Shell script questions
70 shell script questions
exec tempfile
1) How to pass argument to a script ?
./script argument
Example : Script will show filename
./show.sh file1.txt
cat show.sh
#!/bin/bash
cat $1
2) How to use argument in a script ?
First argument: $1,
Second argument : $2
Second argument : $2
Example : Script will copy file (arg1) to destination (arg2)
./copy.sh file1.txt /tmp/
cat copy.sh
#!/bin/bash
cp $1 $2
3) How to calculate number of passed arguments ?
$#
4) How to get script name inside a script ?
$0
5) How to check if previous command run successful ?
$?
6) How to get last line from a file ?
tail -1
7) How to get first line from a file ?
head -1
8) How to get 3rd element from each line from a file ?
awk '{print $3}'
9) How to get 2nd element from each line from a file, if first equal FIND
awk '{ if ($1 == "FIND") print $2}'
10) How to debug bash script
Add -xv to #!/bin/bash
Example
#!/bin/bash –xv
11) Give an example how to write function ?
function example {
echo "Hello world!"
}
echo "Hello world!"
}
12) How to add string to string ?
V1="Hello"
V2="World"
let V3=$V1+$V2
echo $V3
V2="World"
let V3=$V1+$V2
echo $V3
Output
Hello+World
13) How to add two integers ?
V1=1
V2=2
V3=$V1+$V2
echo $V3
V2=2
V3=$V1+$V2
echo $V3
Output
3
3
Remember you need to add "let" to line V3=$V1+$V2
then echo $V3 will give 3if without let , then it will beecho $V3 will give 1+2
14) How to check if file exist on filesystem ?
if [ -f /var/log/messages ]
then
echo "File exists"
fi
then
echo "File exists"
fi
15) Write down syntax for all loops in shell scripting ?
for loop :
for i in $( ls ); do
echo item: $i
done
echo item: $i
done
while loop :
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
until loop :
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done
16) What it means by #!/bin/sh or #!/bin/bash at beginning of every script ?
That line tells which shell to use. #!/bin/bash script to execute using /bin/bash. In case of python script there there will be #!/usr/bin/python
17) How to get 10th line from the text file ?
head -10 file|tail -1
18) What is the first symbol in the bash script file
#
19) What would be the output of command: [ -z "" ] && echo 0 || echo 1
0
20) What command "export" do ?
Makes variable public in subshells
21) How to run script in background ?
add "&" to the end of script
22) What "chmod 500 script" do ?
Makes script executable for script owner
23) What ">" do ?
Redirects output stream to file or another stream.
24) What difference between & and &&
& - we using it when want to put script to background
&& - when we wand to execute command/script if first script was finished successfully
&& - when we wand to execute command/script if first script was finished successfully
25) When we need "if" before [ condition ] ?
When we need to run several commands if condition meets.
26) What would be the output of the command: name=John && echo 'My name is $name'
My name is $name
27) Which is the symbol used for comments in bash shell scripting ?
#
28) What would be the output of command: echo ${new:-variable}
variable
29) What difference between ' and " quotes ?
' - we use it when do not want to evaluate variables to the values
" - all variables will be evaluated and its values will be assigned instead.
" - all variables will be evaluated and its values will be assigned instead.
30) How to redirect stdout and stderr streams to log.txt file from script inside ?
Add "exec >log.txt 2>&1" as the first command in the script
31) How to get part of string variable with echo command only ?
echo ${variable:x:y}
x - start position
y - length
example:
variable="My name is Petras, and I am developer."
echo ${variable:11:6} # will display Petras
x - start position
y - length
example:
variable="My name is Petras, and I am developer."
echo ${variable:11:6} # will display Petras
32) How to get home_dir with echo command only if string variable="User:123:321:/home/dir" is given ?
echo ${variable#*:*:*:}
or
echo ${variable##*:}
or
echo ${variable##*:}
33) How to get “User” from the string above ?
echo ${variable%:*:*:*}
or
echo ${variable%%:*}
or
echo ${variable%%:*}
34) How to list users which UID less that 100 (awk) ?
awk -F: '$3<100' /etc/passwd
35) Write the program which counts unique primary groups for users and displays count and group name only
cat /etc/passwd|cut -d: -f4|sort|uniq -c|while read c g
do
{ echo $c; grep :$g: /etc/group|cut -d: -f1;}|xargs -n 2
done
do
{ echo $c; grep :$g: /etc/group|cut -d: -f1;}|xargs -n 2
done
36) How to change standard field separator to ":" in bash shell ?
IFS=":"
37) How to get variable length ?
${#variable}
38) How to print last 5 characters of variable ?
echo ${variable: -5}
39) What difference between ${variable:-10} and ${variable: -10} ?
${variable:-10} - gives 10 if variable was not assigned before
${variable: -10} - gives last 10 symbols of variable
${variable: -10} - gives last 10 symbols of variable
40) How to substitute part of string with echo command only ?
echo ${variable//pattern/replacement}
41) Which command replaces string to uppercase ?
tr '[:lower:]' '[:upper:]'
42) How to count local accounts ?
wc -l /etc/passwd|cut -d" " -f1
or
cat /etc/passwd|wc -l
or
cat /etc/passwd|wc -l
43) How to count words in a string without wc command ?
set ${string}
echo $#
echo $#
44) Which one is correct "export $variable" or "export variable" ?
export variable
45) How to list files where second letter is a or b ?
ls -d ?[ab]*
46) How to add integers a to b and assign to c ?
c=$((a+b))
or
c=`expr $a + $b`
or
c=`echo "$a+$b"|bc`
or
c=`expr $a + $b`
or
c=`echo "$a+$b"|bc`
47) How to remove all spaces from the string ?
echo $string|tr -d " "
48) Rewrite the command to print the sentence and converting variable to plural: item="car"; echo "I like $item" ?
item="car"; echo "I like ${item}s"
49) Write the command which will print numbers from 0 to 100 and display every third (0 3 6 9 …) ?
for i in {0..100..3}; do echo $i; done
or
for (( i=0; i<=100; i=i+3 )); do echo "Welcome $i times"; done
or
for (( i=0; i<=100; i=i+3 )); do echo "Welcome $i times"; done
50) How to print all arguments provided to the script ?
echo $*
or
echo $@
or
echo $@
51) What difference between [ $a == $b ] and [ $a -eq $b ]
[ $a == $b ] - should be used for string comparison
[ $a -eq $b ] - should be used for number tests
[ $a -eq $b ] - should be used for number tests
52) What difference between = and ==
= - we using to assign value to variable
== - we using for string comparison
== - we using for string comparison
53) Write the command to test if $a greater than 12 ?
[ $a -gt 12 ]
54) Write the command to test if $b les or equal 12 ?
[ $b -le 12 ]
55) How to check if string begins with "abc" letters ?
[[ $string == abc* ]]
56) What difference between [[ $string == abc* ]] and [[ $string == "abc*" ]]
[[ $string == abc* ]] - will check if string begins with abc letters
[[ $string == "abc*" ]] - will check if string is equal exactly to abc*
[[ $string == "abc*" ]] - will check if string is equal exactly to abc*
57) How to list usernames which starts with ab or xy ?
egrep "^ab|^xy" /etc/passwd|cut -d: -f1
58) What $! means in bash ?
Most recent background command PID
59) What $? means ?
Most recent foreground exit status.
60) How to print PID of the current shell ?
echo $$
61) How to get number of passed arguments to the script ?
echo $#
62) What difference between $* and $@
$* - gives all passed arguments to the script as a single string
$@ - gives all passed arguments to the script as delimited list. Delimiter $IFS
$@ - gives all passed arguments to the script as delimited list. Delimiter $IFS
63) How to define array in bash ?
array=("Hi" "my" "name" "is")
64) How to print the first array element ?
echo ${array[0]}
65) How to print all array elements ?
echo ${array[@]}
66) How to print all array indexes ?
echo ${!array[@]}
67) How to remove array element with id 2 ?
unset array[2]
68) How to add new array element with id 333 ?
array[333]="New_element"
69) How shell script get input values ?
a) via parameters
./script param1 param2
b) via read command
read -p "Destination backup Server : " desthost
70) How can we use "expect" command in a script ?
/usr/bin/expect << EOD
spawn rsync -ar ${line} ${desthost}:${destpath}
expect "*?assword:*"
send "${password}\r"
expect eof
EOD
spawn rsync -ar ${line} ${desthost}:${destpath}
expect "*?assword:*"
send "${password}\r"
expect eof
EOD
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