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Unix > ShellScripts > Flashcards

ShellScripts Flashcards

1
Q

My system is running very slow. What do i do ?

A

top command to show CPU utilization per core

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2
Q

How many files are there in my current directory?

A

ls -lrt to see if . and .. come up and then accordingly ls -lrt|wc -l

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3
Q

I am getting an out of space email in one of my mounts, what do i do ?

A

du -ah |sort -rh|head -n 20 to see top 20 files

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4
Q

I want to see how much space is left in a directory

A

df -ah

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5
Q

My directory has multiple sub directories, but i want to show much space is used only for my first 2 directories

A

du -ah –max-depth=2

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6
Q

I want to find all files which contain the line “Published items “

A

grep ‘Published’|grep -v 0

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7
Q

I need to find a file named pwd.txt and i cant remember where it was. How do find it’s location?

A

find / -type f -name pwd.txt

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8
Q

I don’t like the default command prompt, and want to change it to show the hostname/current directory

A

PS1=”[\h \w]”

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9
Q

How do I see status of my last command ?

A

echo $? #0 means success

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10
Q

How do i force delete all files under a directory ?

A

rm -rf

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11
Q

How do i run a long running command so that its not killed ?

A

nohup &

nohup will keep running until it’s done, even if the user that started it logs out.
& to run a process to the background

NO HANGUP - nohup

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12
Q

If i dont want to see a command’s error output , what do i do?

A

2>/dev/null

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13
Q

How do i append a line to a file?

A

echo “line”&raquo_space; filename. #> to insert

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14
Q

How do i put a comment in a shell script

A

Use # like a shebang script

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15
Q

How do i find PID of a process named mysql ?

A

ps -ef|grep mysql|cut -f4 -d” “ # 4th column separated by space

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16
Q

How do you find all processes started by a user ?

A

-u

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17
Q

You have a directory contqaining many files and directories.
You want to search for a line say “CRIT”. How will you search it ?

A 
#cd 
grep -r CRIT .
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18
Q

How to search for a tag in binary files ?

A

grep -a

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19
Q

How to show only filenames ?

A

grep -l

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20
Q

How to read compressed files in a RO shell

A

zgrep

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21
Q

I want to shutdown/decommission a host. I want to see which files are still being written on the server.

A

find . -mtime -7 -ls. #-ls to give more details

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22
Q

If my system is showing OoM exceptions, what do i do ?

A

Can check free memory by free -h, then if more memory is available, can provide more memory to the script

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23
Q

What command should i use to scroll throgh production log ?

A

less +F

Control C to search mode
Control F to watch mode , F = follow

24
Q

How do i go to first line using less

A

g / G : first / lass line

/searchfoward. ?searchbackward

space b to move one page down

25
Q

My file has 1010 many times. i want to replace all of them with 0101, how do i do it ?
I want to replace only those occurrences where the line starts with 1010.
So if the line is
1010client1 -> this i want to replace
21010clietn2 -> this i dont want to replace

A 
sed -i.bak -r 's/1010/0101/g' 
#g to replace all / gloably 
#-i.bak to take a backup,  if you just give -i, no backups are taken.

sed -i.bak -r ‘s/^1010/^0101/g’

26
Q

What do the following regular expressions signify ?

^
$
/t
/r
/n
.
*
\+
A 
^ beginning of line 
$ end of line 
/t tab
/r : carriage return
/n : new line 
usually based on OS - it uses a combinatin of /r/n for new lines

. Matches exactly 1 character
* Matches 0 or more
+ matches 1 or more

27
Q

What is diff bw more and less

A

both used to scroll page by page

more is old utility, loads whole file at once , hence slower

28
Q 
If a file  dummy.txt has the following lines : 
amrita 
anirban 
amrita
anjan

and i want to remove duplicates, what will i do ?

A

sort dummy.txt|uniq
or sort -u dummy.txt

Uniq only removes adjacent duplicates, so need to sort first

uniq -c shows frequency , -d shows dupes only , -u shows unique ones only

29
Q 
What if the file has 
amrita 3 
anirban 2 
ketan 4
anjan 1
and i want to sort by the 2nd column such that 
ketan 4 
amrita 3
anirban 2
anjan 1
A

sort -k 2 -nr

2nd column numerically reverse

30
Q

Which command gives me details about all users who are logged in ?

A

finger

finger

31
Q

How do i login to a remote server ?

A

I can use putty which is a client for SSH and telnet.
I will use SSH as its encrypted

Control + ] , close on telnet command prompt

32
Q

How do i repeat the last command
How do i go to the last directory ?
Home directory ?

A

!!
cd -
cd ~. or just cd will also take me to home directory

33
Q

How do i see all environment variables?

A

env
Usually they are all defined in caps
so if i see echo $NAME it usually means NAME is an ev variable.

An environment variable is available to any child process of the shell

34
Q

i can run cat or ls or any inbuilt command without giving their locatin. How ?

A

Because these are inbuilt executables and their path /locaiton is already configured in PATH environment variable

35
Q

If i have my location of scripts, and i don’t want to always invoke them using their full path, what do i do ?

A

I can put all scripts in a directory say utilityscripts
and i can add the directory to the PATH
echo $PATH # /bin:/usr/bin
PATH=$PATH:

directories on path are separated by : in linux, by ; in windows

36
Q

If i deploy a package, and i dont want to constantly change the location, what will i do ?

A

I will create a symlink or a symbolic link
for eg. ln -s

Use unlink to unlink

37
Q

What is diff. b/w absolute and relative path?

A

Ab. starts with / root

Relative is relative to your current location

38
Q

Client complains too slow, cant connect what do i do ?

A

Ask them to share
traceroute -d or tracert
Maybe they are connecting to incorrect data centers
or there is a high latency hop in between.

39
Q

How do i check all IPs used by a host ?

A

use ifconfig -a or ipconfig -all (windows)

40
Q

How do i copy files from one host to another ?

A

scp
or ftp

cp to copy in the current host

41
Q

How do i rename a file ?

A

mv

42
Q

How do i check how many cores my host has ?

A

lscpu

Will tell me how many cores(CPUs) means how many tasks can run in parallel.

Also tells processor’s clock speed in GHz.

A computer’s processor clock speed determines how quickly the central processing unit (CPU) can retrieve and interpret instructions

43
Q

How do i remove a directory ?

A

rmdir

44
Q

How do i get 99th line of file ?

A

tail -n +99 filename|head -1

cat filename|cut -c 99-100

45
Q

How do i identify blank lines ?
How do i not show them ?
How do i count them

A

grep ‘^$’
| grep -v ‘^$’
| grep -v ‘^$’ |grep -c

46
Q

What is the diff b/w
tail -n 5
and tail -n +5

A

tail -n 5 shows the last 5 lines
tail -n +5 shows from the 5th line

[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] cat testless.txt

1
2
3
3
4
5
6

[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] tail -n 5 testless.txt

2
3
4
5
6

[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] tail -n +5 testless.txt

5
6

47
Q

How do i show match + frequency for CRID in log files

A

grep -c CRID

48
Q

What is a shell ?

What is a kernel ?

A

Shell is an interface where we can write commands to communicate with the kernel.
Kernel is the OS , manages resources.

$ or % is the command prompt
Different types of shell 
like 
Bourne (sh) 
Bourne Again (bash) 
Korn (ksh)

C shell - will have %

49
Q

If i have some common commands like PS1 etc. where do i keep them so that i dont have to write them everytime?

A

.profile OR .bashrc OR … which is executed during shell initialization

50
Q 
$* 
S? 
$1 $2 
$0   
$# 
\$\$ 
$!
A 
$* All arguments 
S? Return status , 0 is successful
$1 $2 First , 2nd argument
$0   File name
$# Number of arguments
\$\$ Current PID  
$! shows the process id of the process that recently went into background
51
Q

If you have to find how many files in a directory how will you find it ?

A

ls -lrta | wc -l will give you 2 more
$(($(ls -lrta|wc-l)-2))

$(()) to run arithmetic expressions : expression substitution
$() to run command : command substituion

52
Q

How do i check equality ?
Check string
Check numbers

A

if [[ $1 == 1 ]]

= and == are for string comparisons, -eq is for numeric ones, but if you use [[ can use < etc

[[ is bash’s improvement to the [ command

53
Q

How do i check for multiple patterns ?

A

egrep ‘|’ |’

54
Q

How do i check for or or and conditions ?

A

aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] [[ 1 > 0 || 2 < 1 ]]; echo $?
0
[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] [[ 1 < 0 || 2 < 1 ]]; echo $?
1
[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] [[ 1 > 0 && 2 < 1 ]]; echo $?
1
[aniamritapc ANIAMRITAPCs-MacBook-Pro /tmp $ ] [[ 1 > 0 && 2 > 1 ]]; echo $?
0

55
Q

If i want to print any metacharacters, how will i do it ?

A

either enclose it in quotes or escape it using a \

Unix (2 decks)

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