Bash notes

 BASH help

The Linux Command Line: https://wiki.lib.sun.ac.za/images/c/ca/TLCL-13.07.pdf

Template
————————
#!/bin/bash                                                                      
                                                                                 
# Replace with description and/or purpose of this shell script                   
                                                                                 
GLOBAL_VAR1="one"                                                                
GLOBAL_VAR2="two"                                                                
                                                                                 
function function_one() {                                                        
    local LOCAL_VAR1="one"                                                       
    # Replace with function code                                                 
    return 0                                                                     
}                                                                                
                                                                                 
# Main body of the shell script starts here                                      
                                                                                 
# Exit with an explicit status                                                   
                                                                                 
exit 0   

————————

Shopt

shopt -s nullglob                                                                
 
Read                                                                             
read -p "Enter the extension that needs to be backed up " EXTENSION  


Date
88e9fe8514cd:shell shjabbar$ date +%F
2021-09-07


88e9fe8514cd:shell shjabbar$ date +%Y
2021
88e9fe8514cd:shell shjabbar$ date +%y
21
88e9fe8514cd:shell shjabbar$


Seq

88e9fe8514cd:shell shjabbar$  seq  -w 02 2 18
02
04
06
08

Declare

$ declare -i x=10
$ echo $x
10
$ x=ok
$ echo $x
0
$ x=15
$ echo $x
15

Case

case "$1" in                                                                     
    start|START)                                                                 
        echo "Command is start"                                                  
        ;;                                                                       
    stop|STOP)                                                                   
        echo "Command is stop"                                                   
        ;;                                                                       
    *)                                                                           
        echo "Usage: $0 start|stop"                                              
        ;;                                                                       
esac          


Shift

88e9fe8514cd:shell shjabbar$ cat shift.sh
#!/bin/bash

# total number of command-line arguments
echo "Total arguments passed are: $#"

# $* is used to show the command line arguments
echo "The arguments are: $*"

echo "The First Argument is: $1"
shift 2

echo "The First Argument After Shift 2 is: $1"
shift

echo "The First Argument After Shift is: $1"
88e9fe8514cd:shell shjabbar$ ./shift.sh shine mohamed jabbar puthiyaveettil house
Total arguments passed are: 5
The arguments are: shine mohamed jabbar puthiyaveettil house
The First Argument is: shine
The First Argument After Shift 2 is: jabbar
The First Argument After Shift is: puthiyaveettil
88e9fe8514cd:shell shjabbar$


Logger

(21-09-10 6:00:34) <0> [/var/log]  
dev-dsk-shjabbar-2a-b6139a9a % pwd
/var/log

(21-09-10 6:00:37) <0> [/var/log]  
dev-dsk-shjabbar-2a-b6139a9a % ls user.log
user.log

(21-09-10 6:00:39) <0> [/var/log]  
dev-dsk-shjabbar-2a-b6139a9a % logger  "Message" -s -t myscript -i
myscript[32594]: Message

(21-09-10 6:00:46) <0> [/var/log]  
dev-dsk-shjabbar-2a-b6139a9a %

Debugging

#/bin/bash -x

Or

set -x
<commands>
set +x  

-e option to stop at error
-v option to print shell command as it is
-x option to print shell command after expansion and substitution

DEBUG=true
If $DEBUG
then
  <<>>>
Else
   <<<>>>
Fi

PS4=‘+ ${BASH_SOURCE}:${LINENO}:$FUNCNAME[0]}()


BASH tricks:

Repeat command (previous command)
!!
!a
!^
!$
!!:3
^old_string^new_string
^old_string^new_string:&
^remove_string

Current command
!#
!#:1

To record a session
script
exit

Clear history
history -c

Strip out blank or comments:
grep -E -v "^#|^$" changename.sh


Remote VIM (pretty cool)
vim scp://dev-dsk-shjabbar-2a-b6139a9a.us-west-2.amazon.com//home/shjabbar/get_snitch_graph

Print out as a table / column:
column -t
88e9fe8514cd:shell shjabbar$ mount | column -t
/dev/disk1s5  on         /                     (apfs,                                local,    read-only,    journaled)
devfs         on         /dev                  (devfs,                               local,    nobrowse)

Less with color:
grep gp_host_names get_snitch_graph
 | less -R

Preserve color when piping to grep
—color=never
ls -l --color=always | grep get_ --color=never

Append text to file using sudo

echo $ENVIRONMENT | sudo tee -a /etc/motd

Display block of strings
awk '/Shadow/,/edit/' exercise4.sh

Delete block of strings

sed '/Shadow/,/edit/d' exercise4.sh
 
Remove a character or a set

cat exercise4.sh | tr -d [aeiou]

uniq -c

Get your public IP using curl

curl ifconfig.me/ip
204.246.162.45

Open network ports:
lsof -Pni
dev-dsk-shjabbar-2a-b6139a9a % sudo lsof -Pni
COMMAND     PID     USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
unbound    2759    named    3u  IPv4    1622      0t0  UDP 127.0.0.1:53


Send mail
mail shjabbar@amazon.com
echo "Isn't this great" | mail -s "Test mail 2" -a get_snitch_graph shjabbar@amazon.com

dev-dsk-shjabbar-2a-b6139a9a % mail shjabbar@amazon.com
Subject: Test mail
Isn't this great
EOT

(21-09-19 5:44:37) <0> [~]  
dev-dsk-shjabbar-2a-b6139a9a %


Create an SSH tunnel:

ssh -N -L local-port-opened-in-local-host:end-host:end-port remote-host-where-the-connection-will-be-initiated

 ssh -N -L 8000:localhost:80 linuxserver

Simplifying multi-hop SSH

In .ssh/config
  # Configuration for CPT                                                          
  host *.cpt*.amazon.com *.cpt*.amazon.com. *.cpt? *.cpt??                         
          ProxyCommand ssh bastion-cpt.amazon.com nc %h %p  

Download webpage:
wget
curl -o file.html www.google.com

Read:
read VAR
 read -n 1 VAR
read -p “Enter Value: “ VAR

Sum a column:

awk '{sum += $3} END {print sum}'


 2021-09-21-05:10:29 big-mac-prod-tools-cmh-cmh52-50001.cmh50 cmh52:prod > df 2> /dev/null | awk '{sum += $3} END {print sum}'
118810236
2021-09-21-05:10:58 big-mac-prod-tools-cmh-cmh52-50001.cmh50 cmh52:prod >


Automatic Yes:
yes | command

 Pkill:
pkill  -9 httpd

Sort process with memory usage:
ps aux | sort -nk 4 | tail -5 | cut -c1-140

Make a quick backup
cp cscope.files{,.bak}

Sequence:
echo 10.0.0.{0..7}
10.0.0.0 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 10.0.0.5 10.0.0.6 10.0.0.7

Truncate a file:
> file
cat /dev/null > file
 
Create multiple directory:
mkdir {1..4}

Delete empty directory
find . -type d -empty -delete

List of all files with matching string
grep -rl bash *

Recursive ls:

ls -lR
find . -type f -ls

Replace string recursively with backup
find . -type f -iname "*.sh" -exec sed -i.bak 's/echo/ECHO/g' {} \;


Print Nth line:
awk 'NR==4' exercise4.sh
cat exercise4.sh | awk 'NR==4'

Convert text to image
sudo yum install ImageMagick
echo shine | convert label:@- hine.png