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Linux Commands

This is the Websoft9 Support Team’s collection of information regarding Linux, that they sometimes use while troubleshoot. It is listed here for transparency, and it may be useful for users with experience with Linux.


# Debian/Ubuntu
uname -a
lsb_release -a

# CentOS/RedHat
cat /etc/centos-release
cat /etc/redhat-release

# This will provide a lot more information
cat /etc/os-release


# Restart

# Memory

# Disk
df -hl

# Docker containers list
sudo docker ps -a

# Network port
netstat -tunlp

# List all environment

File management

# create a new directory and all subdirectories
mkdir -p dir/dir2/dir3

# Send a command's output to file.txt, no STDOUT
ls > file.txt

# Send a command's output to file.txt AND see it in STDOUT
ls | tee /tmp/file.txt

# Search and Replace within a file
sed -i 's/original-text/new-text/g' <filename>

# Download from URL
wget url

# Unzip to pwd path

# Go to directory
cd /data/wwwroot

# Modify the owner of user and group for directory
chown -R nginx.nginx /data/wwwroot
chmod u+x <file>

# Modify the Read, Write and Excuse for directory
find /data/wwwroot/default -type f -exec chmod 640 {} \
find /data/wwwroot/default -type d -exec chmod 750 {} \
# search for a file in a filesystem
find . -name 'filename.rb' -print

# locate a file
locate <filename>

# see command history

# search CLI history

# -B/A = show 2 lines before/after search_term
grep -B 2 -A 2 search_term <filename>

# -<number> shows both before and after
grep -2 search_term <filename>

# Search on all files in directory (recursively)
grep -r search_term <directory>

# search through *.gz files is the same except with zgrep
zgrep search_term <filename>

# Fast grep printing lines containing a string pattern
fgrep -R string_pattern <filename or directory>

# View command history

# Run last command that started with 'his' (3 letters min)

# Search through command history

# Execute last command with sudo
sudo !!


# Restart Systemd Service, e.g nginx, php-fpm, apache
systemctl restart nginx
systemctl restart php-fpm
systemctl restart apache

# Update and Upgrade
yum update -y
yum upgrade -y

# Restart all containers
docker restart $(docker ps -a | awk '{ print $1}' | tail -n +2)

# kill process
kill -9 PID

# Print last lines in log file where 'n'
# is the number of lines to print
tail -n /path/to/log/file


# disk space info. The '-h' gives the data in human-readable values
df -h

# size of each file/dir and its contents in the current dir
du -hd 1

# or alternative
du -h --max-depth=1

# find files greater than certain size(k, M, G) and list them in order
# get rid of the + for exact, - for less than
find / -type f -size +100M -print0 | xargs -0 du -hs | sort -h

# Find free memory on a system
free -m

# Find what processes are using memory/CPU and organize by it
# Load average is 1/CPU for 1, 5, and 15 minutes
top -o %MEM
top -o %CPU

# strace a process
strace -tt -T -f -y -yy -s 1024 -p <pid>

# run strace on all puma processes
ps auwx | grep puma | awk '{ print " -p " $2}' | xargs strace -tt -T -f -y -yy -s 1024 -o /tmp/puma.txt


# Display port and process from TCP, UDP 
netstat -tunlp
netstat -tunlp | grep PORT

# Check the 22 Port
lsof -i:22

# Find the programs that are listening on ports
netstat -plnt
ss -plnt
lsof -i -P | grep <port>

# Show domain IP address
dig +short

# Check DNS using specific nameserver
# = google, = cloudflare, = opendns
dig @

# Find host provider
whois <ip_address> | grep -i "orgname\|netname"

# Curl headers with redirect
curl --head --location ""

# Test if a host is reachable on the network. `ping6` works on IPv6 networks.

# Show the route taken to a host. `traceroute6` works on IPv6 networks.

# List details of network interfaces
ip address

# Check local DNS settings
cat /etc/hosts
cat /etc/resolv.conf
systemd-resolve --status

# Capture traffic to/from a host
sudo tcpdump host


# Debian/Ubuntu

# List packages
dpkg -l
apt list --installed

# Find an installed package
dpkg -l | grep <package>
apt list --installed | grep <package>

# Install a package
dpkg -i <package_name>.deb
apt-get install <package>
apt install <package>

# CentOS/RedHat

# Install a package
yum install <package>
dnf install <package> # RHEL/CentOS 8+

rpm -ivh <package_name>.rpm

# Find an installed package
rpm -qa | grep <package>