``let`` does exactly what ``(( ))`` do, it is for arithmetic expressions. There is [almost no difference](http://wiki.bash-hackers.org/commands/builtin/let#examples) between ``let`` and ``(( ))``.

Your examples are invalid. ``var=${var}bar`` is going to add word ``bar`` to the ``var`` variable (which is a string operation), ``let var+=bar`` is not going to work, because it is not an arithmetic expression:
    
    $ var=&#39;5&#39;; let var+=bar; echo &quot;$var&quot;
    5
Actually, it IS an arithmetic expression, if only variable ``bar`` was set, otherwise ``bar`` is treated as zero.

    $ var=&#39;5&#39;; bar=2; let var+=bar; echo &quot;$var&quot;
    7


I&#39;m trying to convert some Python code into Ruby. Is there an equivalent in Ruby to the `try` statement in Python?

Ruby equivalent for Python&#39;s &quot;try&quot;?

I am googling the question for past hour, but there are only points to Taylor Series or some sample code that is either too slow or does not compile at all. Well, most answer I&#39;ve found over Google is &quot;Google it, it&#39;s already asked&quot;, but sadly *it&#39;s not*...

I am profiling my game on low-end Pentium 4 and found out that ~85% of execution time is wasted on calculating sinus, cosinus and square root (from standard C++ library in Visual Studio), and this seems to be heavily CPU dependent (on my I7 the same functions got only 5% of execution time, and the game is *waaaaaaaaaay* faster). I cannot optimize this three functions out, nor calculate both sine and cosine in one pass (there interdependent), but I don&#39;t need too accurate results for my simulation, so I can live with faster approximation.

So, the question: What are the fastest way to calculate sine, cosine and square root for float in C++?

**EDIT**
Lookup table are more painful as resulting Cache Miss is way more costly on modern CPU than Taylor Series. The CPUs are just so fast these days, and cache is not.

I made a mistake, I though that I need to calculate several factorials for Taylor Series, and I see now they can be implemented as constants.

So the updated question: is there any speedy optimization for square root as well?

**EDIT2**

I am using square root to calculate distance, not normalization - can&#39;t use fast inverse square root algorithm (as pointed in comment: http://en.wikipedia.org/wiki/Fast_inverse_square_root

**EDIT3**

I also cannot operate on squared distances, I need exact distance for calculations

Fastest implementation of sine, cosine and square root in C++ (doesn&#39;t need to be much accurate)

What is the difference between assigning to a variable like `var=foo` and using let like `let var=foo`? Or cases like `var=${var}bar` and `let var+=bar`? What are the advantages and disadvantages of each approach?

Bash: let statement vs assignment

<p>What is the difference between assigning to a variable like <code>var=foo</code> and using let like <code>let var=foo</code>? Or cases like <code>var=${var}bar</code> and <code>let var+=bar</code>? What are the advantages and disadvantages of each approach?</p>


I am getting the &quot;No space left on device&quot; error for pretty much anything i try to do. Even using tab to autocomplete a command!

but when i do df -h i get:

    ubuntu@ip-10-0-2-108:~$ df -h
    Filesystem      Size  Used Avail Use% Mounted on
    /dev/xvda1       99G  6.5G   88G   7% /
    udev            3.7G  8.0K  3.7G   1% /dev
    tmpfs           1.5G  184K  1.5G   1% /run
    none            5.0M     0  5.0M   0% /run/lock
    none            3.7G     0  3.7G   0% /run/shm
    none            100M     0  100M   0% /run/user
    /dev/xvdb       414G  199M  393G   1% /mnt
    overflow        1.0M  1.0M     0 100% /tmp

which to me looks like there is tons of space. df -i also looks similar:

    ubuntu@ip-10-0-2-108:~$ df -i
    Filesystem       Inodes IUsed    IFree IUse% Mounted on
    /dev/xvda1      6553600 94227  6459373    2% /
    udev             951353   393   950960    1% /dev
    tmpfs            953649   274   953375    1% /run
    none             953649     3   953646    1% /run/lock
    none             953649     1   953648    1% /run/shm
    none             953649     1   953648    1% /run/user
    /dev/xvdb      27525120    11 27525109    1% /mnt
    overflow         953649    12   953637    1% /tmp

I am on an Amazon EC2 ubuntu 12.04 instance.

Here are some examples of the error popping up:

    ubuntu@ip-10-0-2-108:~$ sudo crontab -e
    /tmp/crontab.RvYjrR/crontab: No space left on device

    ubuntu@ip-10-0-2-108:~$ ls /va (hit tab for autocomplete)
    
    -bash: cannot create temp file for here-document: No space left on device
    -bash: cannot create temp file for here-document: No space left on device

however the server seems to be running and everything seems to be working.
what on earth is going on??




ubuntu &quot;No space left on device&quot; but there is tons of space

I do not know the service&#39;s name, but would like to stop the service by checking its status. 

For example, if I want to check if the [PostgreSQL][1] service is running or not, but I don&#39;t know the service&#39;s name, then how could I check its status?

I know the command to check the status if the service name is known.

  [1]: http://en.wikipedia.org/wiki/PostgreSQL


How to check if a service that I don&#39;t know the name of is running on Ubuntu

I&#39;m in Linux, Elementary OS, and installed lampp in opt.

My CSS and JS won&#39;t load. When I inspect my page through browser. The console says
Failed to load resource: the server responded with a status of 403 (Forbidden)
I&#39;m really sure that my directories are correct.![This is my workspace][1]

This is the error
![enter image description here][2]


![enter image description here][3]


  [1]: http://i.stack.imgur.com/wD33a.png
  [2]: http://i.stack.imgur.com/rADrb.png
  [3]: http://i.stack.imgur.com/oICLn.png

ERROR 403 in loading resources like CSS and JS in my index.php

I want to tar and all .php and .html files in a directory and its subdirectories. If I use 

`tar -cf my_archive *`
 
it tars all the files, which I don&#39;t want. If I use 

`tar -cf my_archive *.php *.html`

it ignores subdirectories. How can I make it tar recursively but include only two types of files?

How to tar certain file types in all subdirectories?

I know that Python provides an API so you can call Python interpreter in C code, but what I want is the opposite.

My program needs to use some C API, so the code must be written in C. But I also want to package the program with Python. Which means I want to invoke those C function or executables in Python. Is that possible?

If I want the C code to be a library, which means I use it with `#include` and linkage of the `*.o` likely in Python, how to do it? Is that possible? If I write the C code into executable, which means it becomes a command, can I invoke it in Python directly?

Also, I heard that Python code can be compiled, does that mean we can execute the code without the source file? Are the output files binary files? Does it improve performance?

Can we use C code in Python?

What did I do wrong here?

Trying to match any string that contains spaces, lowercase, uppercase, or numbers. Special characters would be nice too, but I think that requires escaping certain characters.

    TEST=&quot;THIS is a TEST title with some numbers 12345 and special char *&amp;^%$#&quot;
    
    if [[ &quot;$TEST&quot; =~ [^a-zA-Z0-9\ ] ]]; then BLAH; fi

This obviously only tests for upper, lower, numbers, and spaces. Doesn&#39;t work though.

*** UPDATE ***

I guess I should have been more specific. Here is the actual real line of code.

    if [[ &quot;$TITLE&quot; =~ [^a-zA-Z0-9\ ] ]]; then RETURN=&quot;FAIL&quot; &amp;&amp; ERROR=&quot;ERROR: Title can only contain upper and lowercase letters, numbers, and spaces!&quot;; fi

*** UPDATE ***

    ./anm.sh: line 265: syntax error in conditional expression
    ./anm.sh: line 265: syntax error near `&amp;*#]&#39;
    ./anm.sh: line 265: `  if [[ ! &quot;$TITLE&quot; =~ [a-zA-Z0-9 $%^\&amp;*#] ]]; then RETURN=&quot;FAIL&quot; &amp;&amp; ERROR=&quot;ERROR: Title can only contain upper and lowercase letters, numbers, and spaces!&quot;; return; fi&#39;

Regex matching in a Bash if statement

I have read so many suggestions about, not putting your customization aka commands in &quot;.profile&quot; file. Rather, create a .bash_profile for yourself and add your alias and etc.

But,when I open the new terminal, if there is only .bash_profile, OS X is not exporting/sourcing the commands mentioned in it. I have to manually source the .bash_profile.

If I create .profile file, on opening a new terminal, all my commands in .profile are executed  and will be available readily.

Could you please help me in understanding, how does it works? Also, when to use .bashrc/.profile/.bash_profile files.

Thanks!

How to make OS X to read .bash_profile not .profile file

For executing a shell script in current shell, we need to use a period `.` or a `source` command. But why does it not work with a `sudo` permission?

I have a script with execute permission called ``setup.sh``. When I use a period, I get this:

    $ sudo . ./setup.sh 
    sudo: .: command not found

The `source` command also produces a similar error. Am I missing out something? What should I do to run the script with `sudo` permission in the **same/current** shell?

Thanks in advance..


Execute a shell script in current shell with sudo permission

I tried the following on the command prompt in bash:

    sudo cat &lt;&lt; EOF &gt; /etc/yum.repos.d/some-name.repo
    #Content
    #....
    #...
    EOF

It complained :

    -bash: /etc/yum.repos.d/some-name.repo: Permission denied

Then I did `sudo su` and tried the exact same thing except the `sudo` before `cat`, and it worked without any problem. What am I missing in the above ?

sudo cat &lt;&lt; EOF &gt; File doesn&#39;t work, sudo su does

        
    array=${ls -d */}
    echo ${array[@]}  

I have three directories: `ww` `ee` `qq`. I want them in an array and then print the array.



How do I assign ls to an array in Linux Bash?

When I did `which groovy`, I got the below output:

    /usr/local/bin/groovy

So I went ahead and created a `helloworld.groovy` with the below content

    #!/usr/local/bin/groovy
    println &quot;hello world&quot;

After that I did `chmod +x helloworld.groovy` and attempted to run the file with `./hellworld.groovy` and sadly, I got this error `./helloworld.groovy: line 2: print: command not found`

I could get rid of the error by changing to

    #!/usr/bin/env groovy
    println &quot;hello world&quot;

Why would the first method cause the error?


Running Groovy script from the command line

Hi I have searched various forums and here as well, I could find some answers for Linux and Mac but not able to find solution for Unix and specially Korn Shell.

How to get process name (command name) from process id (pid)

Below reference I found from SO 
[This one ][1]
[And this one also][2]

I tried below command

  [1]: https://stackoverflow.com/questions/4189717/get-process-name-by-pid
  [2]: https://stackoverflow.com/questions/15545341/process-name-from-its-pid-in-linux

    ps -eaf | awk &#39;{ print substr($0, index($0, $9)) }&#39;

Above command is failing at a point where TIME is given rather than Month and Date (because in this case there will be only 8 columns in string)

Any suggestion would help.

How to get pid given the process name

In Bash, we can set an environment variable for a single command this way:

    FOO=bar somecommand

What if we want to *unset* a variable for a single command?

Unset an environment variable for a single command

I have `abc.sh`:

    exec $ROOT/Subsystem/xyz.sh

On a Unix box, if I print `echo $HOME` then I get `/HOME/COM/FILE`.

I want to replace `$ROOT` with `$HOME` using sed.

Expected Output: 

    exec /HOME/COM/FILE/Subsystem/xyz.sh

I tried, but I&#39;m not getting the expected output:

    sed  &#39;s/$ROOT/&quot;${HOME}&quot;/g&#39; abc.sh &gt; abc.sh.1

----

Addition: 

If I have `abc.sh`

    exec $ROOT/Subsystem/xyz.sh $ROOT/ystem/xyz1.sh

then with 

    sed &quot;s|\$INSTALLROOT/|${INSTALLROOT}|&quot; abc.sh

it is only replacing first `$ROOT`, i.e., output is coming as 

    exec /HOME/COM/FILE/Subsystem/xyz.sh $ROOT/ystem/xyz1.sh

How to use variables in a command in sed?

I am trying to get a specific line from a text file.

So far, online I have only seen stuff like sed, (I can only use the sh -not bash or sed or anything like that). I need to do this only using a basic shell script.

    cat file | while read line
    	do
    	   #do something
    	done

I know how to iterate through lines, as shown above, but what if I just need to get the contents of a particular line


Content Type	Original Author	Original Content on Stackoverflow
Question	IDDQD	View Question on Stackoverflow
Solution 1 - Linux	Aleks-Daniel Jakimenko-A.	View Answer on Stackoverflow

Bash: let statement vs assignment

Linux Problem Overview

Linux Solutions

Solution 1 - Linux

Fastest implementation of sine, cosine and square root in C++ (doesn't need to be much accurate)

Ruby equivalent for Python's "try"?

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