Command Line

strace traces system calls and signals made by a process. It is useful when a program fails but the error message does not explain what happened. Trace a command Attach to a running process Save output to a file Common things to look for are missing files, permission errors, failed network calls and repeated retries.

The ltrace command traces library calls made by a program. It can help when a command fails and you want to see which shared library functions it is calling. Basic usage Example Use ltrace when you are interested in library calls. Use strace when you want to trace system calls such as file, process and … Read more

gcc is the GNU C compiler. It is commonly used to compile C programs on Linux and is often required when building software from source. Compile a simple C file Run the compiled program Show warnings while compiling For larger projects, prefer the project’s documented build process, such as make, cmake or package-specific instructions.

dd copies data at a low level. It is powerful, but it can overwrite disks if the input and output paths are wrong. Create an image of a disk or partition Write an image to a USB drive Always double-check the target device before running dd. A wrong of= value can destroy data.

cron runs commands on a schedule. User cron entries are edited with crontab. Edit the current user crontab Crontab format Run a script every day at 2:30 AM List existing cron jobs Use full paths inside cron jobs because the environment is usually smaller than an interactive shell.

The whatis command shows a short one-line description from the manual page database. It is useful when you know a command name but want a quick reminder of what it does. Examples For more detail, open the full manual page with man.

The renice command changes the nice value of a process that is already running. It can be useful when a process is using too much CPU or when you want to give a task lower priority. Find the process ID Lower priority of a running process A higher nice value means lower scheduling priority. Raising … Read more

The which command shows the path of the executable that will run when you type a command name. It is useful when multiple versions of a tool may be installed. Basic use If the command is found in your PATH, which prints the full path. If nothing is printed, the command may not be installed … Read more

The nice command starts a program with a modified scheduling priority. It is useful when you want a background task to be less aggressive with CPU usage. Run a command with lower priority Example Higher nice values mean lower priority. To adjust a process that is already running, use renice.

cdecl and c++decl translate complicated C and C++ declarations into English and can also generate declarations from plain-language descriptions. They are useful when pointer, function-pointer, or nested type syntax becomes hard to read. Interactive mode cdecl explain int (*signal(int, int (*)(int)))(int) declare p as pointer to integer quit c++decl uses C++ terminology and syntax where … Read more