[Archive] Useful Linux One-Liners for System Administrators

Special Note: I wrote this blog post when I was early in my career as a Linux System Administrator; on an older blog back in February 2014. While I still work with Linux, it’s at a much deeper level, helping companies scale large cloud-based systems. Use these commands at your own risk, and take the time to understand it’s impact on the system

Here is a list of one-liners that I use on a weekly basis. Some of these commands allow for client friendly results, while others serve the purpose of a tool in a toolbox; perfect for troubleshooting.

Let me know what you think about these, if you are using them and if you would like to see more. Contact me on Twitter or using the Contact Form. As an added bonus combine these with tmux and have a great set up.

Disk Usage, Human Readable, 3 Folders Deep #

(Updated 4/5/15 – Updated to account for files less than 1MB)

du -ax --max-depth=3 /tmp/ | sort -n | awk '{if($1 > 102400) print $1/1024 "MB" " " $2; else print $0 }'

Generate Password #

perl -e '@r=(a..z,A..Z,0..9);$p.=$r[int(rand(@r))],$i++while($i<8);print"$p\n"'

Calculate Process Size – Human Readable #

ps aux | grep 'process here' | awk '{print $6/1024 " MB";}'

Average Process Size – Human Readable #

ps aux | grep 'process here ' | awk '{print $6/1024;}' | awk '{avg += ($1 - avg) / NR;} END {print avg " MB";}'

Drop Cache Items to Free Up Memory #

WARNING: Always sync the disk first.

free -m && sync && echo 3 > /proc/sys/vm/drop_caches && free -m

You can signal the Linux Kernel to drop various aspects of cached items by changing the above command. For this command, I echo a 3, but you can echo 1 or 2 as well, see below. In short, this allows for a system administrator to clean up memory of unnecessary cache and will only work with kernel 2.6.16 or newer.

To free pagecache: #

echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes: #

echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes: #

echo 3 > /proc/sys/vm/drop_cache

Sort Local Users by Oldest to Newest #

sort -nk3 -t: /etc/passwd | less

Perl – Global Find/Replace within a File #

perl -p -i -e 's/findme/replaceme/g' file.conf

Human Readable Memory/Swap #

cat /proc/meminfo | grep -E "MemTotal|SwapTotal" | awk '{print $1 ($2/1024)/1024 "GB"}'

Example output:

MemTotal:125.943GB
SwapTotal:32GB

Rename Multiple Files Extensions #

for i in *.txt; do mv "$i" "`basename $i .txt`.conf"; done

See Number of Connections to a Service #

netstat -np | awk '{print $7}' | awk -F/ '{count[$2]++}END{for(j in count) print count[j],j}' | sort -nr