguments to the shell file joined together with spaces.
• “$@” is identical to the arguments received by the shell file: blanks in arguments are ignored and the result is a list of words identical to the original arguments.
 
12) Explain about @@@ lines?
@@@ Lines are counted (but not printed), and as long as the count is not greater than the desired version, the editing commands are passed through. Two ed commands are added after those from the history file: $d deletes the single @@@ line that sed left on the current version.
 
13) Explain about vis?
Vis that copied its standard input to its standard output, except that it makes all non printing characters visible by printing them as \nnn, where nnn is the octal value of the character. Vis is invaluable for detecting strange or unwanted characters that may have crept into files.
 
14) Is the function call to exit at the end of vis necessary?
The call to exit at the end of vis is not necessary to make the program work properly, but it ensures that any caller of the program will see a normal exit status from the program when it completes. An alternate way to return status is to leave main with return 0; the return value from main is the program`s exit status.
 
15) Explain about fgets?
Fgets (buf, size, fp) fetches the next line of input from fp, up to and including a newline, into buf, and adds a terminating \0; at most size-1 characters are copied. A Null value is returned at the end of the file.
 
16) Explain about efopen page?
The routine efopen encapsulates a vey common operation: try to open a file; if it`s not possible, print an error message and exit. To encourage error messages that identify the offending program, efopen refers to an external string program containing the name of the program, which is set in main.

17) Explain about yacc parser generator?
Yacc is a parser generator that is a program for converting a grammatical specification of a language like the one above into a parser that will parse statements in the language.

Posted in: UNIX