A shell is an interactive command-line interpreter that runs
programs on behalf of the user. A shell repeatedly prints a prompt,
waits for a command line on stdin, and then carries out
some action, as directed by the contents of the command line.
The command line is a sequence of ASCII text words delimited by
whitespace. The first word in the command line is either the name of
a built-in command or the pathname of an
executable file. The remaining words are command-line arguments. If
the first word is a built-in command, the shell immediately executes
the command in the current process. Otherwise, the word is assumed to
be the pathname of an executable program. In this case, the shell
forks a child process, then loads and runs the program in the context
of the child. The child processes created as a result of interpreting
a single command line are known collectively as a job. In
general, a job can consist of multiple child processes connected by
Unix pipes.
If the command line ends with an ampersand (&), then the job
runs in the background, which means that the shell does not wait
for the job to terminate before printing the prompt and awaiting the
next command line. Otherwise, the job runs in the foreground,
which means that the shell waits for the job to terminate before
awaiting the next command line. Thus, at any point in time, at most
one job can be running in the foreground. However, an arbitrary number
of jobs can run in the background.
For example, typing the command line
tsh> jobs
causes the shell to execute the built-in jobs command.
Typing the command line
tsh> /bin/ls -l -d
runs the ls program in the foreground. By convention, the shell
ensures that when ls begins executing its main routine:
int main(int argc, char *argv[])
the argc and argv arguments have the following values:
- argc == 3,
- argv[0] == "/bin/ls",
- argv[1]== "-l",
- argv[2]== "-d".
Alternatively, typing the command line
tsh> /bin/ls -l -d &
runs the ls program in the background.
Unix shells support the notion of job control, which allows
users to move jobs back and forth between background and foreground,
and to change the process state (running, stopped, or terminated) of
the processes in a job. Typing ctrl-c causes a SIGINT signal to
be delivered to each process in the foreground job. The default
action for SIGINT is to terminate the process. Similarly, typing
ctrl-z causes a SIGTSTP signal to be delivered to each process in the
foreground job. The default action for SIGTSTP is to place a process in
the stopped state, where it remains until it is awakened by the receipt
of a SIGCONT signal. Unix shells also provide various built-in
commands that support job control. For example:
- jobs: List the running and stopped background jobs.
- bg <job>:
Change a stopped background job to a running background job.
- fg<job>:
Change a stopped or running background job to a running in the
foreground.
- kill<job>: Terminate a job.
