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ln -s <target> <source>     # create symbolic link
echo $?                     # print last return value int
tar -zcf out.tar.gz folder  # compress folder to .tar.gz
tar xzf out.tar.gz folder   # extract .tar.gz to folder
mogrify -format eps *.png   # convert all *.png to *.eps
sh -x <script>              # show all commands executed by given script for debug
df -h                       # show disk usage and free space
apt-mark hold jenkins       # hold a package at the current version

If this fails, the version of tar may not support gzip compression. In this case, you can use the traditional two-stage command: gzip -dc filename.tar.gz | tar xf -

Commit directories to SVN individually

find . -maxdepth 1 -regex ./[A-Za-z0-9].+ -exec svn commit '{}' --message "Adding {} pictures folder to repository" ;

“find .” matches all files and directories in the current directory
“-maxdepth 1” only matches top-level directories
“-regex ./[A-Za-z0-9].+” removes the ./ directory
“-exec svn commit” runs “svn commit” with each result
”{}” is replaced with the result directory name
; is required to end the line

Also see http://www.pixelbeat.org/cmdline.html for a neat reference.

Joining multiple PDFs together

If they’re all the same size, you can use Ghostscript:

gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=out.pdf in1.pdf in2.pdf in3.pdf

Misc Notes

./configure: line 23: syntax error near unexpected token `$'inr''

The configure file line endings has changed from Unix style to Windows style (probably winzip being stupid). To fix:

$ vim ./configure
:set ff=unix
:wq

Adding a New Sudo User

Create a new user:

root@server:~$ sudo adduser username
[sudo] password for root:
Adding user 'username' ...
Adding new group 'username' (1234) ...
Adding new user 'username' (1234) with group 'username' ...
Creating home directory '/home/username' ...
Copying files from '/etc/skel' ...
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Changing the user information for username
Enter the new value, or press ENTER for the default
        Full Name []: Your Name
        Room Number []:
        Work Phone []: 
        Home Phone []:
        Other []: 
Is the information correct? [Y/n] y

Add it to the admin user group (or adm):

sudo usermod -a -G admin username

Or, define it as an explicit member of the admin group (or adm group):

sudo usermod -g admin username

Or, define it explicitly as someone who can sudo (necessary on new Ubuntu releases):

sudo adduser username sudo

Alternatively, you could use visudo, but be careful when editing the sudoers file!

Chaining Greps when Tailing a File

If you are tailing a file and trying to execute multiple greps, you can’t just chain them using pipes:

tail -f /my/file.log | grep -v a | grep -v b | grep -v c

You need to add the flag `–line-buffered” to each grep.

tail -f /my/file.log | grep --line-buffered -v a | grep --line-buffered -v b | ...

spamd: still running as root, safe_lock: cannot create tmp lockfile /nonexistent/.spamassassin

If you have configured your site to use Spamassassain, and your /var/log/mail.log includes messages such as:

spamd: still running as root: user not specified with -u, not found, or set to root, falling back to nobody
plugin: eval failed: bayes: (in learn) locker: safe_lock: cannot create tmp lockfile /nonexistent/.spamassassin/bayes.lock.my.domain.6304 for /nonexistent/.spamassassin/bayes.lock: No such file or directory

This is because spamassassin is not being run as a particular user, or is being run as root. According to /usr/share/doc/spamassassin/README.spamd this is technically OK, but it’s easier just to create a new user for spamd to run as. Create a new user with adduser --system, and then modify /etc/default/spamassassin (designed to be edited on Ubuntu):

OPTIONS="--create-prefs --max-children 5 --helper-home-dir -u spamd"

(Reference)

Installing an X server to startup at boot

I tried installing a basic X server (called a “client”) using sudo apt-get install xauth and sudo apt-get install xorg (as recommended by Ubuntu). However I couldn’t get the X server to start at boot. I even tried creating a file /etc/init.d/xserver with the single command exec startx, but this didn’t do anything unless I executed the script as a user (no, I don’t know enough about init.d yet).

The solution was to install a display manager such as Gnome’s. sudo apt-get install gdm followed by a sudo shutdown -r now, and now an X server was running at startup so I can do things like this:

DISPLAY=:0 sudo aticonfig --adapter=all --odgt

Compare two directories with files recursively

diff -rq dir1 dir2

Based off this article