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Useful Unix Commands and Notes

This is a compilation of commands that I have found particularly useful in a pinch. 👍

Table of Contents


Start by learing safe shell scripting best practices.

Every Day Commands

find commands

  • find all files that were modified less that 2 days ago
find . -mtime -2
  • find all files owned by root
find . -uid `id -u root`
  • find all files 28 days old or newer
find . -name '*.log' -mtime -28
  • find/replace with sed
find . -type f -exec sed -i 's/bad/good/g' {} \;
  • find and remove files
find . -type f -exec rm {} \;
  • remove all files except for those that match a pattern
sudo find . -not -name 'FileA*' -type f -exec rm {} \;
  • remove all files except for those that match a list
find ./ -maxdepth 1 -not \( -name 'A*' -or -name 'B*' \)
  • test network speeds by ping the Google Public DNS IP (

sed commands

  • multiple replacements in one command
sed -i -e 's/string1/val1/' -e '/s/string2/val2/' file.txt
  • delete first 100 lines of a file
sed -i '1,100d' filename


Some file test flags. See also the wikipedia article

  • -e -- file exists
  • -f -- file is a regular file
  • -d -- file is a directory
  • ! -- negate


if test ! -s "$1"
   echo $1 does not exist or is empty.


*    *    *    *    *  command to be executed
┬    ┬    ┬    ┬    ┬
│    │    │    │    │
│    │    │    │    │
│    │    │    │    └───── day of week (0 - 6) (0 or 6 are Sunday to Saturday, or use names)
│    │    │    └────────── month (1 - 12)
│    │    └─────────────── day of month (1 - 31)
│    └──────────────────── hour (0 - 23)
└───────────────────────── min (0 - 59)

(stolen from

misc commands

  • load a property file with periods


if [ -f "$file" ]
  echo "$file found."

  while IFS='=' read -r key value
    key=$(echo $key | tr '.' '_')
    eval "${key}='${value}'"
  done < "$file"

  echo "User Id       = " ${db_uat_user}
  echo "user password = " ${db_uat_passwd}
  echo "$file not found."
  • see memory information on a host in MB
free -m


cat /proc/meminfo
  • what's my IP? (Use to get your external facing IP)
  • find/replace one liner in perl
perl -p -i -e 's/oldstring/newstring/g' `find ./ -name *.html`
  • traverse symlinks for a file and give the actual location
readlink -f /usr/bin/java
  • read a password
read -es password  #reads the next line into $password and masks user input
  • simple replacement for dos2unix (which is not standard)
tr -d '\r' < dosfile > unixfile
  • rename *.bak to *
rename 's/\.bak$//' *.bak
  • find a uid or gid for a user
id -u username
id -g username
  • set http_proxy env variable
export (or set on Windows) $http_proxy
  • list groups (this searches through naming directories as well as /etc/group)
getent group|grep <group name/>
  • use getent to retrieve a service user name
getent passwd|grep maven|awk -F\: '{print $1}'
  • system calls include child processes
strace -f /bin/true		
  • run a simple HTTP server using python (2.7+)
python -m SimpleHTTPServer [port]
echo ${TEST#*.}
  • cut the end of a value stored in a variable
echo ${TEST%.*}
  • use pygmentize to generate an HTML output of a diff file
pygmentize -l diff -f html -O full -o diff.html diff.txt
  • pass env variables using sudo
sudo -E bash -c 'echo $HTTP_PROXY'
  • capture command execution in a variable -- this example is useful for a date format string (man date for formats)
timestamp=$(date '+%Y%m%d%H%M')
  • bash shell default to env value or use positional argument
  • tar up a directory but exclude logs
tar cvfz mydir.tgz mydir --exclude mydir/logs
  • list a directory sorted by size reversed
ls -lSr
  • check a unique file size (better than md5sum)
sha1sum {file_name}
  • human readable file sizes including summary of first level directories
du -h --max-depth=1 --all
  • view configuration of services in /etc/rc[0-9]/. The chkconfig can be used to administer all start up services.
sudo chkconfig --list
  • generate a random 15 character password
head -c 500 /dev/urandom | tr -dc a-zA-Z0-9\.\^\*\? | head -c 15; echo
  • download Java SDK stuff
curl -s | bash
sdk install kotlin

file permissions (chmod)

# Permission rwx
7 full 111
6 read and write 110
5 read and execute 101
4 read only 100
3 write and execute 011
2 write only 010
1 execute only 001
0 none 000



keytool -list -keystore file.pfx -storetype PKCS12
  • Convert the .pfx to a PEM .cer -- this is a prereq to use a Java PFX cert in curl
openssl pkcs12 -in certificate.pfx -out certificate.cer -nodes

TCP commands

  • find what process is associated with a TCP port
$ /usr/sbin/lsof -i :8380
  • See all TCP ports
$ /usr/sbin/lsof -Pnl +M -i4
  • ping Google DNS -- this is very useful for checking network reliability
ping -t
  • test connectivity to a TCP or UPD port

You can start up a port to listen on:

nc -l <port to listen on>

You can connect to an active port using:

nc -z <IP or host> <port> (e.g., nc - z dev1-app1 1234)
sudo iptables -A OUTPUT -p tcp --dport 25 -j REJECT

curl commands

  • run curl with the .cer
curl -k https://{https_url} -E ~/certificate.cer:{cert_password}
  • run curl with the http proxy and authentication
curl -O some_url -x <host/>:<port/> -U <user/>:<pass/>


SSH commands

set up SSH keys

  1. generate keys: ssh-keygen -t rsa
  2. copy the public key: ssh-copy-id user@host

SSH config

  • turn off GSSAPIAuthentication
echo "GSSAPIAuthentication no" >> ~/.ssh/config
  • turn off StrictHostKeyChecking in ssh
ssh -o StrictHostKeyChecking=no <user>@<host> -i <priv_key_loc> 
  • restart sshd
service sshd restart
  • to troubleshoot SSH connections, check /var/log/secure


sudo mount /local/folder
  • set system time
ntpdate (replace with NTP server IP)
  • change default shell to zsh (on OS X)
chsh -s $(which zsh)


  • I/O performance benchmarks
hdparm -tT /dev/sda1




echo "$((OFFSET + 9990))"


AWS specific

alias myip='curl -w "\n"'


Reading List

List of other useful Unix/Linux cheatsheets or tutorials:

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