Tag: Linux

Execute a command whenever a file changes

- by mahmood

You can do this with inotifywait utility from inotify-tools package

while true ; do
  inotifywait -e delete_self "/tmp/fileToMonitor.txt" \
    && cp new_file "/tmp/fileToMonitor.txt"
done
[ "$UID" -eq 0 ] || exec sudo "$0" "$@"

while true ; do
  inotifywait -e modify "/etc/resolv.conf" && ./dns.sh
done

References
https://superuser.com/questions/939600/how-to-get-notified-when-a-specific-file-is-deleted-in-linux
https://superuser.com/questions/181517/how-to-execute-a-command-whenever-a-file-changes
http://man7.org/linux/man-pages/man1/inotifywait.1.html#EVENTS

Command Substitution in bash

- by mahmood

Text between backticks is executed and replaced by the output of the command (minus the trailing newline characters, and beware that shell behaviors vary when there are NUL characters in the output). That is called command substitution because it is substituted with the output of the command.

A=`cat /etc/p2ass2wd2 | head -n1`
echo "$A"
A=$(cat /etc/p2ass2wd2 | head -n1)
echo "$A"

References
https://unix.stackexchange.com/questions/48392/understanding-backtick
https://unix.stackexchange.com/questions/147420/what-is-in-a-command

Get IP Address using bash

- by mahmood

Displaying private IP addresses

ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/'


address=$(ip addr show wlp3s0 | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/')

Displaying the public IP address

If you want to know the public IP address of a Linux server, you can send an HTTP request to one of the following web servers.

  • http://ifconfig.me
  • http://www.icanhazip.com
  • http://ipecho.net/plain
  • http://indent.me
  • http://bot.whatismyipaddress.com
  • https://diagnostic.opendns.com/myip
  • http://checkip.amazonaws.com
curl http://checkip.amazonaws.com
wget -qO- http://checkip.amazonaws.com

References
https://unix.stackexchange.com/questions/119269/how-to-get-ip-address-using-shell-script
https://www.linuxtrainingacademy.com/determine-public-ip-address-command-line-curl/

Staging the changes in git

- by mahmood 
git status

git add -A stages all changes

git add . stages new files and modifications, without deletions

git add -u stages modifications and deletions, without new files

git add -A is equivalent to git add .; git add -u

git add -A is equivalent to git add --all

git add -u is equivalent to git add --update

References
https://stackoverflow.com/questions/572549/difference-between-git-add-a-and-git-add
https://git-scm.com/docs/git-stage
https://git-scm.com/docs/git-add

Signing Commits in Git using GPG Key

- by mahmood 
gpg --list-secret-keys --keyid-format LONG

From the list of GPG keys, copy the GPG key ID you’d like to use. In this example, the GPG key ID is 3AA5C34371567BD2

$ gpg --list-secret-keys --keyid-format LONG
/Users/hubot/.gnupg/secring.gpg
------------------------------------
sec   4096R/3AA5C34371567BD2 2016-03-10 [expires: 2017-03-10]
uid                          Hubot 
ssb   4096R/42B317FD4BA89E7A 2016-03-10
git config --global user.signingkey 3AA5C34371567BD2

or

git commit -S -m your commit message

References
https://help.github.com/en/articles/telling-git-about-your-signing-key
https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work
https://help.github.com/en/articles/signing-commits