How to Create and Open a File in C (Read/Write Mode) In C, the fopen function handles file creation and opening. To get read/write access with creation, you need the right combination of “mode” strings. The Code FILE *fp = fopen(“myfile.txt”, “a+”); // ‘a+’ opens for reading and appending. // It creates the file if…
How to Check for Block Special Files in Python In Linux, hardware like hard drives and SSDs are represented as “block special files” (usually in /dev). Python provides a simple way to identify them using the os module. The Pythonic Way import os import stat path = “/dev/sda” try: mode = os.stat(path).st_mode if stat.S_ISBLK(mode): print(f”{path}…
The problem is with ssh that makes a quoted string to more than one if there are spaces. For example, ssh user@host cmd “my string” The cmd on host will be executed like cmd my string rather than cmd “my string” It will only work if the string is escaped like ssh user@host cmd my…
In many text processing tasks, we often need to check if a given string starts with a specific substring. In this article, we will demonstrate how to achieve this using the std::string::compare() function from the C++ Standard Library. The compare() function has several overloads, but the one of interest for our purpose is: int compare(size_type…
How to split and iterate a string separated by a specific character in C++? For example, “a string separated by space” by ‘ ‘=> [“a”, “string”, “separated”, “by”, “space”] and “a,string,separated,by,comma” by ‘,’ => [“a”, “string”, “separated”, “by”, “comma”] C++ standard library’s getline() function can be used to build a function. getline() can accept a…
How to Generate a Secure Password in the Linux Shell Generating strong, random passwords from the command line is a vital skill for sysadmins. In 2026, we have more tools than ever to do this securely without leaving the terminal. Method 1: openssl (Reliable & Standard) openssl rand -base64 12 generates a 12-character random string….
Vim seems not indent C++11 lambas very well. How to make Vim indent C++11 lambdas correctly? For this following program, Vim indents it as #include <iostream> #include <string> #include <vector> #include <algorithm> int main () { std::vector<std::string> strs({“one”, “two”}); std::vector<std::string> newstrs; std::transform(strs.begin(), strs.end(), std::back_inserter(newstrs), [](const std::string& s) -> std::string { if (s == “one”) {…
How to operator[] access element in a const map in C++? For example, the compiler will report error on this piece of code: #include <iostream> #include <string> #include <map> std::int64_t count(const std::map<std::string, std::int64_t>& map) { return map[“one”] + map[“two”]; } int main () { std::map<std::string, std::int64_t> map = { {“one”, 1}, {“two”, 2} }; std::cout…
How to iterate all dirs and files in a dir in C++? To open a dir, we can use opendir() to open a directory stream. DIR *opendir(const char *name); Then we can use readdir() to iterate the directory stream. struct dirent *readdir(DIR *dirp); Here is an example C++ program using these 2 library functions. #include…
How to Get a File’s Modification Time in C on Linux The stat system call is the standard way to retrieve file metadata. The Code #include <sys/stat.h> struct stat st; stat(“path/to/file”, &st); printf(“Mtime: %ld”, st.st_mtim.tv_sec); C Programming in 2026 In 2026, while stat is still the go-to, modern C code (C11/C17) often uses struct timespec…
How to remove newline characters from a string in C++? For example, a string like line 1 line 3 line 4 should be converted to line 1line 3line 4 Method 1: use `erase()` and `remove()` In short, use this code snippet: input.erase(std::remove(input.begin(), input.end(), ‘\n’), input.end()); std::remove() shifts all elements that are equal to the value…
How to test whether a given path is a directory or a file in C++? For example, pathtype(“/”) –> “a dir” pathtype(“/tmp/file.txt”) –> “a file” # if there is a file file.txt pathtype(“/tmp/dir.txt”) –> “a dir” # if there is a dir named dir.txt The type of a file/dir can be found out using lstat()…
The default target of make is the first target. But can I change the default target in Makefile and how to change the default target of make? The default goal of make is the first target whose name does not start with ‘.’ if .DEFAULT_GOAL is not set. Ref: make manual. To set the default…
Within a Bash script, how to judge whether its STDERR is redirected to a file in Bash on Linux? For example, ./script.sh /tmp/log 2>&1 Can script.sh detect that its STDERR is redirected? Knowing the destination file is better. To test whether a script’s STDERR (or STDOUT) is redirected to a file, check by [[ -f…
HDFS SecondaraNameNode log shows 2017-08-06 10:54:14,488 ERROR org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode: Exception in doCheckpoint java.io.IOException: Inconsistent checkpoint fields. LV = -63 namespaceID = 1920275013 cTime = 0 ; clusterId = CID-f38880ba-3415-4277-8abf-b5c2848b7a63 ; blockpoolId = BP-578888813-10.6.1.2-1497278556180. Expecting respectively: -63; 263120692; 0; CID-d22222fd-e28a-4b2d-bd2a-f60e1f0ad1b1; BP-622207878-10.6.1.2-1497242227638. at org.apache.hadoop.hdfs.server.namenode.CheckpointSignature.validateStorageInfo(CheckpointSignature.java:134) at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.doCheckpoint(SecondaryNameNode.java:531) at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.doWork(SecondaryNameNode.java:395) at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode$1.run(SecondaryNameNode.java:361) at org.apache.hadoop.security.SecurityUtil.doAsLoginUserOrFatal(SecurityUtil.java:415) at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.run(SecondaryNameNode.java:357) It seems the checkpoint…
How to Flush the Stdout Buffer in Python By default, Python buffers stdout when it’s not connected to a terminal (e.g., when redirected to a file). The Methods Python 3.3+: print(“Hello”, flush=True) Manual: import sys; sys.stdout.flush() Command Line: python3 -u script.py (Unbuffered mode) Python in 2026 In 2026, Python 3.12+ is the standard. For logging…
Most time, we need to tune system parameters to achieve better performance but what the general parameters to be tuned in Linux systems. I think you may want to add following parameters to Kernel boot (/etc/default/grub) parameters intel_idle.max_cstate=0 processor.max_cstate=0 idle=poll intel_pstate=disable At the same time, you may also want to shutdown/open Pause Loop Exiting (PLE)….
How to set one task’s CPU affinity quickly? 1, Get this task’s ID # taskset -pc PID 2, Set this task’s CPU affinity # taskset -c affinity ./task There is also another format # taskset -pc affinity pid For example: set process with PID 3783 to CPU 15 # taskset -pc 15 3783
How to get a substring at words boundaries in PHP? For example, for a string $a = “ab cc dde ffg ff”;, I would like to get its substring of length at most 7 from position 0 without breaking words. A simple substring($a, 0, 7) will break the word dde. Here, we consider “words” are…
How to convert a piece of HTML code to plain text without leading and ending spaces in PHP? For example, I would like to convert <div> <b>hello</b> world</div> to a string hello world You may use this piece of code in PHP to strip HTML tags, remove leading and ending spaces and convert special characters…