I am looking for a reliable way to find out the location in the filesystem of the program that runs when I enter a command.

ps -ef gives me list of all currently running processes (-e means every ; -f means full list with relevant parameters).

So, to find the location of the uname program, I tried

grep `uname -n &` `ps -ef`

I'm trying to catch uname -n's process ID (PID) and forward it to grep, which should return the line which contains that PID, so I could see which command is running at the moment.

I know there are commands like tee, pipes... but I haven't succeeded in combining them to achieve my goal.

I suppose there is some shell function which returns stdout of called function, something like:

stdout(some__command)

so I could call:

grep stdout(uname -n &) stdout(ps -ef).

What am I missing here?

If all you want to know is the command location

If you want to know what directory the top level command is stored in you have a number of options:

$ which uname
/bin/uname

$ type -a uname
uname is /bin/uname

$ locate uname
/bin/uname
 (... SNIP dozens of Windows files on C & D ...)
/usr/lib/klibc/bin/uname
/usr/lib/plainbox-provider-resource-generic/bin/uname_resource
/usr/share/man/man1/uname.1.gz
/usr/share/man/man2/oldolduname.2.gz
/usr/share/man/man2/olduname.2.gz
/usr/share/man/man2/uname.2.gz

The last option locate returns all files containing uname not just the program that is run from the command prompt.

strace - trace system calls and interrupts

You don't need the pid of commands called in order to find the names of various commands that are called by a function. With strace all the command names are displayed directly.

For your uname -n example the output is:

$ strace uname -n
execve("/bin/uname", ["uname", "-n"], [/* 62 vars */]) = 0
brk(NULL) = 0x2356000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, st_mode=S_IFREG) = 0
mmap(NULL, 109073, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff2f9a9f000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF21133>1Pt2"..., 832) = 832
fstat(3, 0755, st_size=1868984, ...) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff2f9a9e000
mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ff2f94cb000
mprotect(0x7ff2f968b000, 2097152, PROT_NONE) = 0
mmap(0x7ff2f988b000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7ff2f988b000
mmap(0x7ff2f9891000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff2f9891000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff2f9a9d000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff2f9a9c000
arch_prctl(ARCH_SET_FS, 0x7ff2f9a9d700) = 0
mprotect(0x7ff2f988b000, 16384, PROT_READ) = 0
mprotect(0x606000, 4096, PROT_READ) = 0
mprotect(0x7ff2f9aba000, 4096, PROT_READ) = 0
munmap(0x7ff2f9a9f000, 109073) = 0
brk(NULL) = 0x2356000
brk(0x2377000) = 0x2377000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, 0644, st_size=10219008, ...) = 0
mmap(NULL, 10219008, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff2f8b0c000
close(3) = 0
uname(sysname="Linux", nodename="alien", ...) = 0
fstat(1, 0620, st_rdev=makedev(136, 2), ...) = 0
write(1, "alienn", 6alien
) = 6
close(1) = 0
close(2) = 0
exit_group(0) = ?
+++ exited with 0 +++

For more information refer to man strace:

STRACE(1) General Commands Manual STRACE(1)

NAME
 strace - trace system calls and signals

SYNOPSIS
 strace [-CdffhikqrtttTvVxxy] [-In] [-bexecve] [-eexpr]... [-acolumn] [-ofile] [-sstr‐
 size] [-Ppath]... -ppid... / [-D] [-Evar[=val]]... [-uusername] command [args]

 strace -c[df] [-In] [-bexecve] [-eexpr]... [-Ooverhead] [-Ssortby] -ppid... / [-D]
 [-Evar[=val]]... [-uusername] command [args]

DESCRIPTION
 In the simplest case strace runs the specified command until it exits. It intercepts
 and records the system calls which are called by a process and the signals which are
 received by a process. The name of each system call, its arguments and its return value
 are printed on standard error or to the file specified with the -o option.

 strace is a useful diagnostic, instructional, and debugging tool. System administra‐
 tors, diagnosticians and trouble-shooters will find it invaluable for solving problems
 with programs for which the source is not readily available since they do not need to be
 recompiled in order to trace them. Students, hackers and the overly-curious will find
 that a great deal can be learned about a system and its system calls by tracing even
 ordinary programs. And programmers will find that since system calls and signals are
 events that happen at the user/kernel interface, a close examination of this boundary is
 very useful for bug isolation, sanity checking and attempting to capture race condi‐
 tions.

Above is just the beginning of manpage for strace