6.handy-snippet.md

Table of content ^back

• Bash/Shell handy snippet
  • Run next command if the previous one is failed
  • Read a file line by line assigning the value to a variable
  • Simple multiple files/folders maker
  • Loop in a string array
  • Run multiple replicated command with multiple files
  • Do multiple silent task
  • Backgroud process to file (stdout + stderr)
  • Update multiple files' name in 1 command
  • Manage IP tables
  • telnet
  • sed common use cases
  • Concatenate file common use cases
  • Append content to file with sudo privileges
  • Create json file with variable in bash

Bash/Shell handy snippet

Run next command if the previous one is failed

cmd1 && echo "Success" || echo "Failed"

---

Read a file line by line assigning the value to a variable

while read -r line; do
    echo "Text read from file: $line"
done < my_filename.txt

---

Simple multiple files/folders maker

mkdir -p project/{app,frontend,backend}
touch {main,variables,provider}.tf

---

Loop in a string array

b=("aa" "ab"); for t in ${b[@]} ; do echo $t ; done
echo ${#b[@]} # get array length

---

Run multiple replicated command with multiple files

for a in *.yml; ansible-playbook "$a" --syntax-check ; done

---

Do multiple silent task

for file in $(find /home/ec2-user/ -type f); do cmd1 $file | cmd2 &disown ; done

---

Backgroud process to file (stdout + stderr)

command > file.log 2>&1 &

Explanation:

• > stdout to file
• 2>&1: redirect stderr to stdout
• &: run process in backgroud

---

Update multiple files' name in 1 command

find . -name "*_f"| sed "s/.\///g" | while read f ; do mv "$f" "${f%?????}_f"; done

Manage IP tables

iptables -P INPUT ACCEPT
iptables -I INPUT 1 -p tcp -s 159.89.202.45 -j ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P INPUT DROP

---

telnet

• Without interaction

echo -e '\x1dclose\x0d' | telnet {$IP_ADDRESS} {$PORT}

• Multiple IPs with SSH port

PORT=22 list=("172.29.211.1" "172.29.211.2") ; for ip in "${list[@]}" ; do echo -e '\x1dclose\x0d' | timeout 2 telnet $ip {$PORT} && echo "connect $ip success" || echo "connect $ip failed" ; done

---

sed common use cases

• Replace string directly on file

sed -i 's/pattern1/pattern2/g' filename

---

Concatenate file common use cases

• Insert multiple-lines text in to new file

cat <<EOF > filename
Multiple
Lines
Text
Data
EOF

• Appending multiple-lines text in to existed file

cat <<EOF >> filename
Multiple
Lines
Text
Data
EOF

• Append text with sudoers (persistent mount point)

echo -e "\nnfs-uuid:/  /mnt/efs nfs4 nfsvers=4.1,rsize=1048576,wsize=1048576,hard,timeo=600,retrans=2,noresvport,_netdev 0 0" | sudo tee -a /etc/fstab

Append content to file with sudo privileges

• Create multiple-lines text file as sudoer

sudo tee /etc/systemd/system/custom-service.service > /dev/null <<-EOF
# *****************************************************************
# *********************** Custom Service **************************
# *****************************************************************

[Unit]
Description=Custom Service
After=network.target

[Service]
ExecStart=/start/script/for/custom-service
ExecStop=/stop/script/for/custom-service
User=root

[Install]
WantedBy=multi-user.target
EOF

Create json file with variable in bash

SAMPLE='
{
    "Comment": "Update DNSName.",
    "Changes": [
        {
            "Action": "UPSERT",
            "ResourceRecordSet": {
                "Name": "alex.",
                "Type": "A",
                "AliasTarget": {
                    "HostedZoneId": "######",
                    "DNSName": "$bar",
                    "EvaluateTargetHealth": false
                }
            }
        }
    ]
}
'
echo "${SAMPLE}" | tee output-file-path.json > /dev/null