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11 useful commands for Linux/Unix administrators

Command line is more powerful because you can do  lot with them,  you can tell your computer exactly what you want and get the appropriate answer, while GUI application can only tell your computer what the GUI programmer has defined.  Recently, CLI is mostly used by Linux/Unix administrators since most popular linux distros come with a complete set of GUI applications that make the user use less and less the command line.


1– Save man-page as pdf

 man -t awk | ps2pdf - awk.pdf

2– Duplicate installed packages from one machine to the other (RPM-based systems)

ssh [email protected] "rpm -qa" | xargs yum -y install

3– Stamp a text line on top of the pdf pages to quickly add some remark, comment, stamp text, … on top of (each of) the pages of the input pdf file

echo "This text gets stamped on the top of the pdf pages." | enscript -B -f Courier-Bold16 -o- | ps2pdf - | pdftk input.pdf stamp - output output.pdf

4– Display the number of connections to a MySQL Database

Count the number of active connections to a MySQL database.
The MySQL command “show processlist” gives a list of all the active clients.
However, by using the processlist table, in the information_schema database, we can sort and count the results within MySQL.

mysql -u root -p -BNe "select host,count(host) from processlist group by host;" information_schema

5– Create a local compressed tarball from remote host directory

ssh [email protected] "tar -zcf - /path/to/dir" > dir.tar.gz

This improves on #9892 by compressing the directory on the remote machine so that the amount of data transferred over the network is much smaller. The command uses ssh(1) to get to a remote host, uses tar(1) to archive and compress a remote directory, prints the result to STDOUT, which is written to a local file. In other words, we are archiving and compressing a remote directory to our local box.

6– tail a log over ssh

This is also handy for taking a look at resource usage of a remote box.

ssh -t remotebox "tail -f /var/log/remote.log"

7– Print diagram of user/groups

Parses /etc/group to “dot” format and pases it to “display” (imagemagick) to show a usefull diagram of users and groups (don’t show empty groups).

awk 'BEGIN{FS=":"; print "digraph{"}{split($4, a, ","); for (i in a) printf "\"%s\" [shape=box]\n\"%s\" -> \"%s\"\n", $1, a[i], $1}END{print "}"}' /etc/group|display

8– Draw kernel module dependancy graph.

Parse `lsmod’ output and pass to `dot’ drawing utility then finally pass it to an image viewer

lsmod | perl -e 'print "digraph \"lsmod\" {";<>;while(<>){@_=split/\s+/; print "\"$_[0]\" -> \"$_\"\n" for split/,/,$_[3]}print "}"' | dot -Tpng | display -

9– Create strong, but easy to remember password

Why remember? Generate!
Up to 48 chars, works on any unix-like system

read -s pass; echo $pass | md5sum | base64 | cut -c -16

10– Find all files larger than 500M and less than 1GB

find / -type f -size +500M -size -1G

11– Limit the cpu usage of a process

This will limit the average amount of CPU it consumes.

sudo cpulimit -p pid -l 50

Originally posted 2016-01-24 17:48:57.

About admin

I have been working with Linux based systems since 2002. I hope the article published on this site will be useful to those new to server administration.
  • Bavol Lee

    Very very useful tips!