osh.1 - nroff source
OSH(1) General Commands OSH(1)
NAME
osh - old shell (command interpreter)
SYNOPSIS
osh [-v] [- | -c string | -i | -l | -t | file [arg1 ...]]
DESCRIPTION
Osh is an enhanced, backward-compatible port of the standard command
interpreter from Sixth Edition UNIX. It may be used either as an
interactive shell or as a non-interactive shell. Throughout this man-
ual, `(+)' indicates those cases where osh is known to differ from the
original sh(1), as it appeared in Sixth Edition UNIX.
The options are as follows:
- The shell reads and executes command lines from the standard
input until end-of-file or exit.
-c string
The shell executes string as a command line and exits.
-i (+) The shell behaves as an interactive shell by reading and
executing commands from the appropriate rc files if possible
(see Startup and shutdown below) before prompting the user,
reading, and executing command lines from the standard input.
The shell prints a diagnostic and exits with a non-zero status
if it is not connected to a terminal.
-l (+) The shell behaves as a login shell by reading and executing
commands from the appropriate rc files if possible (see Startup
and shutdown below) before prompting the user, reading, and exe-
cuting command lines from the standard input. The shell prints
a diagnostic and exits with a non-zero status if it is not con-
nected to a terminal.
-t The shell reads a single command line from the standard input,
executes it, and exits.
-v (+) The shell verbosely prints the words of each command line to
the standard error after performing parameter substitution and
word splitting, but before executing the resulting command line.
The shell may also be invoked non-interactively to read, interpret, and
execute a command file. The specified file and any arguments are
treated as positional parameters (see Parameter substitution below)
during execution of the command file.
Otherwise, if no arguments except for -v are specified and if both the
standard input and standard error are connected to a terminal, the
shell is interactive. An interactive shell prompts the user with a
`% ' (or `# ' for the superuser) before reading each command line from
the terminal.
(+) When an interactive shell starts, it reads and executes commands
from the appropriate rc files if possible (see Startup and shutdown
below) before reading and executing command lines from the terminal.
Commands
Each command is a sequence of non-blank command arguments separated by
blanks (spaces or tabs). The first argument specifies the name of a
command to be executed. Except for certain types of special arguments
described below, the arguments other than the command name are passed
without interpretation to the invoked command.
If the first argument names a special command, the shell executes it
(see Special commands below). Otherwise, the shell treats it as an
external command, which is located as follows.
(+) If the command name contains no `/' characters, the sequence of
directories in the environment variable PATH is searched for the first
occurrence of an executable file by that name, which the shell attempts
to execute. However, if the command name contains one or more `/'
characters, the shell attempts to execute it without performing any
PATH search.
(+) If an executable file does not begin with the proper magic number
or a `#!shell' sequence, it is assumed to be a shell command file, and
a new shell is automatically invoked to execute it. The environment
variable EXECSHELL specifies the shell which is invoked to execute such
a file.
If a command cannot be found or executed, a diagnostic is printed.
Command lines
Commands separated by `|' or `^' constitute a chain of filters, or a
pipeline. The standard output of each command but the last is taken as
the standard input of the next command. Each command is run as a sepa-
rate process, connected by pipes (see pipe(2)) to its neighbors.
A command line, or list, consists of one or more pipelines separated,
and perhaps terminated by `;' or `&'. The semicolon designates sequen-
tial execution. The ampersand designates asynchronous execution, which
causes the preceding pipeline to be executed without waiting for it to
finish. The process ID of each command in such a pipeline is reported,
so that it may be used if necessary for a subsequent kill(1).
A list contained within parentheses such as `( list )' is executed in a
subshell and may appear in place of a simple command as a filter.
If a command line is syntactically incorrect, a diagnostic is printed.
Termination reporting
All terminations other than exit and interrupt are considered to be
abnormal. If a sequential process terminates abnormally, a message is
printed. The termination report for an asynchronous process is given
upon execution of the first sequential command subsequent to its termi-
nation, or when the wait special command is executed. The following is
a list of the possible termination messages:
Hangup
Quit
Illegal instruction
Trace/BPT trap
IOT trap
EMT trap
Floating exception
Killed
Bus error
Memory fault
Bad system call
Broken pipe (+)
For an asynchronous process, its process ID is prepended to the appro-
priate message. If a core image is produced, ` -- Core dumped' is
appended to the appropriate message.
I/O redirection
Each of the following argument forms is interpreted as a redirection by
the shell itself. Such a redirection may appear anywhere among the
arguments of a simple command, or before or after a parenthesized com-
mand list, and is associated with that command or command list.
A redirection of the form `<arg' causes the file `arg' to be used as
the standard input (file descriptor 0) for the associated command.
A redirection of the form `>arg' causes the file `arg' to be used as
the standard output (file descriptor 1) for the associated command. If
`arg' does not already exist, it is created; otherwise, it is truncated
at the outset.
A redirection of the form `>>arg' is the same as `>arg', except if
`arg' already exists the command output is always appended to the end
of the file.
For example, either of the following command lines:
% date >dirlist ; pwd >>dirlist ; ls -l >>dirlist
% ( date ; pwd ; ls -l ) >dirlist
creates on the file `dirlist', the current date and time, followed by
the name and a long listing of the current working directory.
(+) A `<-' redirection causes input for the associated command to be
redirected from the standard input which existed when the shell was
invoked. This allows a command file to be used as a filter.
A `>arg' or `>>arg' redirection associated with any but the last com-
mand of a pipeline is ineffectual, as is a `<arg' redirection with any
but the first.
The standard error (file descriptor 2) is never subject to redirection
by the shell itself. Thus, commands may write diagnostics to a loca-
tion where they have a chance to be seen. However, fd2 provides a way
to redirect the diagnostic output to another location.
If the file for a redirection cannot be opened or created, a diagnostic
is printed.
Quoting
The shell treats all quoted characters literally. This includes char-
acters which may have special meaning to the shell such as `|', `^',
`;', `&', `<', `>', and others described in this manual. If such char-
acters are quoted, they represent themselves and may be passed as part
of arguments.
An individual backslash (\) quotes, or escapes, the next individual
character. A backslash followed by a newline is a special case which
allows continuation of command-line input onto the next line. Each
backslash-newline sequence in the input is translated into a blank.
Individual characters, and sequences of characters, are also quoted
when enclosed by a matched pair of double (") or single (') quotes.
For example:
% awk '{ print NR "\t" $0 }' README ^ more
causes awk(1) to write each line in `README', preceded by its line num-
ber and a tab, to the standard output which is piped to more(1) for
viewing. The quotes prevent the shell from trying to interpret any
part of the string, which is then passed as a single argument to awk.
If a double or single quote appears but is not part of a matched pair,
a diagnostic is printed.
Parameter substitution
When the shell is invoked with arguments besides -v, it has additional
string processing capabilities which are not otherwise available. Such
a shell may be invoked as follows:
osh [-v] name [arg1 ...]
If the first character of name is not `-', it is taken as the name of a
command file, or shell script, which is opened as the standard input
for a new shell instance. Thus, the new shell reads and interprets
command lines from the named file.
Otherwise, name is taken as one of the shell options, and a new shell
instance is invoked to read and interpret command lines from its stan-
dard input. However, notice that the -c option followed by a string is
the one case where the shell does not read and interpret command lines
from its standard input. Instead, the string itself is taken as a com-
mand line and executed.
In each command line, an unquoted character sequence of the form `$N',
where N is a digit, is treated as a positional parameter by the shell.
Each occurrence of a positional parameter in the command line is sub-
stituted with the value of the Nth argument to the invocation of the
shell (argN). `$0' is substituted with name.
In all shell instances, `$$' is substituted with the process ID of the
current shell. The value is represented as a 5-digit ASCII string,
padded on the left with zeros when the process ID is less than 10000.
(+) All shell instances attempt to set the special parameters in the
following list. `(*)' indicates those which are always set. Other-
wise, the parameter is unset when the shell cannot determine its value.
$d The value of the environment variable OSHDIR.
$e The value of the environment variable EXECSHELL.
$h The value of the environment variable HOME.
$n (*) The number of positional parameters currently available to
the shell.
$p The value of the environment variable PATH.
$s (*) The exit status of the last sequential command from the pre-
vious command line.
$t The terminal name with which the standard input was associ-
ated when the shell was invoked, as determined by ttyname(3).
The value (if any) is equivalent to that given by `tty <-'.
$u The effective user name of the current user, as determined by
getpwuid(3). The value (if any) is equivalent to that given
by `id -un'.
$v (*) The version of the current shell represented as a one-word,
read-only string.
All substitution on a command line is performed before the line is
interpreted. Thus, no action which alters the value of any parameter
can have any effect on a reference to that parameter occurring on the
same line.
A positional-parameter value may contain any number of characters with
special meaning to the shell. Each one which is unquoted, or
unescaped, within a positional-parameter value retains its special
meaning when the value is substituted in a command line by the invoked
shell.
Take the following two shell invocations for example:
% osh -c '$1' 'echo Hello! >/dev/null'
% osh -c '$1' 'echo Hello! \>/dev/null'
Hello! >/dev/null
In the first invocation, the `>' in the value substituted by `$1'
retains its special meaning. This causes all output from echo to be
redirected to `/dev/null'. However, in the second invocation, the
meaning of `>' is escaped by `\' in the value substituted by `$1'.
This causes the shell to pass `>/dev/null' as an argument to echo
instead of interpreting it as a redirection.
File name generation
Prior to executing a command, the shell scans each argument for
unquoted `*', `?', or `[' characters. If one or more of these charac-
ters appears, the argument is treated as a pattern and causes the shell
to search for file names which match it. Otherwise, the argument is
used as is.
The meaning of each pattern character is as follows:
o The `*' character in a pattern matches any string of characters in
a file name (including the null string).
o The `?' character in a pattern matches any single character in a
file name.
o The `[...]' brackets in a pattern specifies a class of characters
which matches any single file-name character in the class. Within
the brackets, each character is taken to be a member of the class.
A pair of characters separated by an unquoted `-' specifies the
class as a range which matches each character lexically between the
first and second member of the pair, inclusive. A `-' matches
itself when quoted or when first or last in the class.
Any other character in a pattern matches itself in a file name.
Notice that the `.' character at the beginning of a file name, or imme-
diately following a `/', is always special in that it must be matched
explicitly. The same is true of the `/' character itself.
If the pattern contains no `/' characters, the current directory is
always used. Otherwise, the specified directory is the one obtained by
taking the pattern up to the last `/' before the first unquoted `*',
`?', or `['. The matching process matches the remainder of the pattern
after this `/' against the files in the specified directory.
In any event, a list of file names is obtained from the current (or
specified) directory which match the given pattern. This list is
sorted in ascending ASCII order, and the new sequence of arguments
replaces the given pattern. The same process is carried out for each
of the given pattern arguments; the resulting lists are not merged.
Finally, the shell attempts to execute the command with the resulting
argument list.
If a pattern argument refers to a directory which cannot be opened, a
`No directory' diagnostic is printed.
If a command has only one pattern argument, a `No match' diagnostic is
printed if it fails to match any files. However, if a command has more
than one pattern argument, a diagnostic is printed only when they all
fail to match any files. Otherwise, each pattern argument failing to
match any files is simply removed from the argument list.
Startup and shutdown (+)
If the first character of the file name used to invoke an interactive
shell is `-' (e.g., -osh), it is a login shell and tries to read and
execute commands from the following four rc init files in sequence:
/usr/local/etc/osh.login, /usr/local/etc/osh.oshrc, $h/.osh.login, and
$h/.oshrc. The same is true when the shell is invoked with the -l
option, regardless of the shell's file name.
In the case where an interactive shell is not a login shell according
to its file name, it tries to read and execute commands from the fol-
lowing two rc init files in sequence: /usr/local/etc/osh.oshrc and
$h/.oshrc. The same is true when the shell is invoked with the -i
option, regardless of the shell's file name.
In any case, after the shell finishes its startup actions, it then
prompts the user, reads, and executes command lines from the standard
input as usual.
If the shell is invoked as a login shell, it tries to read and execute
commands from /usr/local/etc/osh.logout and $h/.osh.logout in sequence
upon logout. These two rc logout files may be used if necessary for
cleanup upon termination of a login session by an EOT (see End of file
below) or a SIGHUP signal (see Signals below).
Notice that the shell only performs the startup and shutdown actions
described above for readable, regular rc files. If any rc file is not
readable, the shell ignores it and continues as normal. If any rc file
is not a regular file (or a link to a regular file), the shell ignores
it, prints a diagnostic, and continues as normal.
In the normal case, a SIGINT or SIGQUIT signal received by the shell
during execution of any rc file causes it to cease execution of that
file without terminating. Thus, it may be desirable to use the sigign
special command to ignore these and other signals in some cases. For
example, this is particularly true for /usr/local/etc/osh.login,
/usr/local/etc/osh.oshrc, and /usr/local/etc/osh.logout.
The exit special command always causes the shell to terminate if it
occurs in any rc file.
End of file
An end-of-file in the shell's input causes it to exit. If the shell is
interactive, this means it exits when the user types an EOT (^D) at the
prompt.
Special commands
The shell treats the following built-in commands specially.
: [arg ...]
Does nothing and sets the exit status to zero.
chdir [dir]
Changes the shell's current working directory to dir. (+) If
dir is an unquoted `-', the shell's previous working directory
is used instead. Otherwise, if dir is not specified, the user's
home directory is used by default.
echo [-n] [string ...] (+)
Writes its string arguments (if any) separated by blanks and
terminated by a newline to the standard output. If `-n' is
specified, the terminating newline is not written.
exec command [arg ...] (+)
Replaces the current shell with an instance of command, which
must be external to the shell.
exit Causes the shell to cease execution of a file. This means exit
has no effect at the prompt of an interactive shell.
fd2 [-e] [-f file] command [arg ...] (+)
Redirects from/to file descriptor 2 for command. See the fd2(1)
manual page for full details.
goto label (+)
Transfers shell control to the `: label' line of the current
command file. See the goto(1) manual page for full details.
if expression [command [arg ...]] (+)
Evaluates expression and conditionally executes command if
appropriate. See the if(1) manual page for full details.
login [arg ...]
Replaces the current interactive shell with login(1).
newgrp [arg ...]
Replaces the current interactive shell with newgrp(1).
setenv name [value] (+)
Sets the environment variable name to the string value. If
value is not specified, the environment variable name is set to
the empty string.
shift Shifts all positional-parameter values to the left by 1, so that
the old value of `$2' becomes the new value of `$1' and so
forth. The value of `$0' does not shift.
sigign [+ | - signal_number ...] (+)
+ causes the specified signals to be ignored if possible, and -
causes the specified signals to be reset to the default action
if possible. If a signal was already ignored when the shell was
invoked, it can never be reset with -. If no arguments are
specified, a list is printed of those signals which are ignored
by sigign in the current shell.
source file [arg1 ...] (+)
Causes the shell to read and execute commands from file and
return. The specified file and any arguments are treated as
positional parameters (see Parameter substitution above) during
execution of the file. The source command may be nested. As
with command files, most shell-detected errors cause the shell
to cease execution of the file. If the source command is nested
and such an error occurs, all nested source commands terminate.
umask [mask] (+)
Sets the file creation mask (see umask(2)) to the octal value
specified by mask. If the mask is not specified, its current
value is printed.
unsetenv name (+)
Removes the variable name from the environment.
wait Waits for all asynchronous processes to terminate, reporting on
abnormal terminations.
Signals (+)
An interactive or login shell always ignores the SIGINT, SIGQUIT, and
SIGTERM signals (see signal(3)). A login shell also handles the SIGHUP
signal, the receipt of which causes the shell to terminate the login
session and to read and execute its rc logout files if possible.
If SIGHUP, SIGINT, SIGQUIT, or SIGTERM is already ignored when the
shell starts, it is also ignored by the current shell and all of its
child processes. Otherwise, SIGINT and SIGQUIT are reset to the
default action for sequential child processes, whereas SIGHUP and
SIGTERM are reset to the default action for all child processes.
When a non-interactive shell executes a command file, it does not han-
dle or ignore any signal by default. Any other non-interactive shell
ignores SIGINT and SIGQUIT.
For any signal not mentioned above, the shell inherits the signal
action (default or ignore) from its parent process and passes it to its
child processes. Remember that the sigign special command may be used
to ignore signals when the shell does not do so by default.
Asynchronous child processes always ignore both SIGINT and SIGQUIT.
Also, if such a process has not redirected its input with a `<', `|',
or `^', the shell automatically redirects it to come from /dev/null.
EXIT STATUS (+)
The exit status of the shell is generally that of the last command exe-
cuted prior to end-of-file or exit.
However, if the shell is interactive and detects an error, it exits
with a non-zero status if the user types an EOT at the next prompt.
Otherwise, if the shell is non-interactive and is reading commands from
a file, any shell-detected error causes the shell to cease execution of
that file. This results in a non-zero exit status.
A non-zero exit status returned by the shell itself is always one of
the values described in the following list, each of which may be accom-
panied by an appropriate diagnostic:
2 The shell detected a syntax, redirection, or other error not
described in this list.
125 An external command was found but did not begin with the proper
magic number or a `#!shell' sequence, and a valid shell was not
specified by EXECSHELL with which to execute it.
126 An external command was found but could not be executed.
127 An external command was not found.
>128 An external command was terminated by a signal.
ENVIRONMENT (+)
Notice that the concept of `user environment' was not defined in Sixth
Edition UNIX. Thus, use of the following environment variables by this
port of the shell is an enhancement:
EXECSHELL
If set to a non-empty string, the value of this variable is
taken as the path name of the shell which is invoked to execute
an external command when it does not begin with the proper magic
number or a `#!shell' sequence. Its value is available to the
shell via the `$e' special parameter.
HOME If set to a non-empty string, the value of this variable is
taken as the user's home directory. Its value is available to
the shell via the `$h' special parameter and is the default
directory for the chdir special command.
OSHDIR If set to a non-empty string, the value of this variable is
taken as the path name of a directory which may be used for tem-
porary files. Its value is available to the shell via the `$d'
special parameter.
PATH If set to a non-empty string, the value of this variable is
taken as the sequence of directories used by the shell to search
for external commands. Its value is available to the shell via
the `$p' special parameter. Notice that the Sixth Edition UNIX
shell always used the equivalent of `.:/bin:/usr/bin', not PATH.
FILES
/dev/null
default source of input for asynchronous processes
/usr/local/etc/osh.login (+)
system-wide rc init file for login shells
/usr/local/etc/osh.oshrc (+)
system-wide rc init file for all interactive shells
$h/.osh.login (+)
user rc init file for login shells
$h/.oshrc (+)
user rc init file for all interactive shells
/usr/local/etc/osh.logout (+)
system-wide rc logout file for login shells
$h/.osh.logout (+)
user rc logout file for login shells
SEE ALSO
awk(1), env(1), expr(1), fd2(1), goto(1), grep(1), if(1), kill(1),
login(1), newgrp(1), sed(1), sh6(1)
Osh home page: http://v6shell.org/
AUTHORS
This enhanced port of the Thompson shell is derived from Sixth Edition
UNIX /usr/source/s2/sh.c, which was principally written by Ken Thompson
of Bell Labs. Jeffrey Allen Neitzel initially ported it in January
2004 and currently maintains it as sh6(1). In addition, he is the
principal developer and maintainer of this enhanced version of the
shell, which is hereby made available as osh(1).
HISTORY
A sh command appeared as /bin/sh in First Edition UNIX.
The Thompson shell was used as the standard command interpreter through
Sixth Edition UNIX. Then, in the Seventh Edition, it was replaced by
the Bourne shell. However, the Thompson shell was still distributed
with the system as osh because of known portability problems with the
Bourne shell's memory management in Seventh Edition UNIX.
LICENSE
See either the LICENSE file which is distributed with osh or
http://v6shell.org/license/ for full details.
NOTES
Unlike the original, this port of the shell can handle 8-bit character
sets, as well as the UTF-8 encoding. The original, on the other hand,
can only handle 7-bit ASCII.
Notice that certain shell oddities were historically undocumented in
this manual page. Particularly noteworthy is the fact that there is no
such thing as a usage error. Thus, the following shell invocations are
perfectly valid:
osh -cats_are_nice!!! ': "Good kitty =)"'
osh -tabbies_are_too!
osh -s
The first two cases correspond to the -c and -t options respectively;
the third case corresponds to the - option.
SECURITY
This port of the Thompson shell does not support being used in a set-ID
context. If the effective user (group) ID of the shell process is not
equal to its real user (group) ID, the shell prints a diagnostic and
exits with a non-zero status. The reasons for this are as follows.
First, the way in which the shell uses positional parameters (see
Parameter substitution above) makes it a simple matter to invoke an
interactive shell from a command file if the user knows the name of the
current terminal (if any). This is distinctly not a bug and can be
very useful in the normal case.
However, if the shell did support set-ID execution, this could possibly
allow a user to violate the security policy on a host where the shell
is used. For example, if the shell were running a setuid-root command
file, a regular user could invoke an interactive root shell as a
result.
BUGS
The shell makes no attempt to recover from read(2) errors and exits if
this system call fails for any reason.
osh-20081122 November 19, 2008 OSH(1)
