bash_cheatsheet.md

title	description	created
BASH CheatSheet	The most commonly used bash commands are given here.	2022-10-23

Table of Contents

• BASH CheatSheet for Developers
  • Commands
    • tr command
  • One Liners
    • Block Bad IPs
  • If Statements
    • Check if args are passed
    • Check if required variables exist
    • Check if environment variables exists
  • While Loops
    • Run process for 5 Seconds
    • Run until state changes within ttl
  • for Loops
    • Upload all docker image
  • Functions
  • Redirecting Outputs
    • Stdout, Stderr
  • Manipulating Text
    • Remove first 3 characters
    • Only show last 3 characters

BASH CheatSheet for Developers

Commands

tr command

Remove whitespace:

$ echo 'foo - bar' | tr -d '[:space:]'
foo-bar

Convert to uppercase:

$ echo 'HeLLo' | tr '[:lower:]' '[:upper:]'
HELLO

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One Liners

Block Bad IPs

Use iptables to block all bad ip addresses:

$ cat /var/log/maillog | grep 'lost connection after AUTH from unknown' | tail -n 5
May 10 11:19:49 srv4 postfix/smtpd[1486]: lost connection after AUTH from unknown[185.36.81.145]
May 10 11:21:41 srv4 postfix/smtpd[1762]: lost connection after AUTH from unknown[185.36.81.164]
May 10 11:21:56 srv4 postfix/smtpd[1762]: lost connection after AUTH from unknown[175.139.231.129]
May 10 11:23:51 srv4 postfix/smtpd[1838]: lost connection after AUTH from unknown[185.211.245.170]
May 10 11:24:02 srv4 postfix/smtpd[1838]: lost connection after AUTH from unknown[185.211.245.170]

Get the data to show only IPs:

cat /var/log/maillog | grep 'lost connection after AUTH from unknown' | cut -d'[' -f3 | cut -d ']' -f1 | tail -n5
185.36.81.164
175.139.231.129
185.211.245.170
185.211.245.170
185.36.81.173

Get the uniqe IP addresses:

$ cat /var/log/maillog | grep 'lost connection after AUTH from unknown' | cut -d'[' -f3 | cut -d ']' -f1 | sort | uniq
103.194.70.16
112.196.77.202
113.172.210.19
113.173.182.119
139.59.224.234

Redirect the output to iptables:

$ for ip in $(cat /var/log/maillog | grep 'lost connection after AUTH from unknown' | cut -d'[' -f3 | cut -d ']' -f1 | sort | uniq); do iptables -I INPUT -s ${ip} -p tcp --dport 25 -j DROP; done

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If Statements

Check if args are passed

if [[ $# -eq 0 ]] ; then
    echo 'need to pass args'
    exit 0
fi

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Check if required variables exist

if [ $1 == "one" ] || [ $1 == "two" ]
then
  echo "argument 1 has the value one or two"
  exit 0
else
  echo "I require argument 1 to be one or two"
  exit 1
fi

OR

NAME=${1}
if [ -z ${NAME} ]
  then
    echo NAME is undefined
    exit 1
  else
    echo "Hi ${NAME}"
fi

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Check if environment variables exists

if [ -z ${OWNER} ] || [ -z ${NAME} ]
then
  echo "does not meet requirements of both environment variables"
  exit 1
else
  echo "required environment variables exists"
fi

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While Loops

Run process for 5 Seconds

set -ex
count=0
echo "boot"
ping localhost &
while [ $count -le 5 ]
  do
    sleep 1
    count=$((count + 1))
    echo $count
  done
ps aux | grep ping
echo "tear down"
kill $!
sleep 2

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Run until state changes within ttl

UPDATE_COMPLETE=false
UPDATE_STATUS=running
COUNT=0
while [ ${UPDATE_COMPLETE} == false ]
    do
        if [ $count -gt 10 ]
            then
                echo "timed out"
                exit 1
        fi
        if [ ${UPDATE_STATUS} == running ] 
            then
                echo "still running"
                sleep 1
                COUNT=$((COUNT+1))
                if [ $count == 7 ]
                    then
                        UPDATE_COMPLETE=true
                fi
        elif [ ${UPDATE_STATUS} == successful ]
            then
                UPDATE_COMPLETE=successful
        else
            echo "unexpected update response"
            exit 1
        fi
    done
echo "complete"

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for Loops

Upload all docker image

for i in $(ls | grep .tar): do
  docker load -i $i;
done

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Functions

message(){
    NAME=${1}
    echo "Hi ${NAME}"
}

OR pass all args:

message(){
    echo "Hi $@"
}

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Redirecting Outputs

Stdout, Stderr

Redirect stderr to /dev/null:

grep -irl faker . 2>/dev/null

Redirect stdout to one file and stderr to another file:

grep -irl faker . > out 2>error

Redirect stderr to stdout (&1), and then redirect stdout to a file:

grep -irl faker . >out 2>&1

Redirect both to a file:

grep -irl faker . &> file.log

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Manipulating Text

Remove first 3 characters

$ STRING="abcdefghij"
$ echo ${STRING:3}
defghij

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Only show last 3 characters

With tail to only show the last 3 characters:

$ STRING="abcdefghij"
$ echo ${STRING} | tail -c 4
hij

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