Tips and Tricks

Useful commands

Recursively set directory permissions to 755

# find /var/www/path -type d -print0 | xargs -0 chmod -c 755

Recursively rename directory and files from ISO-8859-15 to UTF-8

$ convmv -f iso-8859-15 -t utf8 -r /var/www/path --preserve-mtimes --notest

Recursively search for files of a specitic type, e.g. documents and list them

$ find . -regex '.*\(pdf\|doc\)'

User: Change a users home directory

# usermod -d /home/newhome user

Strip comments from config file

$ grep -v "#" file.conf

SCP file transfer

File transfer via commandline.

Upload file:

# scp /root/htdocsbackup.tar.bz2 root@remote-host.de:/root/

Download file:

# scp root@remote-host.de:/root/htdocsbackup.tar.bz2 /root/htdocsbackup.tar.bz2

Upload directory:

# scp -p -r /root/directory root@remote-host.de:/root

Download directory:

# scp -p -r root@remote-host.de:/root/directory /root

SSH tunnel

Very useful if the server you want to connect to has only the SSH port open and/or a firewall infront and you want to access for example a tomcat server running on port 8080.

# ssh -N -i /home/<user>/.ssh/id_dsa -L 7777:remote-host.de:8080 root@remote-host.de

After establishing the tunnel you can access the tomcat server of the remote machine port 8080 at your localhost port 7777.

Generate SSH Host Keys

# ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key -N

# ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key -N

# ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key -N

SSL: Remove passphrase from a SSL key

$ openssl rsa -in ssl_with_passphrase.key -out ssl_without_passphrase.key

SSL: Add/change passphrase to/from a SSL key

$ openssl rsa -in ssl_without_passphrase.key -out ssl_with_passphrase.key -des3

Postfix: Show mailqueue

# mailq

Postfix: Attempt to deliver all queued mail

# postqueue -f

Postfix: Delete a single mail from queue

# postsuper -d id_of_mail

Postfix: Delete all mails with a specific sender from queue

# mailq | tail -n +2 | awk 'BEGIN { RS = "" } / apache@example\.com$/ { print $1 } ' | tr -d '*!' | postsuper -d -

ImageMagick: Combine images side by side

$ convert image{1,2,3}.png +append output.png

Safely spin down external USB drive AFTER unmounting using sg3_utils

# sg_start --readonly --stop /dev/sdX

Delete content in the underlying directory of a mounted filesystem without having to unmount first

Bind mount root to a temporary mount point, e.g. /mnt/temp. Useful to fix. e.g. following systemd informational notice:

tmp.mount: Directory /tmp to mount over is not empty, mounting anyway.

# mount --bind / /mnt/temp

Browse to /mnt/temp/yourmountpoint and be able to delete the files, afterwards unmount /mnt/temp again.

eyeD3: Remove a specific ID3 tag frame

$ eyeD3 file.mp3 --remove-frame=TPE2

Ghostscript: Merge multiple PDF files

$ gs -dNOPAUSE -sDEVICE=pdfwrite -dPDFA -sOUTPUTFILE=merged.pdf -dBATCH file1.pdf file2.pdf file3.pdf

KDE4 problems and workarounds

Reset to the default Plasma layout

Log out of KDE/X and delete from tty:

$ rm ~/.kde/share/config/plasma*

Change the Dolphin standard view to detailed view for ALL folders

Open Dolphin -> Settings > Configure Dolphin -> Views -> Common Tab -> and mark -> Use the same view for all folders

Package management (Portage)

To which package belongs file X

# equery belongs -f <dateiname>

Which installed packages have USE-Flag X

# equery hasuse <useflag>

Backup an installed package

# quickpkg net-www/apache

After this you can install the package on another machine or do a fast downgrade if something went bad after an update. You can find the file under /usr/portage/packages/All and in this example a symlink to the apache file in /usr/portage/packages/net-www. To install the now precompiled binary use:

# emerge --usepkgonly "<net-www/apache-2.2.4"

Detect and get rid of leftover packages

# emerge depclean --pretend

Change your current profile

# emerge eselect

# eselect profile list

# eselect profile set <nummer>