## Test if a directory is empty in Bash

Posted: 29th January 2015 by Tim in Bash, Linux
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Using Bash, there are a number of ways to test if a directory is empty. One of those ways is to use ls -A to list all files, including those starting with . and see if anything is printed. This can be done like so:

if [ ! "$(ls -A <path>)" ] then echo "<path> is empty!" else echo "<path> is not empty" fi Or inline: if [ ! "$(ls -A <path> ]; then echo "empty!"; fi

Or, for a slightly less readable version:

test "$(ls -A <path>)" || echo "empty!" This works because test "" is true, and any other string value is false. If the directory is empty, ls -A will return an empty string. ## Printing the nth word on each line using awk Posted: 7th January 2015 by Tim in Awk, Bash, Linux Tags: , , , , , , , , , There are many ways in the linux terminal to print the nth word of a given file or output. One way to do this without worrying about tabs, extra spaces or word length is to use awk. With awk, this can be done on one line by using the {print$<n>} syntax.

For example, the ps command may print this:

## Using PostgreSQL in PHP

Posted: 12th July 2014 by Tim in PHP, PostgreSQL
Tags: , , , , , , , , , , , ,

PHP is able to communicate with PostgreSQL databases using some relatively simple calls. In a similar manner to other database systems, the script needs to do the following:

1. connect to the database using pg_connect
2. execute queries using pg_query and pg_free_result
3. close the database connection using pg_close

For example, consider the following script:

<?PHP // database connection $dbhost = "localhost";$dbname = "everyone"; $dbuser = "phptest";$dbpass = "testpassword"; $db = pg_connect("host=$dbhost dbname=$dbname user=$dbuser password=$dbpass") or die("Could not connect to database$dbname on host $dbhost!"); // execute the SQL query$query = "SELECT lanname, lanpltrusted FROM pg_language;"; $result = pg_query($query)     or die ("Query failed: " . pg_last_error());

// print the results echo "<table style=\"width: 500px; border: 1px black solid;\">\n"; echo "\t<tr>\n"; echo "\t\t<th>Language</th>\n"; echo "\t\t<th>Trusted</th>\n"; echo "\t</tr>\n";

while ($row = pg_fetch_array($result, NULL, PGSQL_ASSOC)) {     echo "\t<tr>\n";     echo "\t\t<td>${row['lanname']}</td>\n"; echo "\t\t<td>${row['lanpltrusted']}</td>\n";     echo "\t</tr>\n"; }

echo "</table>\n";

// clean up pg_free_result($result); pg_close($db); ?>

This script fetches all languages supported by this PostgreSQL installation, and notes whether it is ‘trusted’ (ie: whether non-superusers can create scripts using that language). The script above will generate the following HTML table:

Language Trusted
internal f
c f
sql t
plpgsql t

Note that this code does not only work for web environments; it can be used for standalone PHP scripts too.

## LaTeX style (.sty) files

Posted: 27th June 2014 by Tim in LaTeX
Tags: , , , , , ,

When writing LaTeX documents, you may find yourself copying and pasting some common settings such as margins, fonts and paragraph indentation. This is not only tedious, it can be a real headache if you’re writing multiple documents that you want to look the same. To solve this problem, you can use a style (.sty) file.

A style file uses the same syntax as a LaTeX file, but uses the .sty suffix. To use this file in your LaTeX document, load it using the \usepackage{<filename_without_sty_suffix>} syntax.

Consider the following two files:

###### timstyle.sty

% timstyle.sty % This file contains common document settings

% Page margins (2cm wider, 2cm longer) \addtolength{\textwidth}{2cm} \addtolength{\hoffset}{-1cm} \addtolength{\textheight}{2cm} \addtolength{\voffset}{-1cm}

% Font (Times New Roman) \usepackage{times}

% No paragraph indentation \setlength{\parindent}{0in}

###### example.tex

\documentclass[11pt, a4paper]{article} \usepackage{timstyle} % note: no .sty suffix here \begin{document} Hello World! This is the first paragraph in the document. The paragraph is not very long, but it spans multiple lines. As you can see, the first line of the paragraph is not indented.\\

This is the second paragraph. Look --- still no indentation! \end{document}

The code above will produce this document. As you can see, the margin, font and paragraph indentation settings are in timstyle.sty, which simplifies the example.tex file. The style file can now be reused in other files too.

## Command Line Arguments in Bash

Posted: 14th June 2014 by Tim in Bash, Linux
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In Bash, arguments passed in on the command line are stored in numbered variables. For example, the first argument is $1, the second argument is $2, and so on. The total number of arguments passed to the program is stored in $# $0 contains the path to the program. This path may be an absolute path or a relative path, depending on how you called the script. $@ and $* will return all of the arguments passed to the program.

For example:

#/bin/bash echo "Execution command: '$0$@' ($# args)" echo "First 3 arguments:" if [$# -ge 1 ] then     echo "  \$1 =$1" fi if [ $# -ge 2 ] then echo " \$2 = $2" fi if [$# -ge 3 ] then     echo "  \$3 =$3" fi

This script will print this if called using a relative path:
Execution command: './command_line_args.sh one two three' (3 args) First 3 arguments:   $1 = one$2 = two   $3 = three Or, if called using an absolute path: Execution command: '/tmp/command_line_args.sh ichi ni san' (3 args) First 3 arguments:$1 = ichi   $2 = ni$3 = san

## Find the current working directory of a process in Linux

Posted: 28th May 2014 by Tim in Linux
Tags: , , , , , , , , ,

Using tools such as ps or top, you are able to see the processes running on a machine. However, you can’t see the directory from which the process was started. Knowing the working directory can be useful if, for example, you need to move a script or program to stop a fork bomb, if you want to see where a script or program lives or, if a script or program reads files using a relative path, to see which files are being read.

This working directory can be found using the pwdx <pid> [<pid> ...] utility. For example, consider the following output from ps:

UID        PID  PPID  C STIME TTY          TIME CMD everyone  2646     1  2 21:21 ?        00:00:00 terminal everyone  2651  2646  0 21:21 pts/0    00:00:00 bash

We can see that terminal and bash are running, but we don’t know where these processes were started. Using pwdx, we can easily find out:

$pwdx 2646 2651 2646: /home/everyone 2651: /home/everyone ## SQL gotcha – comparisons with NULL Posted: 13th May 2014 by Tim in SQL Tags: , , , , , , , Consider the following query (tested on PostgreSQL – some other systems may require a table to be specified): SELECT 'Yes' AS Value_Returned WHERE 1 != 2; This query returns 1 row: ( Value_Returned = 'Yes' ), as one would expect. But what if we compare against NULL? SELECT 'Yes' AS Value_Returned WHERE 1 != NULL; 0 rows returned, even though 1 is not NULL. This is because of the way logic works for NULLs; <anything> != NULL and <anything> = NULL always return UNKNOWN, which is not TRUE. UNKNOWN AND TRUE equals UNKNOWN, and UNKNOWN AND FALSE equals FALSE. Similarly, any NOT IN operation using a set containing NULL will never return TRUE. For example: SELECT 'Yes' as Value_Returned WHERE 2 NOT IN (1, NULL); does not return any rows. ## Using fork() in C/C++ – a minimum working example Posted: 26th April 2014 by Tim in C, C++ Tags: , , , , In a C or C++ program, fork() can be used to create a new process, known as a child process. This child is initially a copy of the the parent, but can be used to run a different branch of the program or even execute a completely different program. After forking, child and parent processes run in parallel. Any variables local to the parent process will have been copied for the child process, so updating a variable in one process will not affect the other. Consider the following example program: #include <stdio.h> #include <unistd.h> int main(int argc, char **argv) { printf("--beginning of program\n");  int counter = 0; pid_t pid = fork();  if (pid == 0) { // child process int i = 0; for (; i < 5; ++i) { printf("child process: counter=%d\n", ++counter); } } else if (pid > 0) { // parent process int j = 0; for (; j < 5; ++j) { printf("parent process: counter=%d\n", ++counter); } } else { // fork failed printf("fork() failed!\n"); return 1; }  printf("--end of program--\n");  return 0; } This program declares a counter variable, set to zero, before fork()ing. After the fork call, we have two processes running in parallel, both incrementing their own version of counter. Each process will run to completion and exit. Because the processes run in parallel, we have no way of knowing which will finish first. Running this program will print something similar to what is shown below, though results may vary from one run to the next. --beginning of program parent process: counter=1 parent process: counter=2 parent process: counter=3 child process: counter=1 parent process: counter=4 child process: counter=2 parent process: counter=5 child process: counter=3 --end of program-- child process: counter=4 child process: counter=5 --end of program-- ## C/C++ gotcha – using #if true Posted: 14th April 2014 by Tim in C, C++ Tags: , , , , , , , Consider the following code, which compiles without warnings with both gcc and g++: #include <stdio.h> int main(int argc, char **argv) { #if true printf("This does what you expect\n"); #else printf("This does not do what you expect!\n"); #endif  return 0; } When compiling with g++, the program prints This does what you expect. However, when compiling with gcc, This does not do what you expect! The problem here is with the #if true statement. In C++, true is a keyword which (unsurprisingly) evaluates to true. However, in C there is no such keyword, so true is just an undefined macro. #if <undefined_macro> will always evaluate to false, hence why the #else block is evaluated instead. If you’re writing code which is used in both C and C++, use #if 0 or #if 1 instead as this is guaranteed to behave in the same way in both languages. ## Adding lyrics to sheet music with Lilypond Posted: 26th March 2014 by Tim in LilyPond Lilypond is a useful tool for typesetting music. previously, I explained the basics of how to create sheet music for Mary Had A Little Lamb. This post will explain how to add the lyrics. This post follows on from the previous post, so read that first if you haven’t done so already. Adding lyrics to your music only requires two extra steps: ###### 1) Write down the lyrics Lyrics need to be written inside a \lyricmode block. A couple of things to note here: • By default, each word is associated with one note. To skip a note, use "". • If a word spans multiple notes, split the word on the note boundaries and add -- between them. See the example below. The lyrics for Mary Had A Little Lamb would look like this: words = \lyricmode { Ma -- ry had a lit -- tle lamb lit -- tle lamb lit -- tle lamb Ma -- ry had a lit -- tle lamb whose fleece was white as snow } In the above, words is the name given to this block of lyrics, which will be used later. ###### 2) Add the lyrics to the score This can be done with \addlyrics \words where \words is the name given above. The full document would look like this: song = \relative c' { \clef treble \key c \major \time 4/4  e4 d c d e e e2 d4 d d2 e4 e e2 e4 d c d e e e c d d e d c2 r2 } words = \lyricmode Ma -- ry had a lit -- tle lamb lit -- tle lamb lit -- tle lamb Ma -- ry had a lit -- tle lamb whose fleece was white as snow } \score { << \new Staff \song \addlyrics \words >> } which will generate this: ## LaTeX table of contents with clickable links Posted: 11th March 2014 by Tim in LaTeX Tags: , , , , , , , , , , When working with large documents with tens (or hundreds) of pages, it’s useful to be able to scroll directly to the section you’re interested in by clicking the section in the table of contents. In LaTeX, this functionality can be added quickly and easily in just a few lines using the hyperref package (and the color package if you want the links to be colored). This post extends on this post how to add a table of contents to a LaTeX document. If you don’t know how to do that, read that post first. To make the links clickable, we need to add the packages and configuration to the preamble – the part before \begin{document}. A typical configuration may look something like this: \usepackage{color} \usepackage{hyperref} \hypersetup{ colorlinks=true, % make the links colored linkcolor=blue, % color TOC links in blue urlcolor=red, % color URLs in red linktoc=all % 'all' will create links for everything in the TOC } The configuration is fairly self-descriptive. With this, we will have a table of contents with links, as well as clickable website URLs (always useful). The full working example will produce this document: \documentclass[12pt, a4paper]{article} \usepackage{color} \usepackage{hyperref} \hypersetup{ colorlinks=true, linkcolor=blue, urlcolor=red, linktoc=all } \begin{document} \tableofcontents \newpage \section{First Section} \subsection{First part of the first section} Source code for this can be found at \url{http://timmurphy.org/2014/03/11/latex-table-of-contents-with-clickable-links} \subsection{Second part of the first section} \ldots \section{Second Section} \subsection{First part of the second section} \ldots \end{document} ## Using environment variables in C++ Posted: 26th February 2014 by Tim in C++ Tags: , , , , , , Sometimes you need to use environment variables from within your program. There are a few ways to get the environment into your program, but the most portable way is to use the getenv function. The function will return a pointer to the null-terminated string value, or NULL if the variable is not set. For example, the following program will print the value of $HOME if it is set. It will return 0 if the variable is set, or 1 if it’s not.

#include <cstdlib> #include <iostream>

int main() {     char *homePath(getenv("HOME"));     if (homePath == NULL)     {         std::cout << "$HOME is not set!" << std::endl; } else { std::cout << "$HOME is set to '" << homePath << "'" << std::endl;     }

    return homePath == NULL; }

## Replacing newline characters in linux

Posted: 12th February 2014 by Tim in Awk, Linux
Tags: , , , , , , ,

There are many linux tools available to do search and replace, with sed being one of the most commonly used. However, tools like sed work line-by-line. If you need to replace/remove newline characters then things get complicated. It can be done with sed, but it’s not pretty.

The nicest solution I’ve seen is using awk. awk uses a Record Separator (RS) setting to determine how to split each record, and an Output Record Separator (ORS) setting to determine how to split the records as they are output. By default, RS and ORS are both set to '\n' (newline), meaning it reads in text line-by-line and outputs them in the same form. By changing ORS to something else, we can get all of the data printed on one line.

The examples below will use a file named random_data.txt which contains the following data:

18838ef123e f33a244eb1e 4492b3091o9 9o7ef44b22e 77a1194g229

To replace the newline characters with a space, we can use the following:

awk '{ print $0; }' RS='\n' ORS=' ' < random_data.txt 18838ef123e f33a244eb1e 4492b3091o9 9o7ef44b22e 77a1194g229 ORS does not have to be one character: awk '{ print$0; }' RS='\n' ORS=' :: ' < random_data.txt 18838ef123e :: f33a244eb1e :: 4492b3091o9 :: 9o7ef44b22e :: 77a1194g229 ::

The above commands are overly verbose, making it more obvious as to what's going on. However, both the RS value and the print \$0 are default settings. RS can be omitted completely, and the print code can be replaced with the number 1. This 1 is a true condition, indicating to awk to use the default behaviour.

So to repeat the example of replacing newlines with a space, we can shorten the command to:
awk 1 ORS=' ' < random_data.txt 18838ef123e f33a244eb1e 4492b3091o9 9o7ef44b22e 77a1194g229

The shorter command is a bit more abstract but does the same job while cutting the command line length in half.

## Displaying code in LaTeX documents

Posted: 27th January 2014 by Tim in C++, Java, LaTeX
Tags: , , , , , , ,

There are a few ways to do this, but one of the simplest ways to pretty-print code in LaTeX documents is to use the listings package. The package can be configured to use specific colors for different parts of the code, with many programming languages supported.

The following document will display code for both C++ and Java, with settings provided for the most common configuration:

\documentclass{article} \usepackage{listings} \usepackage{xcolor} % for setting colors

% set the default code style \lstset{     frame=tb, % draw a frame at the top and bottom of the code block     tabsize=4, % tab space width     showstringspaces=false, % don't mark spaces in strings     numbers=left, % display line numbers on the left     commentstyle=\color{green}, % comment color     keywordstyle=\color{blue}, % keyword color     stringstyle=\color{red} % string color }

\begin{document}

\begin{lstlisting}[language=C++, caption={C++ code using listings}] #include <iostream> int main() {     // print hello to the console     std::cout << "Hello, world!" << std::endl;     return 0; } \end{lstlisting}

\begin{lstlisting}[language=Java, caption={Java code using listings}] public class Hello {     public static void main(String[] args)     {         // print hello to the console         System.out.println("Hello, world!");     } } \end{lstlisting}

\end{document}

This will produce the following document:

The package is much more flexible than the example above shows; see the full documentation for more details.