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GCP Fix Hosts

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This is a script that fixes /etc/hosts in GCP.

#!/bin/bash
 
# VARIABLES
FILE='/etc/hosts'
 
#BACKUP /etc/hosts before changes
cp -ipHR $FILE $FILE_$(date +%Y-%m-%d_%H-%M-%S)
 
# GET IP ADDRESS
IP=$(/sbin/ifconfig | grep 'inet' | grep -v 'inet6' | grep -v '127.0' | cut -d: -f2 | awk '{print $2}' | head -1)
 
# GET HOSTNAME
HN=$(/bin/hostname)
 
# GET PROJECT ID
echo "169.254.169.254 metadata.google.internal  # Added by Google" >> $FILE
PR=$(curl -f -s http://metadata.google.internal/computeMetadata/v1/project/project-id -H "Metadata-Flavor: Google")
 
# ADD HOSTNAME ENTRY TO /etc/hosts
STRING="$IP $HN.mydomain.com"
 
if grep -q "$STRING" "$FILE"
then
  echo "Valid host entry."
else
  echo "Adding host entry."
  echo $STRING $HN >> $FILE
fi
 
# DELETE GOOGLE ENTRIES.
sed -i '/# Added by Google/d' $FILE
 
# ADD BACK GOOGLE ENTRIES.
echo $IP $HN".c."$PR".internal" $HN"  # Added by Google" >> $FILE
echo "169.254.169.254 metadata.google.internal  # Added by Google" >> $FILE
 
# SET PERMISSIONS.
chmod 644 $FILE
 
# DISPLAY /etc/hosts
cat $FILE

Result

127.0.0.1       localhost
::1             localhost ip6-localhost ip6-loopback
ff02::1         ip6-allnodes
ff02::2         ip6-allrouters
 
10.128.0.30 test.mydomain.com test
10.128.0.30 test.c.project-id.internal test  # Added by Google
169.254.169.254 metadata.google.internal  # Added by Google

It adds a proper host entry before the Google entries. /etc/hosts has top to bottom precedence.

Bash Read File

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How to read a file line by line in Bash.

#!/bin/bash
file=test.txt
while IFS= read -r line; do
  echo $line
done < $file

Contents of test.txt.

one
two
three

Results when running test.sh.

$ bash test.sh
one
two
three

To read multiple strings line by line.

#!/bin/bash
file=test.txt
while read -r a b; do
  echo $b
done < $file

Modified contents of test.txt.

one 1
two 2
three 3

New results.

$ bash test.sh
1
2
3

Create A Swap File

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How to create a swap file.

A 2GB swap file.

dd if=/dev/zero of=/swapfile bs=1k count=2048k

Activate.

mkswap /swapfile
chmod 0600 /swapfile
systemctl daemon-reload
swapon /swapfile

To make swap permanent, add to /etc/fstab.

/swapfile  swap   swap    defaults   0 0

Check if swap is working.

cat /proc/swaps
free -h

CloudFormation Userdata

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Here’s another way to add startup scripts to your instance during creation.

      UserData:
        Fn::Base64: !Sub |
          #cloud-config
          repo_upgrade: none
 
          runcmd:
            # Cloud init startup script
            - "bash /root/setup.sh"
 
          write_files:
            # Cloud init startup script
            - owner: root:root
              permissions: '0644'
              path: /root/setup.sh
              content: |
                #! /bin/bash -x
          write_files:
            # Cloud init startup script
            - owner: root:root
              permissions: '0644'
              path: /root/setup.sh
              content: |
                #! /bin/bash -x
                # run your bash commands here
                date > log.txt
                uptime >>  log.txt

The above script creates a file setup.sh and executes it during instance creation. The output is dumped to log.txt file.

Audacity Copy

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I was getting an error when launching Audacity.

The error was:

“The system has detected that another copy of Audacity is running.”

However, Audacity wasn’t running at all. Not one process. It turned out to be just a locked file located in /var/tmp/audacity-[your-username].

All you have to do is delete the entire directory and rerun Audacity.

rm -rf /var/tmp/audacity=[your-username]/

Once deleted, Audacity starts right up with no issues.

Bash Variables

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Here’s how to read Bash variables from another file.

Contents of config.sh

#!/bin/bash
var1='thisisvariableone'
var2='thisisvariabletwo'

Call config.sh from another script.

#!/bin/bash
source config.sh
echo var1
echo var2

This is extremely helpful if you have multiple scripts using a common variable. You just to update one file instead of multiple files. All the other scripts will read and pick up the same variables from a config file. You can use this to setup your global variables.

Split A Large File

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How to split a large file. 

split -b 500MB httpd.log
ll -lh
total 1.9G
-rw-r--r-- 1 root root 954M Mar 25 12:35 httpd.log
-rw-r--r-- 1 root root 477M Mar 25 12:38 xaa
-rw-r--r-- 1 root root 477M Mar 25 12:38 xab

Split with an output file.

split -b 200M httpd.log split.log
ll -lh
total 1.9G
-rw-r--r-- 1 root root 954M Mar 25 12:35 httpd.log
-rw-r--r-- 1 root root 200M Mar 25 12:52 split.logaa
-rw-r--r-- 1 root root 200M Mar 25 12:52 split.logab
-rw-r--r-- 1 root root 200M Mar 25 12:52 split.logac
-rw-r--r-- 1 root root 200M Mar 25 12:52 split.logad
-rw-r--r-- 1 root root 154M Mar 25 12:52 split.logae