CMPSC 311, Shell Scripts, part 2

CMPSC 311, Introduction to Systems Programming

Shell Scripts, Part 2

References

Large parts of this presentation are taken from the Solaris man page for sh.

Special Commands (but not the complete list)

Items in square brackets [] are optional.

Loop control, etc.

break [n]	Exit from the enclosing for or while loop, if any. If n is specified, break n levels.
continue [n]	Resume the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.
exit [n]	Causes the calling shell or shell script to exit with the exit status specified by n. If n is omitted the exit status is that of the last command executed. An EOF (control-D as keyboard input) will also cause the shell to exit.
return [n]	Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed.
shift [n]	The positional parameters from $n+1 ... are renamed $1 ... . If n is not given, it is assumed to be 1.

Job control, intended for interactive use only.

This is the description for the Bourne shell, but you should use the C shell or Bash for interactive use. These concepts and commands work for those shells also.
This description leaves out some details.

A job is a command or pipeline the user enters at the terminal.
All jobs exist in one of the following states: foreground, background, or stopped.

Foreground and background jobs are running, stopped jobs are not (for now).
A job in the foreground has read and write access to the controlling terminal.
A job in the background is denied read access and has conditional write access to the controlling terminal.
A stopped job is a job that has been placed in a suspended state, usually as a result of a SIGTSTP signal (terminal stop, originated by control-Z, for ex.).

Job numbers (job identifiers) are maintained by the shell; they are not the same as process numbers, but they can be mapped to process numbers by the shell.
%% refers to the current job
%n refers to job n

bg [%jobid ...]

Resumes the execution of a stopped job in the background. If %jobid is omitted the current job is assumed.

fg [%jobid ...]

Resumes the execution of a stopped job in the foreground, or moves an executing background job into the foreground. If %jobid is omitted, the current job is assumed.

jobs [%jobid ...]

Reports jobs that are stopped or executing in the background. If %jobid is omitted, all jobs that are stopped or running in the background will be reported.

jobs -x command [arguments]

Replace any jobid found in command or arguments with the corresponding process group ID, and then execute command passing it arguments.

kill [ -sig ] %job ...

Sends either the TERM (terminate) signal or the specified signal to the specified jobs or processes. Signals are either given by number or by names (as given in <signal.h> stripped of the prefix "SIG" with the exception that SIGCLD is named CHLD). If the signal being sent is TERM (terminate) or HUP (hangup), then the job or process will be sent a CONT (continue) signal if it is stopped. The argument job can be the process id of a process that is not a member of one of the active jobs.

kill -l

List the signal numbers and names.

stop %jobid ...

Stops the execution of a background job(s).

stop pid ...

Stop execution of the process number pid.

suspend

Stops the execution of the current shell (but not if it is the login shell).

wait [%jobid ...]

As next, but with a job identifier.

wait [ n ]

Wait for your background process whose process id is n and report its termination status. If n is omitted, all your shell's currently active background processes are waited for and the return code will be zero.

More special commands

No effect; the command does nothing. A zero exit code is returned.

. filename

Read and execute commands from filename and return. The search path specified by PATH is used to find the directory containing filename.

cd [ argument ]

Change the current directory to argument. The shell parameter HOME is the default argument. The shell parameter CDPATH defines the search path for the directory containing argument. Alternative directory names are separated by a colon (:). The default path is <null> (specifying the current directory). Note: The current directory is specified by a null path name, which can appear immediately after the equal sign or between the colon delimiters anywhere else in the path list. If argument begins with a / the search path is not used. Otherwise, each directory in the path is searched for argument.

echo [ arguments ... ]

The words in arguments are written to the shell's standard output, separated by space characters. See echo(1).

eval [ argument ... ]

The arguments are read as input to the shell and the resulting command(s) executed.

exec [ argument ... ]

The command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and, if no other arguments are given, cause the shell input/output to be modified.

getopts

Use in shell scripts to support command syntax standards (see intro(1)). This command parses positional parameters and checks for legal options. See getoptcvt(1).

Print the current working directory. See pwd(1).

read name ...

One line is read from the standard input and, using the internal field separator, IFS (normally space or tab), to delimit word boundaries, the first word is assigned to the first name, the second word to the second name, and so forth, with leftover words assigned to the last name. Lines can be continued using \newline. Characters other than newline can be quoted by preceding them with a backslash. These backslashes are removed before words are assigned to names, and no interpretation is done on the character that follows the backslash. The return code is 0, unless an EOF is encountered.

Shell variables (not a complete description)

NAME=VALUE
set [ -aefhkntuvx [ argument ... ] ]

Using + rather than - causes these flags to be turned off.
The current set of flags may be found in $- .

unset [ name ... ]
export [ name ... ]

Some more features

$HOME/.profile
/etc/profile

If the shell is invoked through exec(2) and the first character of argument zero is -, commands are initially read from /etc/profile and from $HOME/.profile, if such files exist.

Because the shell implements both foreground and background jobs in the same process group, they all receive the same signals, which can lead to unexpected behavior. It is, therefore, recommended that other job control shells be used, especially in an interactive environment.

The flags below are interpreted by the shell on invocation only.

If the -i flag is present or if the shell input and output are attached to a terminal, this shell is interactive. In this case, TERMINATE is ignored (so that kill 0 does not kill an interactive shell) and INTERRUPT is caught and ignored (so that wait is interruptible). In all cases, QUIT is ignored by the shell.
etc.

Last revised, 25 Feb. 2013