[md] The dd command doesn’t evaluate arithmetic expressions in its parameters. If you try this: $ dd if=/dev/zero of=./4Ktest100M bs=4KB count=25000*9 dd: invalid number ‘25000*9′ You’ll get an error because dd expects literal numeric values, not expressions. The shell doesn’t expand 25000*9 as arithmetic — it’s passed to dd as a literal string. ## Using…
When accepting path input in Bash scripts, users might provide /path/to/dir/ or /path/to/dir. Normalizing these to a consistent format without the trailing slash is a common requirement. The Problem You need a reliable method that: Removes trailing slashes if present Works with both files and directories Handles paths that already lack a trailing slash Works…
Bash provides several straightforward ways to test whether a string contains a substring. The best approach depends on your use case and performance requirements. Using [[ ]] with glob patterns The fastest and most Bash-native method is the [[ conditional construct with glob patterns: if [[ “$string” == *”substring”* ]]; then echo “Found it!” fi…
When a process mmaps and mprotects many memory ranges, you’ll eventually hit the kernel limit and start seeing ENOMEM errors from mprotect() calls. This happens because Linux maintains internal kernel structures for each mapped range, and there’s a hard ceiling on how many the kernel will track per process. The culprit is the vm.max_map_count sysctl…
When working with file I/O in C on Linux, you’ll often need to convert between low-level file descriptors (returned by open(), socket(), etc.) and higher-level FILE pointers (returned by fopen()). This is common when integrating different libraries or mixing POSIX I/O with standard C I/O. Getting a FILE Pointer from a File Descriptor Use fdopen()…
When writing C programs on Linux that handle file paths, you need to allocate buffers safely. The kernel defines maximum path and filename lengths through standard macros in <linux/limits.h>. Getting the limits On Linux systems, <linux/limits.h> provides two key macros: NAME_MAX — maximum length of a single filename component (255 characters) PATH_MAX — maximum length…
When you hit the open files limit on Linux, you’ll see errors like “Too many open files” — typically when running applications that create many temporary file handles. The default limit is often 1024, which isn’t enough for resource-heavy workloads. Check your current limit: ulimit -n Understanding soft and hard limits Linux maintains two types…
When processing text files or command output, non-printable ASCII characters corrupt formatting, break parsing, or create unexpected behavior. You need the right tool for your specific case — whether you’re stripping control characters, handling UTF-8, or cleaning up embedded escape sequences. Understanding Printable ASCII The standard printable ASCII range includes: Tab (hex 09) Line Feed…
To determine file size in C, use stat() to populate a struct stat and read the st_size field. Here’s the correct approach: #include <sys/stat.h> #include <stdio.h> off_t file_length(const char *path) { struct stat st; if (stat(path, &st) == -1) { perror(“stat”); return -1; } return st.st_size; } Why off_t and not long Using off_t as…
Viewing file contents in hexadecimal is essential for debugging binaries, inspecting file headers, analyzing corrupted data, and understanding file formats. Linux provides several tools for this, each with different strengths. xxd xxd is the most commonly used hex viewer. By default, it displays hex on the left and ASCII representation on the right: xxd file.txt…
Bash provides comparison operators for conditionals through the test command (aliased as [), the [[ ]] compound command, and the (( )) arithmetic command. Each has specific use cases and behavior differences worth understanding. String Comparisons String comparisons work with test/[ and [[ ]]: Operator Meaning -n STRING String length is nonzero (true if not…
When you install a JDK on Linux, you need to configure environment variables so your shell uses the correct Java binary and build tools can locate the installation. If you have multiple Java versions installed, proper configuration becomes essential. Why Set Java Environment Variables Setting JAVA_HOME and PATH ensures: Tools and applications can locate the…
Parsing directory and filename components from an absolute path is a common task in systems programming. Linux provides dirname() and basename() functions for this purpose, but they have implementation-specific behaviors that can cause bugs if you’re not careful. Using dirname() and basename() The POSIX standard defines both functions, though the implementations vary. Here’s the safest…
[md] Use setenv to set an environment variable in csh: setenv VARIABLE value Replace VARIABLE with the name of the variable and value with its value. The variable is immediately available in the current shell and inherited by child processes. ## Examples Set a simple variable: setenv PATH /usr/local/bin:/usr/bin:/bin setenv EDITOR vim setenv JAVA_HOME /usr/lib/jvm/java-17-openjdk…
Working with environment variables in C is straightforward using POSIX APIs. The two primary functions are getenv() for retrieval and setenv() for modification. Getting Environment Variables Use getenv() to read an environment variable: #include <stdlib.h> #include <stdio.h> int main(void) { const char *path = getenv(“PATH”); if (path != NULL) { printf(“PATH: %s\n”, path); } else…
The nm command displays symbolic information from object files, libraries, and executables. It’s the standard tool for inspecting what functions and variables are defined or referenced in a static library (.a file). Basic usage List all symbols in a library: nm lib.a This outputs symbol names, but the default format can be hard to parse….
Vim’s regex engine is powerful but requires understanding its syntax rules, which differ significantly from standard POSIX extended regex. Whether you’re doing basic substitutions or complex text transformations, knowing how Vim handles regex patterns will save you hours of frustration. Vim Regex Basics Vim supports multiple regex engines, controlled by the magic setting. By default,…
Spawning background processes in Bash is straightforward with the & operator, but there are several nuances depending on your use case. Basic approach The simplest method is appending & to your command: for i in $(cat ./all-hosts); do ssh “$i” “ifconfig | grep Link” & done Each & launches the process in the background and…
OCaml provides robust FFI (Foreign Function Interface) capabilities for calling C code. This is essential when you need to use existing C libraries or write performance-critical sections in C. Using the Unix Module The simplest approach for Unix/Linux system calls is the built-in Unix module, which wraps common POSIX functions without requiring manual FFI binding….
When working with relative paths in Linux, you often need the absolute path for scripting, logging, or passing to other programs. Several tools can resolve this, each with different strengths. Using readlink The readlink command is the traditional tool for this job. The -f flag follows symlinks and resolves the path to its absolute form:…