BASH CheatSheet: Learn the Most Common BASH Commands

Master BASH with this comprehensive cheatsheet for beginners! Learn the most common BASH commands. This comprehensive BASH cheatsheet for beginners is the perfect resource for anyone who wants to learn the most common BASH commands quickly and easily. Covering a wide range of topics, from basic file management to advanced scripting, this cheatsheet has everything you need to start using BASH effectively.

Whether you're a complete beginner or you have some basic BASH knowledge, this cheatsheet is a must-have for anyone who wants to learn the most common BASH commands.

Commands

tr command

Remove whitespace:

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

Convert to uppercase:

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

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

If Statements

Check if args are passed

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

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

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

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

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"

for Loops

Upload all docker image

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

Functions

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

OR pass all args:

message(){
    echo "Hi $@"
}

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

Manipulating Text

Remove first 3 characters

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

Only show last 3 characters

With tail to only show the last 3 characters:

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

#bash #linux