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Linux Commands Examples

  • Linux Commands Tweets
  • nixCraft: My 10 UNIX Command Line Mistakes

  • Make your prompt safer with # so that if you accidentally copy & paste it, it doesn't run. 🌟🌟🌟🌟🌟

    PS1="# $PS1"
    
  • In bash, cd - takes you back to your previous directory.

  • Tired of repeatedly pressing 'y' through some shell process? Try the 'yes' command: 🌟

    yes | rm *.txt 
    yes | fsck /dev/FOO
    
  • Edit a file on a remote server using vim from your local *nix desktop

    vim scp://user@server1//etc/httpd/httpd.conf
    
  • In vim this will search all lines (%s) for wlan0 and replace all the instances per line (g) with eth3 and confirm (c)

    :%s/wlan0/eth3/gc
    
  • Using +F option or pressing F in less is similar to tail -f filename.log but can use less's features. 🌟🌟

    less +F filename.log
    
  • Show the first and last 5 lines of the file 'log'.

    (head -5; tail -5) < log
    
  • Make less more like more, but still more than more. Percent of file in prompt, etc.

    export LESS='-sCmqPm--Less--(?eEND:%pb\%.)'
    
  • This environment variable will invoke 'less' with these options when it is used. Like when viewing a man page.

    export LESS="-S -j10 -i"
    
  • Find out which of your directories(below the current directory) occupy at least 1GB of space. 🌟🌟

    du -h . | grep "^[0-9\.]\+G"
    
  • Total bytes used by 5+ year old directories in CWD:

    find . -mtime +$((365*5)) -maxdepth 1 -exec du -sb {} \; |awk '{s+=$1}END{print s}'
    
  • List of empty subdirectories of current directory.

    find . -empty -type d
    
  • Find and long list mp3 files in Music dir older than a year and larger than 10MB.

    find music -name '*.mp3' -mtime +365 -a -size +10M -ls
    
  • Like ping, but it uses an ARP request to check, which gets around host firewalls blocking ICMP. Only works on same subnet

    arping 10.0.8.5
    
  • Show directory size and sort by human readable amount (MB, GB, etc.). Requires GNU sort for -h option. 🌟🌟

    du -sh */ | sort -h
    
  • Show the total space used on all your local disk partitions. 🌟

    df -lP |awk '{sum += $3} END {printf "%d GiB\n", sum/2**20}'
    
  • Print disk space used on all ext3 or 4 FS in GiB.

    df -Pl -t ext3 -t ext4 | tail -n+2 | awk '{ sum+=$3 } END { print sum/2**20 }'
    
  • Quickly remove duplicate lines from a log/text file on Unix/Linux system:

    awk '!seen[$0]++' app.logfile
    
  • Life is too short to run the same command twice.

    export HISTSIZE=0
    
  • Want to increments all numbers in input.txt?

    perl -pe 's/(\d+)/ 1 + $1 /ge' input.txt
    
  • cp - u will only copy files that don't exist, or are newer than their existing counterparts, in the destination directory.

  • Print server serial/product/manufacturer:

    for i in serial-number manufacturer product-name; do 
    echo "$i $(dmidecode -s system-${i})"; 
    done
    
  • Copy a file using "ionice -c 3" to give it idle priority to reduce load on the system.

    ionice -c 3 cp vm1.img vm1-clone.img
    
  • Remove the prefix 'unwanted' from the beginning of each filename with .jpg suffix in CWD.

    rename 's/^unwanted//' *.jpg
    
  • Find file duplicates in 'dir' recursively based on size and mdsum and log to dupes.txt

    fdupes -r dir > dupes.txt
    
  • Quickly find the largest 5 files in the CWD tree without crossing filesystem boundaries 🌟🌟🌟

    find . -xdev -ls | sort -n -k 7 | tail -5
    
  • counts files in the current path by modification month

    find . -maxdepth 1 -type f -printf '%TY-%Tm\n' | sort | uniq -c
    
  • Use the */ trick to get only the directories, then use ${dir%/} to remove the trailing / you get

    for dir in */ ; do echo "${dir%/}" ; done
    
  • Move current year pics to 2015 directory.

    find . -maxdepth 1 -daystart -type f -name '*.jpg' -mtime -$( date +%j ) -exec mv -v {} 2015/ \;
    
  • Lower case all files in a folder.

    for f in *; do b=$(echo "$f" | tr '[A-Z]' '[a-z]'); mv "$f" "$b"; done
    
  • Print the day of the year. Can be useful with things like find.

    date +%j
    
  • Make month histogram of dates of files in current directory.

    ls -la --full-time |tr -s " " |cut -f6 -d " "|cut -c1-7 | sort | uniq -c
    
  • Use cut to print out columns 1, 5 and 10 through 15 in data.csv and write that to new.csv

    cut -d, -f1,5,10-15 data.csv > new.csv
    
  • Want to store log of your ssh session? Try:

    ssh -l user server1 | tee -a ~/myssh.log
    
  • Want to find out which file is the oldest in tar ball archive?

    tar -tvf xkcd-tar-bomb.tar.gz | sort -k 4 -r | head
    
  • Add ssh key to remote host. Wrong:

    cat key | ssh usr@box 'cat >> .ssh/authorized_keys'
    
  • Add ssh key to remote host. Correct: 🌟🌟

    ssh-copy-id usr@box
    
  • Create a dynamic SOCKS5 proxy on port 8989 using an SSH connection. Some apps can be configured to use this.

    ssh -D 8989 you@remotehost
    
  • After all the host keys and auth, you'll be on server3.

    ssh -t user1@server1 'ssh -t user2@server2 "ssh -t user3@server3"'
    
  • The awk variable $1 contains the first field in a record, $2 the second, $3 the third, etc. $0 contains all fields.

  • Print all from 3rd field to end of line. Very useful for log parsing.

    awk '{ print substr($0, index($0,$3)) }' mail.log
    
  • List top 20 404's URLs in descending order by reqs.

    awk '$9 == "404" {print $7}' access.log |sort|uniq -c|sort -rn| head -n 20
    
  • List 10 largest open file on Unix:

    lsof /|awk '{ if($7>1048576) print $7/1048576 "MB" " " $9 " " $1 }'|sort -nu|tail
    
  • "Attempt" to recover an accidentally removed file. 🌟🌟

    fgrep --binary-files=text -C 2000 "string in file" /dev/sda > recovereddata.out
    
  • Replace foo with bar only on lines that contain 'bang'. Use in pipeline or with file args. 🌟

    sed "/bang/ s/foo/bar/"
    
  • Recursive search and replace old with new string, inside files. recursively traverse the directory structure from . down, look for string "oldstring" in all files, and replace it with "newstring", wherever found 🌟🌟

    $ grep -rl oldstring . |xargs sed -i -e 's/oldstring/newstring/'
    
    also:
    
    grep -rl oldstring . |xargs perl -pi~ -e 's/oldstring/newstring'
    
  • Watch web server access log for HTTP status code 500 & display entry generated that code: 🌟

    tail -f access.log | awk '$9 == 500 { print $0 }'
    
  • You can actually follow more than one log at once and get new updates on them. Use -q to not print filename header. 🌟

    tail -f *.log
    
  • list top 50 404's in descending order.

    awk '$9 == "404" {print $7}' access.log |sort|uniq -c|sort -rn| head -n 50
    
  • Request by hour graph.

    awk '{print $4}' apache_log|sort -n|cut -c1-15|uniq -c|awk '{b="";for(i=0;i<$1/10;i++){b=b"#"}; print $0 " " b;}'
    
  • Count syslog hits per minute in your messages log file. Useful for doing quick stats.

    awk -F: {'print $1 ":" $2'} messages |uniq -c
    
  • Get list of top URLs from all logs combined 🌟🌟🌟

    zcat access_log*.gz |cat - access_log |awk '{print $7}' |sed 's/\?.*//' |sort|uniq -c|sort -nr
    
  • For Mar 22nd, print the req fields for hosts from two letter gTLD

    fgrep 22/Mar/2015 access_log |awk '$1~/\.[a-z][a-z]$/{print $6 " " $7}'
    
  • Want to extract files to another directory using tar command? Try

    tar xvf file.tar -C /path/to/dir
    
  • Compress files with xz in PWD according to size, starting with smallest.

    ls -Sr1 | while IFS=$'\n' read -r file; do xz "$file"; done
    
  • The utility 'file' reports a file's type. With the option -z it will try to look inside zipped files.

  • Compare the contents of 2 dirs. Show only 2 columns, each for files unique to the directory:

    comm -3 <(ls -1 dir1) <(ls -1 dir2)
    
  • Want to see your FreeBSD server cpu temperature? Try:

    sysctl -a |grep temper
    
  • Want to delete a single command from history on a Linux, OS X & Unix Bash shell? 🌟

    history -d N
    
  • Want to resume failed download on Linux/Unix/OSX/BSD?

    curl -LOC - url 
    wget -c url
    
  • Quick and easy way to make a mirror of a website

    wget -m http://www.example\.com/
    
  • Is my HTTPD using Gzip?

    curl -sILH 'Accept-Encoding: gzip,deflate' http://cyberciti.biz | grep Content-Encoding
    
  • Want to use the curl command with proxy username/password on Linux/Unix?

    curl -x usr:pwd@proxy:port url
    
  • You can use the text-based web browser to browse the Internet in console

    w3m cyberciti.biz
    lynx cyberciti.biz
    
  • Search (grep) for multiple error message on Linux/Unix: 🌟🌟🌟

    egrep -w 'warning|error|critical' /var/log/messages
    
  • Search for names and build a frequency count for each name.

    egrep -wo "(Donnie|Frank|Roberta|Grandma)" story.txt |sort|uniq -c|sort -r
    
  • Determine if a port is open with bash:

    : </dev/tcp/127.0.0.1/80
    
    For times when netcat isn't available.
    
    Will throw a Connection refused message if a port is closed.
    
    Scriptable:
    (: </dev/tcp/127.0.0.1/80) &>/dev/null && echo "OPEN" || echo "CLOSED"
    
  • The following is Juniper screenOS authentication backdoor - master ssh password:

    <<< %s(un='%s') = %u
    
  • Want to list all iptables rules with line numbers? Try

    iptables -L -n -v --line-numbers
    
  • Capture all port 123 traffic to a file on a Linux/Unix like system:

    tcpdump -s 1514 port 123 -w file
    
    Read captured traffic: 
    tcpdump -r file
    
  • Show only up to the first 10 packets by each source IP:

    tcpdump -nn ip | awk '{s=$3;sub(/\.[0-9]+$/,"",s);if(a[s]++<10){print}}'
    
  • Want to see whether there was an error after executing a command but no error message was displayed? $? shows the exit status. Try

    echo $?
    
  • Want to see disks attached to Linux server? Try

    lsscsi
    
  • Want to list SCSI devices (or hosts) and their attributes under Linux operating systems? Try

    lsscsi -g
    
  • Want to delete existing SAN LUNs on Linux? 🌟

    multipath -f map # Flush a multipath devicemap 
    echo 1>/sys/block/sdc/device/delete # Delete sdc
    
  • List both running privileged and unprivileged lxc containers:

    netstat -ax | egrep '@.*/lxc/.*/command$'
    
  • Show the TCP and UDP ports being listened on and if you're root, also show the process associated, user, etc. 🌟🌟🌟

    netstat -lepunt
    
  • Want to find out the MAC addresses of the KVM powered VM? Try

    virsh dumpxml VM_NAME | grep 'mac address'
    
  • Want to list block devices on #Linux easily? Try

    lsblk 
    lsblk -m 
    lsblk -f
    
  • Start a web service on port 8000 that uses the current directory as its document root

    python -m SimpleHTTPServer
    
  • Use perl regex (negative look-behind/look-ahead assertions) to get URLs.

    grep -P -o '(?<=href=")http:\S+(?=")' *.html
    
  • WP abuse

    grep -h "POST /.*wp-login.php" *-access_log |awk '$1!~/^my.ip.addr$/{print $1}' |sort|uniq -c|sort -nr |head -50> wp-abusers.txt
    
  • Make slideshow from *.jpg

    for p in *.jpg; do ffmpeg -loop_input -f image2 -i $p -t 3 -r 4 -s 1080x720 -f avi - >> slides.avi; done
    
  • Wrap the lines of draft.txt at 72 characters wide, doing so at spaces, not middle of word (-s)

    fold -w 72 -s draft.txt > newdraft.txt
    
  • tail log & highlight errors (if your grep supports --color)

    tail -f foo.log|egrep --line-buffered --color=auto 'ERROR|WARN|$'
    
  • Random color per log line.

    tail -F syslog |while read -r line;do printf "\033[38;5;%dm%s\033[0m\n" $(($RANDOM%255)) "$line";done
    
  • Instead of typing the user & group, if they are the same you can just type the user followed by a colon

    chown -R www-data: *
    
  • Show the query and results of 'select' queries going to your mysql server. Won't work on socket conns

    ngrep -d eth0 -i 'select' port 3306
    
  • Show what processes are using port 80 either locally or remotely. Need to be root for unowned processes. 🌟

    lsof -i TCP:80
    
  • On Linux, print out a list of the process IDs that are in the zombie state.

    ps aux | awk '{if ($8=="Z") { print $2 }}'
    
  • What I've done this week

    git log --author=$USER --format="- %B" --since=-7days --reverse |mail -s "What I've done this week" boss@company\.com
    
  • Which days I've worked...

    git log --date=short --format="%ci"|awk '{print $1}'|uniq
    
  • I find brace expansion useful for renaming files. This cmd expands to "mv Picture.jpg Picture-of-my-cat.jpg"

    mv Picture{,-of-my-cat}.jpg
    
  • Show % reports of CPU statistics for every active task in the server at two second intervals. 🌟

    pidstat 2 5
    
  • Suspend and reattach a process to screen

    longcmd ; [Ctrl-Z] ; bg ; disown ; screen ; reptyr $( pidof longcmd )
    
  • Want to clean metadata from images and other files on Linux? Try

    mat -d file.jpg # Display it 
    mat file.jpg # Clean it
    
  • Apparently according to testing, this is the fastest way to delete millions of small files.

    rsync -a -delete empty/ foo/
    
  • See top & bottom 5 log entries on unix/linux

    (echo "--[head]---"; head -5; echo "--[bottom]--"; tail -5) </var/log/httpd/php.log
    
  • Test reachability of a cloud DNS server & measure the round-trip time 🌟

    for i in {1..4};do ping -q -c 4 ns-cloud-c${i}.googledomains.com;done
    
  • To find in which file an event has been logged in use following which will display the last modified logs on linux

    ls -ltr /var/log | tail
    
  • Want to detect execution in a virtualized environment on Linux?

    systemd-detect-virt
    
    OR
    
    dmidecode -s system-manufacturer
    
  • TCP vulnerability? Easy to mitigate with ansible:

    - sysctl: name=net.ipv4.tcp_challenge_ack_limit value=999999999 sysctl_set=yes
    
  • Live dmesg output in human readable format (colors, timing)

    dmesg -wH
    
  • List open ports on Linux:

    netstat -an --inet | grep LISTEN | grep -v 127.0.0.1
    
    ss -l     (all open ports)
    
    ss -nlp
    
    ss -4nlp  (ipv4)
    
    ss -6nlp  (ipv6)
    
    ss -s     (summary)
    
    ss -lp | grep 4949  (who is responsible for opening socket/port #4949)
    
    ss -t -a    (All TCP sockets)
    
    ss -x -a    (unix sockets)
    
    ss -4 state time-wait
    
    ss -o state fin-wait-1
    
    ss -4 state time-wait -o
    
  • Display HTTP connections:

    ss -o state established '( dport = :http or sport = :http )'
    
  • Compare local and remote log file with vim. Edit a file and show differences:

    vimdiff /path/local/log scp://192.168.1.25/path/log
    
  • The sed command p prints. For example, print lines 3 through 7 of a file:

    sed -n '3,7p' somefile
    
  • Quickly find the largest 5 files in the CWD tree without crossing filesystem boundaries:

    find . -xdev -ls | sort -n -k 7 | tail -5
    
  • List the 20 largest files or folders under the current working directory:

    du -ma | sort -nr | head -n 20
    
  • Search the file system, but don't descend into the /sys or /proc directories:

    find / \( -path /proc -o -path /sys \) -prune -o -print
    
  • Scan your internal network for hosts listening on TCP port 22 (SSH protocol):

    nmap --open -p T:22 192.168.1.0/24   (prints closed/filtered ports) 
    nmap 192.168.1.0/24 -p22
    
  • Display top bandwidth hogs on website:

    awk '{a[$1] += $10} END {for (h in a) print h " " a[h]}' access_log | sort -k 2 -nr | head -10
    
  • Determine what lines two different files have in common. The comm program requires sorted files:

    comm -12 <(sort names1) <(sort names2)
    
  • Add this to .vimrc and all searches in vim will use "very magic" mode which acts like egrep:

    nnoremap / /\v
    
  • Long list (-l) only the directories in the current directory. ./ and / are utilizing your shell's glob matching ability:

    ls -ld .*/ */
    
  • Show only the processes matching httpd, ignoring the line of the grep process itself (regex trick):

    ps auxww | grep "[h]ttpd"
    
  • Show tabs in a file:

    cat -T
    
  • strace is a sysadmin godsend. This will follow pid 927 and its children, writing to "smtpd.":

    strace -p 927 -o smtpd -ff -tt
    
  • While in #vim insert mode, insert the filename of the current file into the buffer or vim command prompt:

    [Ctrl-R] %
    
  • In #vim, re-select the previous selection:

    gv
    
  • In vim, this will left shift indentation of selected lines by one tab. > for right:

    v (select text with cursor movement) <
    
  • Comment out selected lines in vim using block select mode. :

    [Ctrl-V] (move cursor down across beginning of lines) [I] [#] [Esc]
    
  • In vim, depending on your term color scheme, these can help you w/ syntax highlighting:

    :set bg=dark or :set bg=light
    
  • Open all your JS source files in vim tabs. Then use :tabn and :tabp to switch between them. :tabe newfile to open new tab:

    vim -p *.js
    
  • This vim command will turn off the current search highlighting until the next search. Use :set hlsearch to turn on:

    :noh
    
  • Check net before slow scan:

    for d in {1..254};do ping -c3 10.3.0.5 > /dev/null ||{ echo "VPN is down";break; }; nmap -T1 10.3.0.$d ; done
    
  • You can use the -n option with make to preview what it will do first before actually doing it:

    make -n install
    
  • Who can see your 8.8.8.8 requests?:

    traceroute 8.8.8.8|awk -F[\(\)] '$2~/[0-9]/{print $2}'|while read i;do echo $i;geoiplookup $i;done
    
  • The sed command p prints. For example, print lines 3 through 7 of a file:

    sed -n '3,7p' somefile
    
  • Filter out lines containing 'spam':

    sed '/spam/d' somefile
    
  • Show some full file paths referenced in the process table to try to find out where systems are storing data:

    ps auxww | grep -o "/[^\ ]*"
    
  • Did they really only change what they say they did?:

    diff <(docx2txt < agreement.docx) <(docx2txt < newagreement.docx)
    
  • Handy Bash feature: Process Substitution:

    $ some-command <(another-command)
    $ diff <(curl http://somesite/file1) <(curl http://somesite/file2)
    https://medium.com/@joewalnes/handy-bash-feature-process-substitution-8eb6dce68133#.pfgkrudmb
    
  • Use the -w option in diff to ignore differences in whitespace (tabs instead of spaces, etc.)

    diff -w index.html bookexample/index.html
    
  • mping nic.uz:

    mping(){ ping $@|awk -F[=\ ] '/time=/{t=$(NF-1);f=2000-14*log(t^18);c="play -q -n synth 1 pl "f"&";print $0;system(c)}';}
    
  • Prefix each line with the local time based on epoch time in 1st column:

    |gawk '{printf("%s %s\n",strftime("%Y-%m-%d_%T", $1),$0)}'
    
  • Optimize all png files for faster loading site:

    find . -type f -iname "*.png" -print0|xargs -I {} -0 optipng "{}"
    
  • Backslashing the * glob instead of quoting the expression:

    find ./music -name \*.mp3 -exec cp {} ./new \;
    
  • Find files under /var/log that are readable by the current user. Takes groups and ACLs into account:

    find /var/log -readable -ls
    
  • Prefix the epoch time in column 1 with the local time:

    tail -f udp.log|gawk '{printf("%s %s\n",strftime("%Y-%m-%d_%T", $1),$0)}'
    
  • Want to display date/time for each bash command history on a Linux/Unix?

    HISTTIMEFORMAT="%d/%m/%y %T "
    
  • Regular Expression: will match one of the first five letters of the alphabet

    [a-eA-E]
    
  • Regular Expression: The -f option in grep lets you specify a file of regular expressions, one per line.

  • Regular Expression: The awk variable NR contains the current record number.

  • curl supports numeric ranges. This is the full 14 days of unix-mas from 2012:

    curl -Ns http://www.climagic\.org/uxmas/[1-14]
    
  • Skip first 2 fields & print rest of the line on Linux/Unix:

    awk '{ $1=""; $2=""; print}' input
    
  • Automatic Security Updates in RHEL/CentOS 7:

    yum --security update
    
  • The GNU version of date lets you do things like the following:

    date -d "today + 90 days"
    date -d "+90 days"
    
  • Search man pages for a given string:

    man -k string
    
  • Output a file, displaying non-printing characters:

    cat -v
    cat -A
    
  • Print lines of file without printing ones already seen. $0 means whole line in awk. 'a' is an array:

    awk '!a[$0]++' file
    
  • Use jq to pretty print some json data with ANSI color coded syntax and use -R in less to process the color:

    jq -C '.' data.json | less -R
    
  • Replace all variables $val with $pid in getproc.pl, except on commented lines. FIXED:

    sed -i -e '/^\s*#/!s/$val\>/$pid/g' getproc.pl
    
  • Gen 20 random users/passwords:

    for i in {1..20} ; do rig|head -1 |tr A-Z a-z;done |while read f l;do echo ${f:0:1}${l}:$(pwgen 12 1);done
    
  • Quickly create a large file on a linux system:

    fallocate -l 10G 10Gigfile
    truncate -s 10M output.file
    truncate -s 10G output.file
    https://www.cyberciti.biz/faq/howto-create-lage-files-with-dd-command/
    
  • How to reboot a systemd linux box forcefully:

    systemctl reboot --force --force
    
  • Shell scripts run in their own subshell, so a cd in a script effects only the subshell. To script a cd, use an alias instead.

  • Directly attach screen/tmux over ssh session & save a useless parent bash process:

    ssh -t box screen -r s
    ssh -t box tmux a
    https://github.com/brejoc/ssht
    
  • alias this to ssh and it will (re)open a screen session named like your current user on the remote host:

    #!/usr/bin/env bash
    
    ###
    # call `SSH_SCREEN=1234 ./ssh user@host` to connect to screen 1234
    # for example when there are multiple detached sessions with the same name
    ###
    
    #set -o xtrace #debug
    
    set -o nounset
    set -o errexit
    set -o pipefail
    
    /usr/bin/ssh -t $@ "screen -R ${SSH_SCREEN:-${USER}}"
    
  • How the Network Time Protocol will handle the leap second:

    2016-12-31 23.59.59 
    2016-12-31 23.59.60 <-- leap second 
    2017-01-01 00.00.00
    
  • Analyze your whole lastlog to see the different remote hosts for each user:

    last -da | awk '{print $1 " " $NF}' | sort | uniq -c
    
  • Show top count for col3, 2016-07 - 2017-01:

    sed -n '/^2017-01/,/^2016-06/p' logs.csv |sed '$d' |awk -F, '{print $3}' |sort|uniq -c|sort -rn
    
  • grep gziped log files from .gz files in dirs starting with 2017-01-1 & 1 wild character:

    zgrep SSH::Password_Guessing 2017-01-1?/notice.*gz
    
  • Make URLs from data before foo:

    grep -B3 "foo" multilinedata.txt |egrep "^Id:" |while read x x id; do echo "https://hostname/${id}"; done
    
  • Filename with colons giving you problems with scp or rsync? Use the ./ path prefix B4 filename:

    scp ./file:with:colons.txt user@otherhost
    
  • SSH ProxyCommand Examples. Use "ProxyCommand" ~/.ssh/config to easily access servers hidden behind port knocking and jump hosts:

    Host unreachable_host 
        ServerAliveInterval 10
        ForwardAgent yes
        ForwardX11 yes
        User username
        IdentityFile ~/.ssh/id_rsa
        #ProxyCommand ssh -i ~/.ssh/id_rsa username@gateway_host exec /usr/bin/nc %h %p
        #Since OpenSSH 5.4, the ssh client has it's own way of reproducing the Netcat behaviour:
        ProxyCommand ssh -i ~/.ssh/id_rsa -W %h:%p username@gateway_host
    
  • Verify ssl cert and get info the CLI:

    true | openssl s_client -connect www\.cyberciti\.biz:443 -showcerts
    
  • Remove wanted Debian/Ubuntu/Mint Linux kernel and free up disk space:

    #Sysadmin apt-get --purge autoremove
    
  • Remove wanted CentOS/RHEL Linux kernel and free up disk space

    yum -y remove $(rpm -qa | grep ^kernel | grep -v $(uname -r) | grep -v kernel-firmware)
    
    Next to the package-cleanup you can add to /etc/yum.conf:
    
    installonly_limit=2
    
  • Get a very fast list of the ip addresses of hosts on your local network:

    cat /proc/net/arp | awk '{print $1}'
    
  • List traffic from inside to outside:

    awk '$3~/^172\.16\./ && $5!~/^172\.16\./{printf("%s %s\n",strftime("%Y-%m-%d_%T", $1),$0)}' conn.log
    
  • Know your limits:

    getconf -a | sort -k2nr | grep MAX
    

largest open files




Got inspired and made this wallpaper today. Hope you enjoy.