Ever need to look into a Unix process to see exactly what it's doing? The ps and top commands only give you a bird's eye view of what's going on. But strace will let you peer into a process' inner workings.
The strace utility is a lightweight debugger. It allows you to look at what a Unix process is doing step by step, whether it is opening files, invoking system calls or writing output to your screen. A process which appears to be hung might, in fact, be incredibly busy, but you might not know it unless you use a tool which is able to tell you what is happening step by step. The strace command lets you see what a process is doing, how it is interacting with the operating system. It monitors system calls and signals. As others have pointed out, using strace’s output can be like drinking from a fire hose. You’re likely to get a LOT of output and it’s going to come shooting at you — especially if your process is busy. If you’re going to make good use of strace, it’s good to have a basic understanding of what a system call is and some of the calls that you’re likely to see. To gain some familiarity, you can use strace to look at a simple process or start a process and then watch everything it does. Here’s an extremely simple example:
$ strace echo hello
execve("/bin/echo", ["echo", "hello"], [/* 28 vars */]) = 0
brk(0) = 0x8f58000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=60257, ...}) = 0
mmap2(NULL, 60257, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f94000
close(3) = 0
open("/lib/libc.so.6", O_RDONLY) = 3
read(3, "177ELF111
