myCMD

Mini shell written in C for an Operating Systems course. The constraint was to use only POSIX system calls — no system(), and no shelling out to top/ps/uptime for the monitoring features.

It supports three things:

• running a command with output or input redirection (>, <)
• piping the output of one command into another (|)
• a top mode that prints CPU load average and the number of running processes, read directly from /proc

Linux only — it depends on /proc and POSIX APIs. On Windows it runs fine under WSL.

Build

gcc -Wall -Wextra -O2 -o mycmd myCMD.c

Usage

./mycmd ls -la
./mycmd ls -la > out.txt
./mycmd wc -l < in.txt
./mycmd ls -la | grep ".c"
./mycmd top

In top mode the screen refreshes every 10 seconds. Press q then Enter to quit.

How it works

Everything is in a single file (myCMD.c). The main pieces:

• parse() — walks through argv looking for >, < or | and splits the arguments into "first command" and "second command".
• executarComandos() — fork + execvp. For pipes, it sets up the pipe with pipe() and uses dup2 to wire the child's stdout into the parent's stdin before running the second command.
• executarComandosComRedirecionamentoOutput / Input() — opens the target file with open() and uses dup2 to swap STDOUT_FILENO or STDIN_FILENO before execvp.
• mostrarCargaMediaCPU() — reads /proc/loadavg.
• mostrarStatusProcessos() — opens /proc, treats numeric directory names as PIDs, and parses /proc//stat to count processes in state R.
• lmparMemoria() — frees the buffers allocated during parsing.

Project layout

. ├── myCMD.c └── README.md

Limitations

• Only one operator per invocation. No chained pipes, and you can't mix a pipe with redirection in the same command.
• Source comments are in Portuguese (kept as submitted).
• There's a typo in one function name (lmparMemoria instead of limparMemoria) — left as-is to keep the code in its original state.