Error handling in Bash

To check if a command was successfully run in our bash script, we have to check the value of its output, which is stored in the $? variable. If its value equals to 0, then the command was successful, otherwise, an error was thrown:

rm ~/data.tar.bz2
if [ "$?" != 0 ]; then
   echo "ERROR: Could not delete file"
   exit 1
fi

$ _

The kill that revives

Let's say your working on a remote console and send a process to sleep (CTRL-Z), all of the sudden your connection freezes and you loose the ability to interact with the slept process and bring it back to life. Don't connect again to kill it and restart it. Connect again to revive it with kill

$ kill -SIGCONT pid
$_

:)

disown ...

For the bash lovers:

It happens to all of us ...
You launch a command that is taking too long, you can't logout 'cause you'll lose your session and all the running hours...

what to do??

$ ./myprocess.bash
^Z
[1]+  Stopped ./myprocess.bash
$ bg
$ disown %1
$ logout
$ _

Now INIT will take care of your son, and you are free to leave ...
(see "man disown", a bash built-in).

Adding en_US.utf8 locale to an Ubuntu

sudo localedef –no-archive -i en_US -c -f UTF-8 en_US.UTF-8


locale -a

C
en_US.utf8
POSIX

For the next time you log into the server the following two lines have to vi added to /etc/environment

LANGUAGE=”en_US.utf8″
LANG=”en_US.utf8″

double/single quotes in bash

Single Quotes (' '): Preserve the literal value for each character within the quotes. For example,

$ string="Hello World"
$ echo $string
$ echo '$string "string"'
$ _

Double Quotes (" "): Preserve the literal value of all characters within the quotes. Rules Exception are: ‘$’, ‘`’ and‘\’ (shell expansion). For example,

string="Hello World"
echo $string
echo "My first script: $string, \"string\", "string""
$ _

Back Quotes or Back Sticks(``):Command Substitution (idem $() )
For example,

echo `date`, $(date)
$ _

Adding Google projection to your PostGIS

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 900913, 'spatialreference.org', 900913, '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs', 'PROJCS["unnamed",GEOGCS["unnamed ellipse",DATUM["unknown",SPHEROID["unnamed",6378137,0]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Mercator_2SP"],PARAMETER["standard_parallel_1",0],PARAMETER["central_meridian",0],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1],EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"]]');

Files being accessed by a process

$ ls -l /proc/pid_number/fd_

Try with the pid of your firefox process and you'll see all the sqlite files where firefox stores places, cookies, downloads, permissions, etc.. All the jar files, certificates, extensions and more.

Try:

$ echo ".dump" | sqlite3 /path_to_firefox_profile/formhistory.sqlite

:P

Request Layer with another SLD from OpenLayers

Reports is the layer name in the map and SLD is the sld name

layer=Map.getLayersByName('Reports')[0];
layer.params['STYLES']='SLD';
later.redraw();

Kill'em all >:|

$ kill -9 -1
$ _

Kill all processes owned by the user except the shell from where it's being launched, it's a nice cleanup shortcut.

Code Highlighting

Now you can post your snippets codes with a nice highlight.
Example:

   from datetime import datetime
   print datetime.now()
 

To use it, just write your code inside of <pre> or <textarea>
with tags name="code" and class="language" where language could be:

• Javascript (js)
  
• Bash (bash)
  
• Python (python)
  
• Sql (sql)
  
• C++ (cpp)
  
• Java (java)
  
• Php (php)
  

If you are looking for a console style, use class="console", and <blink>_</blink>
for your cursor :)

How many fields has my CSV file?

$ head -1 my.csv|tr -cd , | wc -c
$ _

(add 1 to the result)

dr.chamberlain:

$ head -n1 my.csv|awk -F, '{ print NF }'
$ _

Expanding strings in shell

$ a="my string"
$ _

Simple quote doesn't expand the variable:

$ echo 'This is $a'
This is $a
$ _

Double quote does expand the variable:

$ echo "This is  $a"
This is my string
$ _

Paico

$ printf 8;printf "=%.0s" {1..15};echo D _

list processes with open sockets

lsof -i -r30 -n +c0

-r30: list every 30 secs
-n: do not resolve names
-i: sockets
+c0: do NOT truncate chars

The only time bash has been unable to help me.

$ [ Where is my brain?
bash: [: missing `]'

Days between 2 unix epoch

#!/bin/bash
# Calculate "days diff" (unix epoch)
function daysdiff {
    begin=${1:-0}
    end=${2:-0}
    let Sec=$end-$begin

    if ((Sec < 0)); then
      echo $((Sec/-86400))
    else
      echo $((Sec/86400));
    fi
    return 0
}

Mount a directory through ssh

# Mount as read only directory from
# remote host (username@host:/dirnamehost) to
# local host (dirnamelocalhost)

sshfs -o ro username@host:/dirnamehost dirnamelocalhost


# remember that the user must belong to the fuse group
# before doing the sshfs
usermod -a -G fuse username

Working with date in shell

If you don't know what time is the unix time 1245257034

<br /><br />$ date -d @1245257034<br />Wed Jun 17 12:43:54 EDT 2009<br /><br />

Or in UTC

$ date -d @1245257034 -u
Wed Jun 17 16:43:54 UTC 2009

To know what is the current unix time in your machine

$ date +%s
1245257175

Getting last part of a string in shell script

# If we have the string

string="my string.hello.world"

# to get the last part considering the field separator = "." execute:

echo ${string##*.}