Archive for the ‘Software Development’ Category

Imagine you have two text files, one with a list of names and another with a list of birth dates, which each name corresponding to the date of birth on the same line number in the other file, like so: names.txt Anthony Kiedis Flea Chad Smith John Frusciante dob.txt 1-Nov-1962 16-Oct-1962 25-Oct-1961 5-Mar-1970 You would […]

Bash gotcha: function variable scope

Posted: 11th October 2015 by Tim in Bash, Linux
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Consider the following bash script: #!/usr/bin/env bash function myFunc {     myvar=123     echo “myFunc: setting myvar=$myvar” } myvar=1 echo “before myFunc: myvar=$myvar” myFunc echo “after myFunc: myvar=$myvar” The code here is fairly simple – we set a variable myvar, call a function and print the value of myvar to the terminal. However, even though we don’t […]

How to get a Makefile directory path

Posted: 27th September 2015 by Tim in Linux, Make
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Sometimes in a Makefile you need to reference something from the directory where the Makefile lives. $(PWD) won’t work in this instance, since that will return the path from where make was called which may be different if, for example, you use the –directory option. In this case, we can use the following code to […]

If you’re using a linux terminal and need to convert all upper case characters to lowercase, there are a number of ways you can do it. One of the easiest is to use tr: tr ‘[:upper:]’ ‘[:lower:]’ For example: $ echo “HeLlo WOrlD” | tr ‘[:upper:]’ ‘[:lower:]’ hello world :upper: and :lower: can be reversed […]

Accessing fields by index in Awk

Posted: 11th August 2015 by Tim in Awk
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In awk, fields are accessed by number; $1 for the first field, $2 for the second, etc. But sometimes the field number you want to access is not known until run time. In these cases, you can access the field using the $(<index>) syntax. The constant NF contains the number of fields available. For example, […]

Reading a file line by line in Bash

Posted: 26th July 2015 by Tim in Bash, Linux
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There are a few ways to read a file in bash, each with their own caveats. If you’re looking to read a file line-by-line verbatim, including blank lines, then using a simple while loop should do the trick. For example, the following code will print the contents of a file with line numbers: line_no=0 while […]

Creating temporary files in Java

Posted: 9th July 2015 by Tim in Java
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Sometimes it is necessary to use temporary files in your program. One concern when creating these files is thread safety; two processes creating a temporary file with the same name. This concern can be largely ignored using Java’s File.createTempFile, which guarantees the file path is unique amongst all java processes running on that JVM. For […]

Pause in Bash

Posted: 27th June 2015 by Tim in Bash, Linux
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Sometimes it is useful to pause a script until the user is ready to proceed. In Windows Batch scripting, this can be done with the PAUSE command. In Bash, the same thing can be done using read with the right parameters: read -rn1 -p “Press any key to continue” This command is doing the following: […]

Awk is a useful language for many command line tasks. Often awk is used on the command line on its own or with strings piped to it, but it is possible to turn that awk code into an executable script. Consider the following script. This file contains awk code with a shebang of awk -f. […]

Exit command in R

Posted: 26th April 2015 by Tim in R
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R is quite different from most other programming languages. One common feature of other languages is the exit routine, or something with a similar name. In R, that function is stop, and takes an error message string as an argument. For example: stop(“I don’t work on weekends”) will print: Error: I don’t work on weekends […]

Linux Terminal Clock

Posted: 9th March 2015 by Tim in Bash, Linux
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Ever wanted to have a real-time clock on your linux terminal? We can create one with a single line of bash, like so: while echo -en “$(date)\r”; do sleep 1; done Let’s look at how this works: date is a common unix tool used to print the current date and time. The $(…) means that […]

With Docker, you can specify the command to run inside the container on the command line. But what if you want to run multiple commands? You can’t escape the && syntax, or wrap the command in quotes, as Docker won’t recognise it. The trick here is to use sh -c ‘<command1 && command2 [&& command3 […]

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> […]

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 […]

There are a number of ways to print a number with zero-padding (leading zeros), depending on the variable type you are wanting to print. One easy way to add zero padding to any type is to use the str(<number>).zfill(<length>). This will convert any number type to a string before adding the extra zeros. For example: […]

To highlight any results found when searching in Vim, you can use :set hlsearch. To turn off the highlighting, use :nohlsearch. If search highlighting is something you always want turned on, you can add set hlsearch (no colon) to your .vimrc file. If, like me, you like to turn off the search highlighting after you’ve […]

Disabling auto indent in Vim

Posted: 12th October 2014 by Tim in Vim
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Auto indent is disabled by default in Vim, but some systems have this feature enabled in the system-wide vimrc file (found in /usr/share/vim/vimrc or a similar location). There are two ways to disable this: remove the system-wide setting, or remove the setting for your user only. To remove the setting system-wide, search for and remove […]

HTTP GET request in Python

Posted: 28th September 2014 by Tim in Python
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There are a number of ways to make a GET request in Python, but the easiest (in my opinion) is via urllib2. With this library, you can make a request with only one line of code, storing the result for use later. For example: import urllib2 data = urllib2.urlopen(“”).read() print “Website size (bytes): ” + […]

HTML link coloring with CSS

Posted: 12th September 2014 by Tim in CSS, HTML
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HTML link colors can be changed easily using CSS. The properties which can be set are: link – a link to a page which has not been visited visited – a visited link hover – a link which has the mouse hovering over it active – a link which is being clicked (mouse button held […]

Passing function pointers as a parameter to another function can be tedious work. The function pointer definitions can be long and cumbersome to write, and obscure to read. Using pointers to member functions can be even more ambiguous. Fortunately, we can leverage the power of templates to make this work easier for us by making […]

If you have a JAR file and want to print the details from MANIFEST.MF, this can be done with one command in linux, using the unzip utility. For example: $ unzip -p /usr/share/java/hsqldb.jar META-INF/MANIFEST.MF Manifest-Version: 1.0 Created-By: 1.7.0_03-b147 (Oracle Corporation) Specification-Title: HSQLDB Implementation-Title: Standard runtime Class-Path: /usr/share/java/servlet-api-3.0.jar Main-Class: org.hsqldb.util.SqlTool Ant-Version: Apache Ant 1.8.2 Implementation-Vendor: buildd […]

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: connect to the database using pg_connect execute queries using pg_query and pg_free_result close the database connection using pg_close For example, consider the following script: <?PHP // database […]

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 […]

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 […]

C/C++ gotcha – using #if true

Posted: 14th April 2014 by Tim in C, C++
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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 […]