The best unix shell scripting interview questions 2018 learn now!

4/3/2018 The Best Unix Shell Scripting Interview Questions 2018 - Learn Now!
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The Best Unix Shell Scripting Interview
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C Shell – Beginner
Q. What must you do before you are able to run your new script for the
first time by its name or with an alias?
You must make it executable, that is to execute the command:
chmod +x scriptname
Q. The following command is included in the .login script of a user:
alias whois ´grep !ˆ /etc/passwd´
What will be the output, when the user issues the following?
who is guru
If there is a defined user account named “guru”, or the string guru is contained
elsewhere in /etc/passwd file, then the output will be the entry which contains
the string “guru”, otherwise, it will be an empty line.
Q. If the condition If ( -r filename ) fails (returns false), what are the
possible reasons?
The possible reasons are:
a) filename is not readable by the owner of the process
b) filename does not exist
Q. Which is the difference between the next two statements?
set var = 99
@ var = 99
Using the typical assignment form (set …), the value assigned in var is the string
99. Using the @, the value is the integer 99.
Q. Given the code snippet:
@ n = 5
while ($n)
# actions
…
end

What actions should be performed inside the loop, in order to get out of
this loop?
Any command that changes the value of variable n, for it to become 0 sometime.
E.g., @ n—
Q. What will the output of the following commands be? Explain.
set names = (Kathrin Chris Jacob)
shift names
echo $#names
The output will be 2.
shift command gets rid of the first element of the array names. So, the echo
command will display 2 as the number of elements of the array.
Q. What does the command rehash do?
rehash recomputes the internal hash table for the PATH variable. If the new
command resides in a directory not listed in PATH, add this directory to PATH,
and then use rehash.
Q. How could you ensure that a script will be run in csh?
The first line of the script could be used to define the shell you want to use, as
follows:
#!/bin/csh
This is sufficient to run the script in csh.
Q. Given that script1 is an executable C shell script situated in directory
/home/myhomedir/project1/data/dir1, use three ways to run it, explaining the
pros and cons.
a) cd/home/myhomedir/project1/data/dir1/script1 (You should first cd to the
directory path)
b) /home/myhomedir/project1/data/dir1/script1 (You should include the
absolute directory path)
c) script1 (shortest form, but it works only if the directory path is added to the
PATH environment variable of the user)

Q. What will be the value of the sixrem variable, after executing this
command?
@ sixrem = $data[2] % 6
The expression divides the value of second element of the data array by 6 and
assigns the remainder of the division to the sixrem variable.
Q. Name two ways to obtain the length of a string, giving a simple
example for each one.
The two ways to obtain the length of a string are:
a) Using the wc command:
set string = “any string”
@ ln = ècho $string | wc -c` -1
b) Using the awk function length:
set ln = ècho $string | awk ‘{print length($0)}’`
Q. Create a script that displays a list of regular files from the current
directory.
#!/bin/csh -f
foreach i (*)
if ( -f $i ) then
print $i
endif
end
Q. Describe in short the word completion feature of the tcsh shell.
Completion works anywhere in the command line, not at just the end, for both
commands and filenames. Type part of a word and hit the Tab key, and the shell
replaces the incomplete word with the complete one in the input buffer. The
completion also adds a “/” to the end of completed directories and a space to the
end of other words. The shell parses the buffer to determine whether the word
you try to complete should be completed as a command, filename or variable.

The first word in the buffer and the first word following ‘;’, ‘|’, ‘|&’, ‘&&’ or ‘||’ is
considered to be a command. A word beginning with ‘$’ is considered to be a
variable. Anything else is a filename. An empty line is ‘completed’ as a filename.
Q. In tcsh, how are the remaining choices (if any) listed whenever the
word completion fails?
The remaining choices (if any), are listed only if the shell variable autolist is set.
Q. In tcsh, how do you disable filename substitution?
noglob shell variable can be set to disable the filename substitution feature.
Q. Compare the sched tcsh built-in command with the UNIX/Linux at
command.
These commands are similar but not the same. sched command runs directly
from the shell, so it has access to shell variables and settings at command can
run a scheduled command at exactly the specified time.
Q. Schedule a prompt change at 10:55 as a reminder for an oncoming
event.
sched 10:55 set prompt = ‘It’s time for the important meeting: >’
Q. What is the impact of -f option in the first line of a csh script?
(#!/bin/csh
versus
#!/bin/csh -f)
Using option -f, the shell does not load any resource or startup files (.cshrc for a
script), or perform any command hashing, and thus starts faster.
Q. How can you start a job in the background, and then terminate your
login session, without terminating the background job?
We can start a job in the background and then terminate our login session,
without terminating the background job using “no hangup” command, nohup:
nohup command > output_file &

Q. Which is the difference between
echo c{1,4,2,5,1}
and
echo [c]{1,4,2,5,1}?
The first echo will display c1 c4 c2 c5 c1, while the second displays only c1.
Q. Display the first and last arguments of a script, regardless of the
number of arguments, and without a loop.
my_var3 = $#argv
echomy_var1: $argv[$1]
echomy_last_var: $argv[$my_var3]
Q. How will you set the ‘search path’ in csv?
Search path in csv can be set as follows:
a) setenv PATH “/myfolder1 /bin: /myfolder2 /myfile3”
b) using list
set path = ( /myfolder1 /bin /myfolder2 /myfile3 )
Q. Create a tar archive into /home/user1/myarch.tar, including all files
ending in .c, .h, .l, .y,.o and .cc and also the Makefile from two
directories, ~/dir1 and ~/dir2.
tar cvf /home/user1/myarch.tar ~/{dir1,dir2}/{Makefile,*.{c,h,l,o,y,cc}}
or
tar cvf /home/user1/myarch.tar ~/dir1/Makefile ~/dir1/*.[chloy] ~/dir1/*.cc
~/dir2/Makefile ~/dir2/*.[chloy] ~/dir2/*.cc
Q. Your script must be executed with exactly two arguments, otherwise
would be terminated. Write a code to implement these checks.
if ($#argv <> 2) then
echo “Usage: $0 arg1 arg2”
echo “You must give exactly two parameters”
exit 20
endif

Q. Write a pipeline that reads from the j-th line up to the k-th line of a text
file, without using awk.
set total = `cat textfile | wc -l`
set j = 10
set k = 18
@ count = $k – $j
head -$k textfile | tail -$count
C Shell – Intermediate
Q. Explain the following commands:
set names = (John Kathrin Chris Jacob)
set names = ($names[1-2] Angela $names[3-])
First command creates an array named names with the four names as its
elements.
Second command adds name Angela between Kathrin and Chris.
Q. How could you move cursor to specified coordinates on screen? (tcsh)
We can move the cursor to specified coordinates on the screen using echotc cm
column row
Q. What is the result of this loop?
foreach i ([A-Z]*) ? mv $i $i.csh ? end
The loop renames all files that begin with a capital letter, adding the “extension”
.csh.
Q. Assuming there is a label cleanup somewhere in a script, explain the
command onintr cleanup
The script will branch to label cleanup if it catches an interrupt signal.
Q. Is there a way to repeat a command for a predefined number of times,
without using a counter-controlled loop?
We can repeat a command for a predefined number of times, without using a

counter-controlled loop using the repeat command, e.g.,
repeat 5 ls >> listings
Q. csh and tcsh both support the filename & command completion
feature. But the feature works differently in csh than in tcsh. Name the
differences.
tcsh automatically completes filenames and commands when the Tab key is hit.
csh does so only when the filec variable is set, after the Esc key is hit.
Q. Name the special login files for csh&tcsh in the order used by each
shell.
If the command is executed from the (“/etc/csh.cshrc& /etc/csh.login”) system
files, the login shell will be started.
a) It will run the commands given in the files present in the home directory of the
user.~/.cshrc or ~/.tcshrc:During shell startup, it gets executed for each instance
of the shell. If ~/.cshrcs present, ‘tcsh’ uses this file else executes ‘~/.tcshrc’
b) ~/.login: at login it is executed after .cshrcby login shell
c) ~/.cshdirs: After .login (tcsh), it is executed by login shell
Q. What do the following lines do? Explain the differences.
ls > filename
ls >! Filename
In both forms, the output of ls command is redirected to filename. If filename
does not exist, it will be created, otherwise it will be truncated. When the first
form is used, if shell parameter noclobber is set and filename is an existing file,
an error results. The ‘!’ in the second form is used to suppress that check.
Q. You can run a script by its name, using an alias or using source.
Explain the differences in using each of the three methods. When is it
suitable to use each method?
A script executed by name is not run in current process (a child process is created
to run the script), so this method is suitable to be used only if the environment
variables and globally defined aliases (in $HOME/.cshrc) should be known to

the script.
The method that executes a script using an alias is a variant of executing the
script by name. In addition, if the alias is defined from shell prompt, it applies
only to the current process. To make the alias global, you must define it in
$HOME/.cshrc, but be careful to keep the number of aliases included there to a
relatively small number.
With the third method, the script runs in the current process, thus, any aliases
defined in the current process will be known to your script.
Q. How could you override a defined alias? Give a simple example.
To override an alias, precede the alias with backslash. Fox example, if you have
an alias named rm which runs a custom script, if you want to run the command
rm instead of the rm alias, you can do it as shown below:
rm filename
Q. You plan to write a script that will process the file passed to it as the
only argument on the command line. So, your script must accept at least
one argument and this single or first argument must be an existing file.
Write the necessary checks, displaying the appropriate messages.
#!/bin/csh
if ( $#argv == 0 ) then
echo Error: A file name must be supplied as argument
exit 10
else if ( ! -e $1 ) then
echo Error: $1 is not an existing file
exit 11
endif
commands to process the file
Q. Write a code excerpt that processes (here, just displays) the elements
of an array, from the first one to the last one.
set myarray = (value1, value2, value3, value4, value5)

set i = 1
while ( $#myarray > 0 )
echo “$i array’s element is: $myarray[1]”
shift myarray
@ i++
end
or,
set myarray = (value1, value2, value3, value4, value5)
set i = 1
foreach val ( $myarray[*] )
echo “$i array’s element is: $val”
@ i++
End
Q. Complete the last echo command with a descriptive message in the
following script. In other words, explain the value of pct variable.
#!/bin/csh
set duout = (`du -sk ~`)
@ dir_size = $duout[1]
set dfout = (`df -k | grep /home`)
@ home_size = $dfout[2]
@ pct = $dir_size * 100 / $home_size
echo “… $pct …”
echo “Your home directory takes $pct % of /home filesystem”
Q. Extract just the mode of a given file, using two different ways.
The mode of a given file can be extracted in the following ways:
a) set file_mode = `ls -l filename | tail -1 | cut -f1 -d” ” | cut -c2-`
b) set file_mode = `ls -l filename | awk ‘ /^-/ {print substr($1,2)}’`

Q. Which is the output of the following excerpt?
netstat -an | awk ‘/SHED/ {split($4,c,”.”); print “Connection to ” c[4] ”
from ” $5}’ | sort -nt” ” -k 3
Lines in the format “Connection to port_number from IP_Address”, for each
established connection, sorted by port_number.
Q. Find the position of a substring in a given string. Display a message if
the string does not contain this substring.
set sub = “str”
set pos = ` echo $string | awk -v s=$sub ‘{print index($0,s)}’`
if ( $pos == 0 ) then
echo “$string does not contain the substring $sub”
else
echo “$sub first occurrence in $string starts in position $pos”
endif
Q. Change the case of a string.
set string = “C Shell Programming”
set ustring = ècho $string | awk ‘{print toupper($0)}’` # = “C SHELL
PROGRAMMING”
set lstring = ècho $string | awk ‘{print tolower($0)}’` # = “c shell programming”
Q. Assume that in a script the value of a variable limt becomes equal to
92.1. Display the message:
Upper limit 92.10% in your_system
set system = `hostname`
printf “Upper limit %f.2%% in the %s” $limt $system
Q. Suppose a script contains the following snippet:
set fl = /home/dbuser5/reports/fs_report1.txt
echo $fl:e
echo $fl:r

echo $fl:t
echo $fl:h
What do you expect to be displayed?
txt
fs_report1
fs_report1.txt
/home/dbuser5/reports
Q. Create a script that converts the filenames from current directory to
lower case letters.
#!/bin/csh -f
foreachmy_old_file1 (`ls`)
setmy_new_ file1 = ècho $my_old_file1 | tr ´[A-Z]´ ´[a-z]´`
if (“$my_new_ file1” == “$my_old_file1”) then
continue
endif
mv $ my_old_file1 $my_new_ file1
end
Q. Name some basic differences between csh and tcsh.
tcsh includes a command-line editor, file name and command completion
features, and enhanced job control, in comparison with the Berkeley csh.
Q. Compare the tcsh shell variables correct and autocorrect.
autocorrect can be set to correct the word to be completed before any
completion attempt. correct can be set to ‘cmd’ to correct command names or to
‘all’ to correct the entire line, each time return is hit.
Q. What is the purpose of the special alias shell?
The shell special alias can be set to specify an interpreter other than the shell
itself.

Q. Which is the method to bind the keys to the standard vi or emacs
bindings?
Shell’s built-in command is bindkey. Its -e option binds all keys to standard
emacs bindings, while -v option binds to standard vi bindings.
Q. Which is the purpose of shell’s variable color?
Shell’s variable color, if set, enables color display for the built in ls-F and it passes
–color=auto to ls.
Q. Set your prompt to display username@hostname: pwd>
It can be done as follows:
set prompt = “%n@%m: %/ >”
Q. How can you start (from shell prompt) 2 commands “in the
background”, ensuring that the second command will start after the
completion of the ﬁrst one?
It can be done as follows:
(command 1 ; command 2) &
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