Jobs and Concurrency

commands

• jobs

• ps

• fg bring background process to the foreground

• bg resume a stopped background process if it is stopped

• disown

• kill

Running Tasks in the background

Tasks can be run concurrently in the background with the & operator.

# run 3 commands in a background jobs
tail -f /var/log/dmesg &
tail -f /var/log/syslog &
nano /tmp/file &

# Three jobs in the background

# list background jobs and their statuses and their PIDs
jobs -l

# list the PIDs only
jobs -p

# list jobs in the current process and its children
ps

UNIX Signals

Resources

• man 7 signal

• kill -L

SIGHUP	1	Term	Hangup or death of controlling terminal or process
SIGINT	2	Term	Interrupt from keyboard
SIGQUIT	3	Core	Quit from keyboard
SIGILL	4	Core	Illegal Instruction
SIGTRAP	5	Core	Trace/breakpoint trap
SIGABRT	6	Core	Abort signal from abort(3)
SIGBUS	7	Core	Bus error (bad memory access)
SIGFPE	8	Core	Floating point exception
SIGKILL	9	Term	Kill signal
SIGSEGV	11	Core	Invalid memory reference
SIGPIPE	13	Term	Broken pipe: write to pipe with no readers
SIGALRM	14	Term	Timer signal from alarm(2)
SIGTERM	15	Term	Termination signal
SIGUSR1	10	Term	User-defined signal 1
SIGUSR2	12	Term	User-defined signal 2
SIGCHLD	17	Ign	Child stopped or terminated
SIGCONT	18	Cont	Continue if stopped
SIGSTOP	19	Stop	Stop process
SIGTSTP	20	Stop	Stop typed at terminal
SIGTTIN	21	Stop	Terminal input for background process
SIGTTOU	22	Stop	Terminal output for background process

Stopping and Resuming

The main signals we are concerned about are:

• kill -19 stop process

• kill -18 continue if stopped

• kill -15 soft termination

• kill -9 hard termination

jobs can be selected by their order in which they were started with %1 being the first job, %2 being the second job and onwards.

tail -f /var/log/dmesg &
tail -f /var/log/syslog &
nano /tmp/file &

# stop the second job
kill -19 %2

# stop the first job
kill -19 %1

# resume the first job
kill -18 %1

# when we terminate the second job, jobs 3 onward maintain their order in the job list
kill -15 %2

Current and Child PID

# prints the current PID VS a child subshell PID
echo $BASHPID; ( echo $BASHPID )