System
| Signal Name | Number | Description |
|---|---|---|
| SIGHUP | 1 | Hangup (POSIX) |
| SIGINT | 2 | Terminal interrupt (ANSI) |
| SIGQUIT | 3 | Terminal quit (POSIX) |
| SIGILL | 4 | Illegal instruction (ANSI) |
| SIGTRAP | 5 | Trace trap (POSIX) |
| SIGIOT | 6 | IOT Trap (4.2 BSD) |
| SIGBUS | 7 | BUS error (4.2 BSD) |
| SIGFPE | 8 | Floating point exception (ANSI) |
| SIGKILL | 9 | Kill(can't be caught or ignored) (POSIX) |
| SIGUSR1 | 10 | User defined signal 1 (POSIX) |
| SIGSEGV | 11 | Invalid memory segment access (ANSI) |
| SIGUSR2 | 12 | User defined signal 2 (POSIX) |
| SIGPIPE | 13 | Write on a pipe with no reader, Broken pipe (POSIX) |
| SIGALRM | 14 | Alarm clock (POSIX) |
| SIGTERM | 15 | Termination (ANSI) |
| SIGSTKFLT | 16 | Stack fault |
| SIGCHLD | 17 | Child process has stopped or exited, changed (POSIX) |
| SIGCONT | 18 | Continue executing, if stopped (POSIX) |
| SIGSTOP | 19 | Stop executing(can't be caught or ignored) (POSIX) |
| SIGTSTP | 20 | Terminal stop signal (POSIX) |
| SIGTTIN | 21 | Background process trying to read, from TTY (POSIX) |
| SIGTTOU | 22 | Background process trying to write, to TTY (POSIX) |
| SIGURG | 23 | Urgent condition on socket (4.2 BSD) |
| SIGXCPU | 24 | CPU limit exceeded (4.2 BSD) |
| SIGXFSZ | 25 | File size limit exceeded (4.2 BSD) |
| SIGVTALRM | 26 | Virtual alarm clock (4.2 BSD) |
| SIGPROF | 27 | Profiling alarm clock (4.2 BSD) |
| SIGWINCH | 28 | Window size change (4.3 BSD, Sun) |
| SIGPWR | 30 | Power failure restart (System V) |
| Redirection | fd 0 (stdin) | fd 1 (stdout) | fd 2 (stderr) | fd 3 | Description |
|---|---|---|---|---|---|
| initial | /dev/tty | /dev/tty | /dev/tty | Let's assume this is run in a terminal, so stdin, stdout and stderr are all initially connected to the terminal (tty). | |
| $(...) | /dev/tty | pipe | /dev/tty | First, the command substitution is set up. Command's stdout (FileDescriptor 1) gets captured (by using a pipe internally). Command's stderr (FD 2) still points to its regular place (the script's stderr). | |
| 3>&2 | /dev/tty | pipe | /dev/tty | /dev/tty | Next, FD 3 should point to what FD 2 points to at this very moment, meaning FD 3 will point to the script's stderr ("save stderr in FD 3"). |
| 2>&1 | /dev/tty | pipe | pipe | /dev/tty | Next, FD 2 should point to what FD 1 currently points to, meaning FD 2 will point to stdout. Right now, both FD 2 and FD 1 would be captured. |
| 1>&3 | /dev/tty | /dev/tty | pipe | /dev/tty | Next, FD 1 should point to what FD 3 currently points to, meaning FD 1 will point to the script's stderr. FD 1 is no longer captured. We have "swapped" FD 1 and FD 2. |
| 3>&- | /dev/tty | /dev/tty | pipe | Finally, we close FD 3 as it is no longer necessary. |
| Run Level | Mode | Action |
|---|---|---|
| 0 | Halt | Shuts down system |
| 1 | Single-User Mode | Does not configure network interfaces, start daemons, or allow non-root logins |
| 2 | Multi-User Mode | Does not configure network interfaces or start daemons. |
| 3 | Multi-User Mode with Networking | Starts the system normally. |
| 4 | Undefined | Not used/User-definable |
| 5 | X11 | As runlevel 3 + display manager(X) |
| 6 | Reboot | Reboots the system |
Standard run levels for Red Hat based distributions
The run level is configured in the /etc/inittab
/etc/rc.d contains a folder for each run level: rcX.d.
rcX.d contains a sym link to /etc/init.d/daemon which is the daemon.
chkconfig is the tool used for that purpose
