Autoconf: example of checking for a feature in configure.ac

AC_MSG_CHECKING([for string])

AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
                           #include <string>
                           string str = "a string";
                      ]])], 
                  [AC_DEFINE([HAS_STRING], [1], [support string])
                   AC_MSG_RESULT([yes])
                  ],
                  [AC_MSG_RESULT([no])])

 

PHP: list all files and sub-directories under the current directory

Create a file index.php:

<?php
$allItems = scandir(".");
$toHide = array('.', '..', 'index.php');
$toShow = array_diff($allItems, $toHide);

foreach ($toShow as $oneItem) {
    echo "<a href='$oneItem'>$oneItem</a><br>";
}
?>

GNU Autoconf issues

For the following errors in configure and make:

WARNING: 'aclocal-1.1x' is missing on your system.
WARNING: 'automake-1.1x' is missing on your system.
WARNING: 'autoheader' is probably too old.

We may probably install the required version of the GNU Autoconf tools and run the reconfiguration:

$ autoreconf -fi

before running ./configure again.

If the problem is only caused by the timestamps of the autoconf-generated files that make autoconf think that a re-configuration is necessary, we can fool autoconf by changing the timestamps of those files in the following order:

touch m4/*
touch build-aux/*
touch Makefile.am
touch confugure.ac
touch aclocal.m4
touch configure
touch Makefile.in
touch config.h.in

Another trick is to assign no-op to the required autoconf commands to allow the build go through without running the actual reconfiguration:

make AUTOCONF=: AUTOHEADER=: AUTOMAKE=: ACLOCAL=:

Java programming: use Matcher to replace patterns with captured groups (code example)

String str = "=123-abc=   =7890-ABCD=";
Pattern pattern = Pattern.compile("=([0-9]*)-([A-Za-z]*)=");
Matcher matcher = pattern.matcher(str);
if (matcher.find())
{
    str = matcher.replaceAll("# $2 + $1 #");
}

System.out.println(str);

The output will be:
# abc + 123 #   # ABCD + 7890 #

Python: redirect output to a file

To redirect the standard output to a file by the source code, add this in the beginning of the Python script:

import sys
sys.stdout = open('/path/to/outputfile', 'a')

Windows 10: check the starting time of the system

Method 1:

1. Right click on the Start icon and select "Run"
2. Run "cmd" to open a terminal.
3. In the terminal run command "net stats workstation"
4. In the beginning of the output, it says: "Statistics since 2019-08-20 7:22:39 PM". The highlighted part is the last reboot time.


Method 2:

1. Right click on the task bar and select "Task Manager"
2. Click on the Performance tab, then CPU
3. At the bottom of the CPU data, it shows "UP time", e.g. "3:01:01:16". It tells you the OS has been running for 3 days 1 hour 1 minute and 16 seconds.

Git error: cannot spawn .git/hooks/post-commit: No such file or directory

This error could happen when on the Windows.

error: cannot spawn .git/hooks/post-commit: No such file or directory

To fix it, simply add this as the first line of the file .git/hooks/post-commit:

#!/bin/sh

Linux: skip a directory in find command

For example, to skip a sub-directory named .git and all the contents under it, run command:

$ find . -path ./.git -prune -o -print

option -prune is used to skip the directory specified by -path.

Perl: Convert hex string to the character

my $hx = "41";
my $ch = chr(hex($hx));
print $ch;

The output will be:
A

Linux: base64 encoding command

Encode a string with base64:

$ echo -n abc | base64

Decode a base64 encoded string:

$ echo -n YWJj | base64 -d