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Unix Interview Questions and Answers
Construct pipes to execute the following jobs.
1. Output of who should be displayed on the screen with
value of total number of users who have logged in
displayed at the bottom of the list.
2. Output of ls should be displayed on the screen and
from this output the lines containing the word ‘poem’
should be counted and the count should be stored in a
file.
3. Contents of file1 and file2 should be displayed on
the screen and this output should be appended in a file
.
From output of ls the lines containing ‘poem’ should be
displayed on the screen along with the count.
4. Name of cities should be accepted from the keyboard .
This list should be combined with the list present in a
file. This combined list should be sorted and the sorted
list
should be stored in a file ‘newcity’.
5. All files present in a directory dir1 should be
deleted any error while deleting should be stored in a
file ‘errorlog’.
Explain the following commands.
$ ls > file1
$ banner hi-fi > message
$ cat par.3 par.4 par.5 >> report
$ cat file1>file1
$ date ; who
$ date ; who > logfile
$ (date ; who) > logfile
What is the significance of the “tee” command?
It reads the standard input and sends it to the standard
output while redirecting a copy of what it has read to
the file specified by the user.
What does the command “ $who | sort –logfile > newfile”
do?
The input from a pipe can be combined with the input
from a file . The trick is to use the special symbol “-“
(a hyphen) for those commands that recognize the hyphen
as std input.
In the above command the output from who becomes the std
input to sort , meanwhile sort opens the file logfile,
the contents of this file is sorted together with the
output of who (rep by the hyphen) and the sorted output
is redirected to the file newfile.
What does the command “$ls | wc –l > file1” do?
ls becomes the input to wc which counts the number of
lines it receives as input and instead of displaying
this count , the value is stored in file1.
Which of the following commands is not a filter man ,
(b) cat , (c) pg , (d) head
man A filter is a program which can receive a flow
of data from std input, process (or filter) it and send
the result to the std output.
How is the command “$cat file2 “ different from “$cat
>file2 and >> redirection operators ?
is the output redirection operator when used it
overwrites while >> operator appends into the file.
Explain the steps that a shell follows while processing
a command.
After the command line is terminated by the key, the
shell goes ahead with processing the command line in one
or more passes. The sequence is well defined and assumes
the following order.
Parsing: The shell first breaks up the command line into
words, using spaces and the delimiters, unless quoted.
All consecutive occurrences of a space or tab are
replaced here with a single space.
Variable evaluation: All words preceded by a $ are
valuated as variables, unless quoted or escaped.
Command substitution: Any command surrounded by
back quotes is executed by the shell which then replaces
the standard output of the command into the command
line.
Wild-card interpretation: The shell finally scans the
command line for wild-cards (the characters *, ?, [, ]).
Any word containing a wild-card is replaced by a sorted
list of
filenames that match the pattern. The list of these
filenames then forms the arguments to the command.
PATH evaluation: It finally looks for the PATH variable
to determine the sequence of directories it has to
search in order to hunt for the command.
What difference between cmp and diff commands?
cmp - Compares two files byte by byte and displays the
first mismatch diff - tells the changes to be made to
make the files identical
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