I want to know how to get a log file of a program. This is useful to find the faults in an application when it crashes.
I mean opening the application through the terminal and at the same time writing a log file.
Are there special commands to do this, or is it automatically done whenever an application is run? If so, where is that log file stored?
Not all applications will leave a log behind, it depends on the coding behind them, even complex ones might not have logging capabilities.
If an application has a debug, verbose or log capabilities that option will be declared in the man page of that application, ie: man ssh will show you that if you use ssh -v you will get a verbose output of what the command is doing on the background when opening a tunnel to a host, ssh -y will output to syslog instead of stderr.
But some applications are just not programmed with that capability. For those strace can be used as a debugging utility for any application though, it logs all system calls and signals, I find it useful when I have any crashes or unexpected faults while running an application, and most of the times it helps me sort issues with applications that do not have a debug or a log option built in to their code, ie strace ls outputs:
execve("/usr/bin/ls", ["ls"], [/* 95 vars */]) = 0
brk(0) = 0x20b6000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd7e5ca3000
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|0644, st_size=118369, ...}) = 0
mmap(NULL, 118369, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd7e5c86000
close(3) = 0
open("/lib64/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF21100000000030>0100020l000000"..., 832) = 832
...
...
...
This allows you to have an idea of what is happening and why is an application failing to halting in its code.
