Tracing system calls

$ strace ls

Traces all the system calls invoked by the process “ls”

Let’s say we are in directory dir and we execute ls

$ ls
file1  file2

And now, let’s see what the command actually does:

$ strace ls

open(".", O_RDONLY|O_NONBLOCK|O_DIRECTORY|0x80000) = 3

getdents(3, /* 4 entries */, 4096)      = 112

write(1, "file1  file2\n", 13file1  file2
)          = 13

It opens the current directory, reads the number of entries (4, this is: our two files, plus “.” plus “..”) and writes the result using write
You should keep in mind that strace traces system calls and not libc functions (that are actually wrappers for system calls). So if you want to find information about some of the syscalls reported by strace you should look up in man section 2

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