Another list that will grow as I need it…
Remove blank lines from output
sed '/^$/d'
Replace data with new data
sed 's/DATA/NEWDATA/g'
Replace multiple spaces with single space
sed 's/ */ /g'
Replace first instance of string
sed -e 's/pattern/REPLACEMENT/1'
This will be a growing list as I find commands I need…
Find files of .ext that are 0 bytes and rm them.
Written by 
find . -name "*.ext" -size 0 -exec rm {} \;
Check the variable is a whole number
if [ $VARIABLE -eq $VARIABLE 2> /dev/null ]; then echo $VARIABLE is a number else echo $VARIABLE is not a number fi
Check the variable is not empty
if [ $VARIABLE != "" ] ; then <code_to_execute> else echo "Error: VARIABLE is empty" fi
or as provided by solstice:
if [ -z "$VARIABLE" ] ; then echo “VARIABLE is empty” else echo “VARIABLE is not empty” fi
or -n to test if it”s not null
if [ -n "$VARIABLE" ] ; then echo “VARIABLE is not empty” else echo “VARIABLE is empty” fi
Check the variable is alpha only
if [[ "$VARIABLE" =~ ^[a-zA-Z]+$ ]] ; then echo "Variable contains letters only" else echo "Error" fi
Recently, I was helping another Admin and I wanted to be able to share our screens but our IS department won’t allow for it. Being that we were working in a terminal session I decided to go this route instead.
Needed:
- Screen
- Local account on host computer/server for remote user
Install screen
sudo apt-get install screen
Set the screen binary (/usr/bin/screen) setuid root. By default, screen is installed with the setuid bit turned off, as this is a potential security hole.
sudo chmod +s /usr/bin/screen sudo chmod 755 /var/run/screen
The host starts screen in a local xterm, using the command screen -S SessionName. The -S switch gives the session a name, which makes multiple screen sessions easier to manage.
screen -S screen-test
The remote user (remote_user) uses SSH to connect to the host computer (host_user).
ssh remote_user@server
The host (host_user) then has to allow multiuser access in the screen session via the command ^A :multiuser on (all ‘screen’ commands start with the screen escape sequence, ^A).
^A :multiuser on
The host (host_user) must grant permission to the remote user (remote_user) to access the screen session using the command ^A :acladd user_name where user_name is the remote user’s login ID (remote_user).
^A :acladd remote_user
The remote user can now connect to the hosts ‘screen’ session. The syntax to connect to another user’s screen session is screen -x host_user/sessionname.
screen -x host_user/screen-test
Install all dependent packages for CPAN
sudo apt-get install build-essential
Invoke the cpan command as a normal user
cpan
Once you hit on enter for “cpan” to execute, you be asked of some few questions. To make it simple for yourself, answer “no” for the first question so that the latter ones will be done for you automatically.
Enter the commands below
make install install Bundle::CPAN
Now all is set and you can install any perl module you want. examples of what installed below:
cpan prompt> install IO::File cpan prompt> install Net::SMTP_auth cpan prompt> install Email::MIME::Attachment::Stripper cpan prompt> install Mail::POP3Client
In order to remove tabs from your output (say to get a specific variable) use the ‘tr’ command, for example
In my zone.file the line containing “Serial” looks like the following:
If I performed
grep Serial zone.file
I would get the following output
2009110904 ; Serial (yyyymmdd##)
You can’t see them here but there are several tabs before that date. In my example I want to get just the actual serial number (2009110904). To do this I’ll first strip off the “; Serial…” information;
grep Serial zone.file | awk -F; {'print $1'}
This command is telling it to look for the ; special character and break up the output there, then from there it is told to print the first column of information this results in:
2009110904
The problem here is that if we want this to be our variable we are actually working with “ 2009110904″ instead of the desired “2009110904″ In this particular case we know those are tabs not spaces. So we’ll need to use tr in order to remove those tabs:
grep Serial zone.file | awk -F\; {'print $1'} | tr -d 't'
This will return to us exactly what we want
2009110904
So you’ve either recently come to the Unix world or you are finally getting around to some shell scripting to ease your daily operations (Say hi Jim!). So for starters what *IS* bash scripting? Scripting in general is making a program or “script” do all or most of your work for you. For instance if you wanted to find every instance of the word “hosting” in a particular file that is 30,000 lines long. That would take a LONG time doing that by hand and you would likely miss several along the way (being a fallible human and all). So instead we would use a grep (or similiar command) from the command line.
Well that may be easy enough for just one command but what if we then need to check all of those lines that are found and seperate them by date or some other criteria, then sort them out etc? For this we will use scripting. We can also use scripting for events that we want to take place every single day, whether it be simple or complex. This can be done via a cronjob, but we can get to that in a later article. So first off we’ll need to find where our “bash” resides…
which bash > helloWorld.sh
“which bash” will give us the location of the bash that is being used within our path. The > will send that result into a new file titled “helloWorld.sh” that we will use for our script. Now if we open up that file we will have something like the following:
/usr/bin/bash
Nothing else should be in our file at this point. This is good but not quite what we need. We need to add #! in front of this. This tells our script that we will be using /usr/bin/bash as our shell to run the rest of this script.
#!/usr/bin/bash
To complete our very first script all we’ll need to do is enter in the following a few lines (doesn’t matter how many) after our #!/usr/bin/bash line, like so:
#!/usr/bin/bash echo "Hello World!"
Now we have our script ready, we just need to execute it. There are two ways to accomplish this, first we can simply chmod the file to give it the execute (x) permission. For simplicity sake we’ll say its:
chmod 755 helloWorld.sh
The 7 sets it so that the owner (you) can do anything with the file, the first 5 sets it so that anyone in its group can read and execute it and likewise for the last 5 but for this is for everyone, not just the group. More on chmod and chown in future articles.
Now that we have our file chmod’d to be executable we can run it right from where we are by typing
./helloWorld.sh
and our output should be simply:
Hello World!
As mentioned there are two ways to execute the file. For the second method we could type the following and not have to worry about permissions (only ownership – but since we own it we really don’t have to worry about that either).
/usr/bin/bash ./helloWorld.sh
And, again, our output would be the same:
Hello World!
Congratulations, you’ve just made your first Bourne Again Shell (Bash) Script.
Recently I had need of cutting up domain names such as domain.com, this is easy enough for as simple a domain as this. However, the problem arises when you need to cut up something more like “education.k12.ia.us”. In order to get the extension alone in either case you can do the following:
echo domain.com | awk -F. {'print $2'}
com
Now normally if you have a domain as simple as example.com with only one period then you can can just do the opposite of this and print $1:
echo domain.com | awk -F. {'print $1'}
domain
The problem with more complex URLs is that awk’s field delimiter is now looking at multiple periods to look at, could be just one or it could be two, three, four or more.
To get the final extension in this case then we’ll use the following:
echo education.k12.ia.us | awk -F. {'print $NF'}
us
Then for the rest of it we’ll have to reverse it, then cut off the first field then reverse it again:
echo education.k12.ia.us | rev | cut -d. -f2- | rev education.k12.ia
Check if an array contains a given string:
teams=(chiefs broncos chargers raiders)
if [[ ${teams[*]} =~ broncos ]]
then
echo "VICTORY!"
fi
Or, if you don’t want it to contain the string then:
teams=(cardinals 49ers seahawks rams)
if [[ ${teams[*]} =~ broncos ]]
then
echo "This conference sucks"
else
echo "VICTORY!"
fi
For Poderosa/Putty
- Create or edit the ~/.inputrc file with the following code:
"\e[3~": delete-char
For SecureCRT
Click Tools Keymap Editor Click the "DEL" button Click Map Selected Key enter in "\e[3~" (no quotes) Hit OK Save
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