A collection of awesome lists, manuals, blogs, hacks, one-liners and tools for Awesome Ninja Admins.

_{Created by trimstray and contributors}

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Who is Ninja Admins?

• race of pure evil who rule the network through a monarchistic feudelic system
• they never opened the door for strangers (or anyone at all)
• they know very nasty piece of code like a fork bombs
• they can make dd is not a destroyer of disks
• they know that #!/usr/bin/env bash superior to #!/bin/bash
• they know that su - logs in completely as root
• they miss and cry for Slackware on production
• they love the old admin nix-world

☑️ Todo

• Add useful shell functions
• Add one-liners for collection tools (eg. CLI Tools)
• Add Ninja Admins T-Shirt stickers
• Generate Awesome Ninja Admins book (eg. pdf format)

Ninja Admins Collection

CLI Tools

▪️ Shells

  🔸 Oh My ZSH! - the best framework for managing your Zsh configuration.
  🔸 bash-it - a community Bash framework.

▪️ Managers

  🔸 Midnight Commander - visual file manager, licensed under GNU General Public License.
  🔸 screen - full-screen window manager that multiplexes a physical terminal.
  🔸 tmux - terminal multiplexer, lets you switch easily between several programs in one terminal.

▪️ Network

  🔸 Curl - command line tool and library for transferring data with URLs.
  🔸 HTTPie - a user-friendly HTTP client.
  🔸 gnutls-cli - client program to set up a TLS connection to some other computer.
  🔸 netcat - networking utility which reads and writes data across network connections, using the TCP/IP protocol.
  🔸 tcpdump - powerful command-line packet analyzer.

▪️ Databases

  🔸 pgcli - postgres CLI with autocompletion and syntax highlighting.
  🔸 mycli - terminal client for MySQL with autocompletion and syntax highlighting.

Web Tools

▪️ SSL

  🔸 SSL Server Test - free online service performs a deep analysis of the configuration of any SSL web server.
  🔸 SSL Server Test (DEV) - free online service performs a deep analysis of the configuration of any SSL web server.
  🔸 ImmuniWeb® SSLScan - test SSL/TLS (PCI DSS, HIPAA and NIST).
  🔸 Report URI - monitoring security policies like CSP and HPKP.
  🔸 CSP Evaluator - allows developers and security experts to check if a Content Security Policy.
  🔸 Common CA Database - repository of information about CAs, and their root and intermediate certificates.
  🔸 CERTSTREAM - real-time certificate transparency log update stream.

▪️ HTTP Headers

  🔸 Security Headers - analyse the HTTP response headers (with rating system to the results).
  🔸 Observatory by Mozilla - set of tools to analyze your website.

▪️ DNS

  🔸 ViewDNS - one source for free DNS related tools and information.
  🔸 DNS Spy - monitor, validate and verify your DNS configurations.
  🔸 DNSlytics - online investigation tool.

▪️ Mail

  🔸 MX Toolbox - all of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.

▪️ Mass scanners (search engines)

  🔸 Censys - platform that helps information security practitioners discover, monitor, and analyze devices.
  🔸 Shodan - the world's first search engine for Internet-connected devices.
  🔸 GreyNoise - mass scanner (such as Shodan and Censys).

▪️ Net-tools

  🔸 Netcraft - detailed report about the site, helping you to make informed choices about their integrity.
  🔸 Security Trails - APIs for Security Companies, Researchers and Teams.
  🔸 Online Curl - curl test, analyze HTTP Response Headers.
  🔸 Ping.eu - online Ping, Traceroute, DNS lookup, WHOIS and others.
  🔸 Network-Tools - network tools for webmasters, IT technicians & geeks.
  🔸 URL Encode/Decode - tool from above to either encode or decode a string of text.
  🔸 Hardenize - deploy the security standards.

▪️ Performance

  🔸 GTmetrix - analyze your site’s speed and make it faster.
  🔸 Sucuri loadtimetester - test here the performance of any of your sites from across the globe.

▪️ Passwords

  🔸 Random.org - generate random passwords.
  🔸 Gotcha? - list of 1.4 billion accounts circulates around the Internet.
  🔸 have i been pwned? - check if you have an account that has been compromised in a data breach.

Manuals/Howtos/Tutorials

▪️ Bash

  🔸 pure-bash-bible - a collection of pure bash alternatives to external processes.
  🔸 The Bash Hackers Wiki - hold documentation of any kind about GNU Bash.

▪️ Unix tutorials

  🔸 nixCraft - linux and unix tutorials for new and seasoned sysadmin.
  🔸 TecMint - the ideal Linux blog for Sysadmins & Geeks.

▪️ Hacking

  🔸 Hacking Articles - LRaj Chandel's Security & Hacking Blog.

Blogs

  🔸 Brendan Gregg's Blog - Brendan Gregg is an industry expert in computing performance and cloud computing.
  🔸 Gynvael "GynDream" Coldwind - Gynvael is a IT security engineer at Google.
  🔸 Michał "lcamtuf" Zalewski - "white hat" hacker, computer security expert.
  🔸 Mattias Geniar - developer, Sysadmin, Blogger, Podcaster and Public Speaker.
  🔸 Nick Craver - Software Developer and Systems Administrator for Stack Exchange.
  🔸 Robert Penz - IT security Expert.
  🔸 Scott Helme - Security Researcher, international speaker and founder of securityheaders.com and report-uri.com.
  🔸 Kacper Szurek - Detection Engineer at ESET.
  🔸 Troy Hunt - Microsoft Regional Director and Microsoft Most Valuable Professional for Developer Security.
  🔸 Linux Audit - the Linux security blog about Auditing, Hardening, and Compliance by Michael Boelen.
  🔸 The Grymoire - collection of useful incantations for wizards, be you computer wizards, magicians, or whatever.

Systems/Services

▪️ Systems

  🔸 Slackware - the most "Unix-like" Linux distribution.
  🔸 OpenBSD - multi-platform 4.4BSD-based UNIX-like operating system.
  🔸 HardenedBSD - HardenedBSD aims to implement innovative exploit mitigation and security solutions.

▪️ HTTP(s) Services

  🔸 Varnish HTTP Cache - HTTP accelerator designed for content-heavy dynamic web sites.

▪️ Security/hardening

  🔸 Emerald Onion - Seattle-based encrypted-transit internet service provider.

Lists

  🔸 Awesome Sysadmin - amazingly awesome open source sysadmin resources.
  🔸 Awesome Shell - awesome command-line frameworks, toolkits, guides and gizmos.
  🔸 Awesome-Hacking - awesome lists for hackers, pentesters and security researchers.

Hacking/Penetration testing

▪️ Bounty programs

  🔸 Openbugbounty - allows any security researcher reporting a vulnerability on any website.
  🔸 hackerone - global hacker community to surface the most relevant security issues.
  🔸 bugcrowd - crowdsourced cybersecurity for the enterprise.
  🔸 Crowdshield - crowdsourced Security & Bug Bounty Management.

▪️ Web Training Apps

  🔸 DVWA - PHP/MySQL web application that is damn vulnerable.
  🔸 OWASP Mutillidae II - free, open source, deliberately vulnerable web-application.
  🔸 OWASP Juice Shop Project - the most bug-free vulnerable application in existence.
  🔸 OWASP WebGoat Project - insecure web application maintained by OWASP designed to teach web app security.
  🔸 Security Ninjas - open source application security training program.

One-liners

Table of Contents

• System
  • terminal
  • mount
  • fuser
  • ps
  • top
  • find
  • diff
  • tail
  • cpulimit
  • pwdx
  • tr
  • chmod
  • who
  • screen
  • du
  • inotifywait
  • openssl
• HTTP/HTTPS
  • curl
  • httpie
• Network
  • ssh
  • linux-dev
  • tcpdump
  • ngrep
  • hping3
  • netcat
  • socat
  • lsof
  • netstat
  • rsync
• Programming
  • awk
  • sed
  • grep

System

Tool: terminal

Close shell keeping all subprocess running

disown -a && exit

Exit without saving shell history

kill -9 $$
unset HISTFILE && exit

Perform a branching conditional

true && { echo success;} || { echo failed; }

Pipe stdout and stderr to separate commands

some_command > >(/bin/cmd_for_stdout) 2> >(/bin/cmd_for_stderr)

Pipe stdout and stderr to separate commands

(some_command 2>&1 1>&3 | tee errorlog ) 3>&1 1>&2 | tee stdoutlog

List of commands you use most often

history | awk '{ a[$2]++ } END { for(i in a) { print a[i] " " i } }' | sort -rn | head

Quickly backup a file

cp filename{,.orig}

Delete all files in a folder that don't match a certain file extension

rm !(*.foo|*.bar|*.baz)

Edit a file on a remote host using vim

vim scp://user@host//etc/fstab

Create a directory and change into it at the same time

mkd () { mkdir -p "$@" && cd "$@"; }

Convert uppercase files to lowercase files

rename 'y/A-Z/a-z/' *

Print a row of characters across the terminal

printf "%`tput cols`s" | tr ' ' '#'

Show shell history without line numbers

history | cut -c 8-
fc -l -n 1 | sed 's/^\s*//'

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Tool: mount

Mount a temporary ram partition

mount -t tmpfs tmpfs /mnt -o size=64M

• -t - filesystem type
• -o - mount options

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Tool: fuser

Kills a process that is locking a file

fuser -k filename

Show what PID is listening on specific port

fuser -v 53/udp

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Tool: ps

Show a 4-way scrollable process tree with full details

ps awwfux | less -S

Processes per user counter

ps hax -o user | sort | uniq -c | sort -r

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Tool: find

Find files that have been modified on your system in the past 60 minutes

find / -mmin 60 -type f

Find all files larger than 20M

find / -type f -size +20M

Find duplicate files (based on MD5 hash)

find -type f -exec md5sum '{}' ';' | sort | uniq --all-repeated=separate -w 33

Change permission only for files

cd /var/www/site && find . -type f -exec chmod 766 {} \;
cd /var/www/site && find . -type f -exec chmod 664 {} +

Change permission only for directories

cd /var/www/site && find . -type d -exec chmod g+x {} \;
cd /var/www/site && find . -type d -exec chmod g+rwx {} +

Find files and directories for specific user

find . -user <username> -print

Find files and directories for all without specific user

find . \!-user <username> -print

Delete older files than 60 days

find . -type f -mtime +60 -delete

---

Tool: top

Use top to monitor only all processes with the specific string

top -p $(pgrep -d , <str>)

• <str> - process containing str (eg. nginx, worker)

---

Tool: diff

Compare two directory trees

diff <(cd directory1 && find | sort) <(cd directory2 && find | sort)

---

Tool: tail

Annotate tail -f with timestamps

tail -f file | while read; do echo "$(date +%T.%N) $REPLY"; done

Analyse an Apache access log for the most common IP addresses

tail -10000 access_log | awk '{print $1}' | sort | uniq -c | sort -n | tail

---

Tool: cpulimit

Limit the cpu usage of a process

cpulimit -p pid -l 50

Tool: pwdx

Show current working directory of a process

pwdx <pid>

---

Tool: taskset

Start a command on only one CPU core

taskset -c 0 <command>

---

Tool: tr

Show directories in the PATH, one per line

tr : '\n' <<<$PATH

Tool: chmod

Remove executable bit from all files in the current directory

chmod -R -x+X *

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Tool: who

Find last reboot time

who -b

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Tool: screen

Start screen in detached mode

screen -d -m [<command>]

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Tool: du

Show 20 biggest directories with 'K M G'

du | sort -r -n | awk '{split("K M G",v); s=1; while($1>1024){$1/=1024; s++} print int($1)" "v[s]"\t"$2}' | head -n 20

Tool: inotifywait

Init tool everytime a file in a directory is modified

while true ; do inotifywait -r -e MODIFY dir/ && ls dir/ ; done;

Tool: openssl

Testing connection to remote host

echo | openssl s_client -connect google.com:443 -showcerts

Testing connection to remote host with specific ssl version

openssl s_client -tls1_2 -connect google.com:443

Testing connection to remote host with specific ssl cipher

openssl s_client -cipher 'AES128-SHA' -connect google.com:443

Generate private key

# _ciph: des3, aes
( _ciph="des3" ; _fd="private.key" ; _len="2048" ; \
openssl genrsa -${_ciph} -out ${_fd} ${_len} )

Remove password from private key

( _fd="private.key" ; _fd_unp="private_unp.key" ; \
openssl rsa -in ${_fd} -out ${_fd_unp} )

Get public key from private key

( _fd="private.key" ; _fd_pub="public.key" ; \
openssl rsa -pubout -in ${_fd} -out ${_fd_pub} )

Generate private key + csr

( _fd="private.key" ; _fd_csr="request.csr" ; _len="2048" ; \
openssl req -out ${_fd_csr} -new -newkey rsa:${_len} -nodes -keyout ${_fd} )

Generate csr

( _fd="private.key" ; _fd_csr="request.csr" ; \
openssl req -out ${_fd_csr} -new -key ${_fd} )

Generate csr (metadata from exist certificate)

( _fd="private.key" ; _fd_csr="request.csr" ; _fd_crt="cert.crt" ; \
openssl x509 -x509toreq -in ${_fd_crt} -out ${_fd_csr} -signkey ${_fd} )

Generate csr with -config param

( _fd="private.key" ; _fd_csr="request.csr" ; \
openssl req -new -sha256 -key ${_fd} -out ${_fd_csr} \
-config <(
cat <<-EOF
[req]
default_bits = 2048
prompt = no
default_md = sha256
req_extensions = req_ext
distinguished_name = dn

[ dn ]
C=<two-letter ISO abbreviation for your country>
ST=<state or province where your organization is legally located>
L=<city where your organization is legally located>
O=<legal name of your organization>
OU=<section of the organization>
CN=<fully qualified domain name>

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = <fully qualified domain name>
DNS.2 = <next domain>
DNS.3 = <next domain>
EOF
))

Convert DER to PEM

( _fd_der="cert.crt" ; _fd_pem="cert.pem" ; \
openssl x509 -in ${_fd_der} -inform der -outform pem -out ${_fd_pem} )

Convert PEM to DER

( _fd_der="cert.crt" ; _fd_pem="cert.pem" ; \
openssl x509 -in ${_fd_pem} -outform der -out ${_fd_der} )

Checking whether the private key and the certificate match

(openssl rsa -noout -modulus -in private.key | openssl md5 ; openssl x509 -noout -modulus -in certificate.crt | openssl md5) | uniq

---

Tool: gnutls-cli

Testing connection to remote host (with sni)

gnutls-cli -p 443 google.com

Testing connection to remote host (without sni)

gnutls-cli --disable-sni -p 443 google.com

HTTP/HTTPS

Tool: curl

curl -Iks https://www.google.com

• -I - show response headers only
• -k - insecure connection when using ssl
• -s - silent mode (not display body)

curl -Iks --location -X GET -A "x-agent" https://www.google.com

• --location - follow redirects
• -X - set method
• -A - set user-agent

curl -Iks --location -X GET -A "x-agent" --proxy http://127.0.0.1:16379 https://www.google.com

• --proxy [socks5://|http://] - set proxy server

---

Tool: httpie

http -p Hh https://www.google.com

• -p - print request and response headers
  • H - request headers
  • B - request body
  • h - response headers
  • b - response body

http -p Hh --follow --max-redirects 5 --verify no https://www.google.com

• -F, --follow - follow redirects
• --max-redirects N - maximum for --follow
• --verify no - skip SSL verification

http -p Hh --follow --max-redirects 5 --verify no --proxy http:http://127.0.0.1:16379 https://www.google.com

• --proxy [http:] - set proxy server

Network

Tool: ssh

Compare a remote file with a local file

ssh user@host cat /path/to/remotefile | diff /path/to/localfile -

SSH connection through host in the middle

ssh -t reachable_host ssh unreachable_host

Run command over ssh on remote host

cat > cmd.txt << __EOF__
cat /etc/hosts
__EOF__

ssh host -l user $(<cmd.txt)

Get public key from private key

ssh-keygen -y -f ~/.ssh/id_rsa

---

Tool: linux-dev

Testing remote connection to port

timeout 1 bash -c "</dev/<proto>/<host>/<port>" >/dev/null 2>&1 ; echo $?

• <proto - set protocol (tcp/udp)
• <host> - set remote host
• <port> - set destination port

Read and write to TCP or UDP sockets with common bash tools

exec 5<>/dev/tcp/<host>/<port>; cat <&5 & cat >&5; exec 5>&-

---

Tool: tcpdump

tcpdump -ne -i eth0 -Q in host 192.168.252.1 and port 443

• -n - don't convert addresses
• -e - print the link-level headers
• -i [iface] - set interface
• -Q|-D [in|out|inout] - choose send/receive direction (-D - for old tcpdump versions)
• host [ip|hostname] - set host, also [host not]
• [and|or] - set logic
• port [1-65535] - set port number, also [port not]

tcpdump -ne -i eth0 -Q in host 192.168.252.1 and port 443 -c 5 -w tcpdump.pcap

• -c [num] - capture only num number of packets
• -w [filename] - write packets to file, -r [filename] - reading from file

---

Tool: ngrep

ngrep -d eth0 "www.google.com" port 443

• -d [iface|any] - set interface
• [domain] - set hostname
• port [1-65535] - set port number

ngrep -d eth0 "www.google.com" (host 10.240.20.2) and (port 443)

• (host [ip|hostname]) - filter by ip or hostname
• (port [1-65535]) - filter by port number

ngrep -d eth0 -qt -O ngrep.pcap "www.google.com" port 443

• -q - quiet mode (only payloads)
• -t - added timestamps
• -O [filename] - save output to file, -I [filename] - reading from file

ngrep -d eth0 -qt 'HTTP' 'tcp'

• HTTP - show http headers
• tcp|udp - set protocol
• [src|dst] host [ip|hostname] - set direction for specific node

ngrep -l -q -d eth0 -i "User-Agent: curl*"

• -l - stdout line buffered
• -i - case-insensitive search

---

Tool: hping3

hping3 -V -p 80 -s 5050 <scan_type> www.google.com

• -V|--verbose - verbose mode
• -p|--destport - set destination port
• -s|--baseport - set source port
• <scan_type> - set scan type
  • -F|--fin - set FIN flag, port open if no reply
  • -S|--syn - set SYN flag
  • -P|--push - set PUSH flag
  • -A|--ack - set ACK flag (use when ping is blocked, RST response back if the port is open)
  • -U|--urg - set URG flag
  • -Y|--ymas - set Y unused flag (0x80 - nullscan), port open if no reply
  • -M 0 -UPF - set TCP sequence number and scan type (URG+PUSH+FIN), port open if no reply

hping3 -V -c 1 -1 -C 8 www.google.com

• -c [num] - packet count
• -1 - set ICMP mode
• -C|--icmptype [icmp-num] - set icmp type (default icmp-echo = 8)

hping3 -V -c 1000000 -d 120 -S -w 64 -p 80 --flood --rand-source <remote_host>

• --flood - sent packets as fast as possible (don't show replies)
• --rand-source - random source address mode
• -d --data - data size
• -w|--win - winsize (default 64)

---

Tool: netcat

nc -kl 5000

• -l - listen for an incoming connection
• -k - listening after client has disconnected
• >filename.out - save receive data to file (optional)

nc 192.168.0.1 5051 < filename.in

• < filename.in - send data to remote host

nc -vz 10.240.30.3 5000

• -v - verbose output
• -z - scan for listening daemons

nc -vzu 10.240.30.3 1-65535

• -u - scan only udp ports

Transfer data file (archive)

server> nc -l 5000 | tar xzvfp -
client> tar czvfp - /path/to/dir | nc 10.240.30.3 5000

Launch remote shell

server> nc -l 5000 -e /bin/bash
client> nc 10.240.30.3 5000

Simple file server

while true ; do nc -l 5000 | tar -xvf - ; done

Simple HTTP Server

Restarts web server after each request - remove while condition for only single connection.

cat > index.html << __EOF__
<!doctype html>
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
        <title></title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
    </head>
    <body>

    <p>

      Hello! It's a site.

    </p>

    </body>
</html>
__EOF__

server> while : ; do \
(echo -ne "HTTP/1.1 200 OK\r\nContent-Length: $(wc -c <index.html)\r\n\r\n" ; cat index.html;) \
| nc -l -p 5000 \
; done

• -p - port number

Simple HTTP Proxy (single connection)

#!/usr/bin/env bash

if [[ $# != 2 ]] ; then
  printf "%s\\n" \
         "usage: ./nc-proxy listen-port bk_host:bk_port"
fi

_listen_port="$1"
_bk_host=$(echo "$2" | cut -d ":" -f1)
_bk_port=$(echo "$2" | cut -d ":" -f2)

printf "  lport: %s\\nbk_host: %s\\nbk_port: %s\\n\\n" \
       "$_listen_port" "$_bk_host" "$_bk_port"

_tmp=$(mktemp -d)
_back="$_tmp/pipe.back"
_sent="$_tmp/pipe.sent"
_recv="$_tmp/pipe.recv"

trap 'rm -rf "$_tmp"' EXIT

mkfifo -m 0600 "$_back" "$_sent" "$_recv"

sed "s/^/=> /" <"$_sent" &
sed "s/^/<=  /" <"$_recv" &
nc -l -p "$_listen_port" <"$_back" \
  | tee "$_sent" \
  | nc "$_bk_host" "$_bk_port" \
  | tee "$_recv" >"$_back"

server> chmod +x nc-proxy && ./nc-proxy 8080 192.168.252.10:8000
  lport: 8080
bk_host: 192.168.252.10
bk_port: 8000

client> http -p h 10.240.30.3:8080
HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=31536000
Content-Length: 2748
Content-Type: text/html; charset=utf-8
Date: Sun, 01 Jul 2018 20:12:08 GMT
Last-Modified: Sun, 01 Apr 2018 21:53:37 GMT

Create a single-use TCP or UDP proxy

### TCP -> TCP
nc -l -p 2000 -c "nc [ip|hostname] 3000"

### TCP -> UDP
nc -l -p 2000 -c "nc -u [ip|hostname] 3000"

### UDP -> UDP
nc -l -u -p 2000 -c "nc -u [ip|hostname] 3000"

### UDP -> TCP
nc -l -u -p 2000 -c "nc [ip|hostname] 3000"

---

Tool: socat

Testing remote connection to port

socat - TCP4:10.240.30.3:22

• - - standard input (STDIO)
• TCP4:<params> - set tcp4 connection with specific params
  • [hostname|ip] - set hostname/ip
  • [1-65535] - set port number

Redirecting TCP-traffic to a UNIX domain socket under Linux

socat TCP-LISTEN:1234,bind=127.0.0.1,reuseaddr,fork,su=nobody,range=127.0.0.0/8 UNIX-CLIENT:/tmp/foo

• TCP-LISTEN:<params> - set tcp listen with specific params
  • [1-65535] - set port number
  • bind=[hostname|ip] - set bind hostname/ip
  • reuseaddr - allows other sockets to bind to an address
  • fork - keeps the parent process attempting to produce more connections
  • su=nobody - set user
  • range=[ip-range] - ip range
• UNIX-CLIENT:<params> - communicates with the specified peer socket
  • filename - define socket

---

Tool: lsof

Show process that use internet connection at the moment

lsof -P -i -n

Show process that use specific port number

lsof -i tcp:443

Lists all listening ports together with the PID of the associated process

lsof -Pan -i tcp -i udp

List all open ports and their owning executables

lsof -i -P | grep -i "listen"

Show open ports

lsof -Pni4 | grep LISTEN | column -t

List all files opened by a particular command

lsof -c "process"

View user activity per directory

lsof -u username -a +D /etc

---

Tool: netstat**

Graph # of connections for each hosts

netstat -an | grep ESTABLISHED | awk '{print $5}' | awk -F: '{print $1}' | grep -v -e '^[[:space:]]*$' | sort | uniq -c | awk '{ printf("%s\t%s\t",$2,$1) ; for (i = 0; i < $1; i++) {printf("*")}; print "" }'

Monitor open connections for specific port including listen, count and sort it per IP

watch "netstat -plan | grep :443 | awk {'print \$5'} | cut -d: -f 1 | sort | uniq -c | sort -nk 1"

---

Tool: rsync**

Rsync remote data as root using sudo

rsync --rsync-path 'sudo rsync' username@hostname:/path/to/dir/ /local/

Programming

Tool: awk

Remove duplicate entries in a file without sorting

awk '!x[$0]++' filename

Exclude multiple columns using AWK

awk '{$1=$3=""}1' filename

---

Tool: sed

To print a specific line from a file

sed -n 10p /path/to/file

Remove a specific line from a file

sed -i 10d /path/to/file

Remove a range of lines from a file

sed -i <file> -re '<start>,<end>d'

---

Tool: grep

Search for a "pattern" inside all files in the current directory

grep -RnisI "pattern" *
fgrep "pattern" * -R

Remove blank lines from a file and save output to new file

grep . filename > newfilename

Except multiple patterns

grep -vE '(error|critical|warning)' filename

Show data from file without comments

grep -v ^[[:space:]]*# filename

Show data from file without comments and new lines

egrep -v '#|^$' filename