ash(1)
NAME
ash, command, getopts, hash, jobs, local, return, setvar, unset - a shell
SYNOPSIS
ash [ -efIijnsxz ] [ +efIijnsxz ] [ -c command ] [ arg ] ...
COPYRIGHT
Copyright 1989 by Kenneth Almquist.
DESCRIPTION
Ash is a version of sh with features similar to those of the System V
shell. This manual page lists all the features of ash but concentrates
on the ones not in other shells.
Invocation
If the -c options is given, then the shell executes the specified shell
command. The -s flag cause the shell to read commands from the standard
input (after executing any command specified with the -c option. If
neither the -s or -c options are set, then the first arg is taken as the
name of a file to read commands from. If this is impossible because
there are no arguments following the options, then ash will set the -s
flag and will read commands from the standard input.
The shell sets the initial value of the positional parameters from the
args remaining after any arg used as the name of a file of commands is
deleted.
The flags (other than -c) are set by preceding them with ``-'' and
cleared by preceding them with ``+''; see the set builtin command for a
list of flags. If no value is specified for the -i flag, the -s flag is
set, and the standard input and output of the shell are connected to
terminals, then the -i flag will be set. If no value is specified for
the -j flag, then the -j flag will be set if the -i flag is set.
When the shell is invoked with the -c option, it is good practice to
include the -i flag if the command was entered interactively by a user.
For compatibility with the System V shell, the -i option should come
after the -c option.
If the first character of argument zero to the shell is ``-'', the shell
is assumed to be a login shell, and the files /etc/profile and .profile
are read if they exist. If the environment variable SHINIT is set on
entry to the shell, the commands in SHINIT are normally parsed and
executed. SHINIT is not examined if the shell is a login shell, or if it
the shell is running a shell procedure. (A shell is considered to be
running a shell procedure if neither the -s nor the -c options are set.)
Control Structures
A list is a sequence of zero or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these
three characters. (This differs from the System V shell, which requires
a list to contain at least one command in most cases.) The commands in a
list are executed in the order they are written. If command is followed
by an ampersand, the shell starts the command and immediately proceed
onto the next command; otherwise it waits for the command to terminate
before proceeding to the next one.
``&&'' and ``||'' are binary operators. ``&&'' executes the first
command, and then executes the second command iff the exit status of the
first command is zero. ``||'' is similar, but executes the second
command iff the exit status of the first command is nonzero. ``&&'' and
``||'' both have the same priority.
The ``|'' operator is a binary operator which feeds the standard output
of the first command into the standard input of the second command. The
exit status of the ``|'' operator is the exit status of the second
command. ``|'' has a higher priority than ``||'' or ``&&''.
An if command looks like
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
A while command looks like
while list
do list
done
The two lists are executed repeatedly while the exit status of the first
list is zero. The until command is similar, but has the word until in
place of while
repeats until the exit status of the first list is zero.
The for command looks like
for variable in word...
do list
done
The words are expanded, and then the list is executed repeatedly with the
variable set to each word in turn. do and done may be replaced with
``{'' and ``}''.
The break and continue commands look like
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue
continues with the next iteration of the num'th innermost loop. These
are implemented as builtin commands.
The case command looks like
case word in
pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see Patterns below),
separated by ``|'' characters.
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell.
A function definition looks like
name ( ) command
A function definition is an executable statement; when executed it
installs a function named name and returns an exit status of zero. The
command is normally a list enclosed between ``{'' and ``}''.
Variables may be declared to be local to a function by using a local
command. This should appear as the first staement of a function, and
looks like
local [ variable | - ] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and exported
and readonly flags from the variable with the same name in the
surrounding scope, if there is one. Otherwise, the variable is initially
unset. Ash uses dynamic scoping, so that if you make the variable x
local to function f, which then calls function g, references to the
variable x made inside g will refer to the variable x declared inside f,
not to the global variable named x.
The only special parameter than can be made local is ``-''. Making ``-''
local any shell options that are changed via the set command inside the
function to be restored to their original values when the function
returns.
The return command looks like
return [ exitstatus ]
It terminates the currently executing function. Return is implemented as
a builtin command.
Simple Commands
A simple command is a sequence of words. The execution of a simple
command proceeds as follows. First, the leading words of the form
``name=value'' are stripped off and assigned to the environment of the
command. Second, the words are expanded. Third, the first remaining
word is taken as the command name that command is located. Fourth, any
redirections are performed. Fifth, the command is executed. We look at
these operations in reverse order.
The execution of the command varies with the type of command. There are
three types of commands: shell functions, builtin commands, and normal
programs.
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the parameters to the
shell function. The variables which are explicitly placed in the
environment of the command (by placing assignments to them before the
function name) are made local to the function and are set to values
given. Then the command given in the function definition is executed.
The positional parameters are restored to their original values when the
command completes.
Shell builtins are executed internally to the shell, without spawning a
new process.
When a normal program is executed, the shell runs the program, passing
the parameters and the environment to the program. If the program is a
shell procedure, the shell will interpret the program in a subshell. The
shell will reinitialize itself in this case, so that the effect will be
as if a new shell had been invoked to handle the shell procedure, except
that the location of commands located in the parent shell will be
remembered by the child. If the program is a file beginning with ``#!'',
the remainder of the first line specifies an interpreter for the program.
The shell (or the operating system, under Berkeley UNIX) will run the
interpreter in this case. The arguments to the interpreter will consist
of any arguments given on the first line of the program, followed by the
name of the program, followed by the arguments passed to the program.
Redirection
Input/output redirections can be intermixed with the words in a simple
command and can be placed following any of the other commands. When
redirection occurs, the shell saves the old values of the file
descriptors and restores them when the command completes. The ``<'',
``>'', and ``>>'' redirections open a file for input, output, and
appending, respectively. The ``<&digit'' and ``>&digit'' makes the input
or output a duplicate of the file descriptor numbered by the digit. If a
minus sign is used in place of a digit, the standard input or standard
output are closed.
The ``<< word'' redirection takes input from a here document. As the
shell encounters ``<<'' redirections, it collects them. The next time it
encounters an unescaped newline, it reads the documents in turn. The
word following the ``<<'' specifies the contents of the line that
terminates the document. If none of the quoting methods ('', "", or \)
are used to enter the word, then the document is treated like a word
inside double quotes: ``$'' and backquote are expanded and backslash can
be used to escape these and to continue long lines. The word cannot
contain any variable or command substitutions, and its length (after
quoting) must be in the range of 1 to 79 characters. If ``<<-'' is used
in place of ``<<'', then leading tabs are deleted from the lines of the
document. (This is to allow you do indent shell procedures containing
here documents in a natural fashion.)
Any of the preceding redirection operators may be preceded by a single
digit specifying the file descriptor to be redirected. There cannot be
any white space between the digit and the redirection operator.
Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name. Then, if PATH does not contain an entry for
"%builtin", it looks for a builtin command by that name. Finally, it
searches each entry in PATH in turn for the command.
The value of the PATH variable should be a series of entries separated by
colons. Each entry consists of a directory name, or a directory name
followed by a flag beginning with a percent sign. The current directory
should be indicated by an empty directory name.
If no percent sign is present, then the entry causes the shell to search
for the command in the specified directory. If the flag is ``%builtin''
then the list of shell builtin commands is searched. If the flag is
``%func'' then the directory is searched for a file which is read as
input to the shell. This file should define a function whose name is the
name of the command being searched for.
Command names containing a slash are simply executed without performing
any of the above searches.
The Environment
The environment of a command is a set of name/value pairs. When the
shell is invoked, it reads these names and values, sets the shell
variables with these names to the corresponding values, and marks the
variables as exported. The export command can be used to mark additional
variables as exported.
The environment of a command is constructed by constructing name/value
pairs from all the exported shell variables, and then modifying this set
by the assignments which precede the command, if any.
Expansion
The process of evaluating words when a shell procedure is executed is
called expansion. Expansion consists of four steps: variable
substitution, command substitution, word splitting, and file name
generation. If a word is the expression following the word case in a
case statement, the file name which follows a redirection symbol, or an
assignment to the environment of a command, then the word cannot be split
into multiple words. In these cases, the last two steps of the expansion
process are omitted.
Variable Substitution
To be written.
Command Substitution
Ash accepts two syntaxes for command substitution:
`list`
and
$(list)
Either of these may be included in a word. During the command
substitution process, the command (syntactly a list) will be executed and
anything that the command writes to the standard output will be captured
by the shell. The final newline (if any) of the output will be deleted;
the rest of the output will be substituted for the command in the word.
Word Splitting
When the value of a variable or the output of a command is substituted,
the resulting text is subject to word splitting, unless the dollar sign
introducing the variable or backquotes containing the text were enclosed
in double quotes. In addition, ``$@'' is subject to a special type of
splitting, even in the presence of double quotes.
Ash uses two different splitting algorithms. The normal approach, which
is intended for splitting text separated by which space, is used if the
first character of the shell variable IFS is a space. Otherwise an
alternative experimental algorithm, which is useful for splitting
(possibly empty) fields separated by a separator character, is used.
When performing splitting, the shell scans the replacement text looking
for a character (when IFS does not begin with a space) or a sequence of
characters (when IFS does begin with a space), deletes the character or
sequence of characters, and spits the word into two strings at that
point. When IFS begins with a space, the shell deletes either of the
strings if they are null. As a special case, if the word containing the
replacement text is the null string, the word is deleted.
The variable ``$@'' is special in two ways. First, splitting takes place
between the positional parameters, even if the text is enclosed in double
quotes. Second, if the word containing the replacement text is the null
string and there are no positional parameters, then the word is deleted.
The result of these rules is that "$@" is equivalent to "$1" "$2" ...
"$n", where n is the number of positional parameters. (Note that this
differs from the System V shell. The System V documentation claims that
"$@" behaves this way; in fact on the System V shell "$@" is equivalent
to "" when there are no positional paramteters.)
File Name Generation
Unless the -f flag is set, file name generation is performed after word
splitting is complete. Each word is viewed as a series of patterns,
separated by slashes. The process of expansion replaces the word with
the names of all existing files whose names can be formed by replacing
each pattern with a string that matches the specified pattern. There are
two restrictions on this: first, a pattern cannot match a string
containing a slash, and second, a pattern cannot match a string starting
with a period unless the first character of the pattern is a period.
If a word fails to match any files and the -z flag is not set, then the
word will be left unchanged (except that the meta-characters will be
converted to normal characters). If the -z flag is set, then the word is
only left unchanged if none of the patterns contain a character that can
match anything besides itself. Otherwise the -z flag forces the word to
be replaced with the names of the files that it matches, even if there
are zero names.
Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are ``!'', ``*'', ``?'', and ``[''.
These characters lose there special meanings if they are quoted. When
command or variable substitution is performed and the dollar sign or back
quotes are not double quoted, the value of the variable or the output of
the command is scanned for these characters and they are turned into
meta-characters.
Two exclamation points at the beginning of a pattern function as a
``not'' operator, causing the pattern to match any string that the
remainder of the pattern does not match. Other occurances of exclamation
points in a pattern match exclamation points. Two exclamation points are
required rather than one to decrease the incompatibility with the System
V shell (which does not treat exclamation points specially).
An asterisk (``*'') matches any string of characters. A question mark
matches any single character. A left bracket (``['') introduces a
character class. The end of the character class is indicated by a ``]'';
if the ``]'' is missing then the ``['' matches a ``['' rather than
introducing a character class. A character class matches any of the
characters between the square brackets. A range of characters may be
specified using a minus sign. The character class may be complemented by
making an exclamation point the first character of the character class.
To include a ``]'' in a character class, make it the first character
listed (after the ``!'', if any). To include a minus sign, make it the
first or last character listed.
The /u Directory
By convention, the name ``/u/user'' refers to the home directory of the
specified user. There are good reasons why this feature should be
supported by the file system (using a feature such as symbolic links)
rather than by the shell, but ash is capable of performing this mapping
if the file system doesn't. If the mapping is done by ash, setting the
-f flag will turn it off.
Character Set
Ash silently discards nul characters. Any other character will be
handled correctly by ash, including characters with the high order bit
set.
Job Names and Job Control
The term job refers to a process created by a shell command, or in the
case of a pipeline, to the set of processes in the pipeline. The ways to
refer to a job are:
%number
%string
%%
process_id
The first form identifies a job by job number. When a command is run,
ash assigns it a job number (the lowest unused number is assigned). The
second form identifies a job by giving a prefix of the command used to
create the job. The prefix must be unique. If there is only one job,
then the null prefix will identify the job, so you can refer to the job
by writing ``%''. The third form refers to the current job. The current
job is the last job to be stopped while it was in the foreground. (See
the next paragraph.) The last form identifies a job by giving the
process id of the last process in the job.
If the operating system that ash is running on supports job control, ash
will allow you to use it. In this case, typing the suspend character
(typically ^Z) while running a command will return you to ash and will
make the suspended command the current job. You can then continue the
job in the background by typing bg, or you can continue it in the
foreground by typing fg.
Atty
If the shell variable ATTY is set, and the shell variable TERM is not set
to ``emacs'', then ash generates appropriate escape sequences to talk to
atty(1).
Exit Statuses
By tradition, an exit status of zero means that a command has succeeded
and a nonzero exit status indicates that the command failed. This is
better than no convention at all, but in practice it is extremely useful
to allow commands that succeed to use the exit status to return
information to the caller. A variety of better conventions have been
proposed, but none of them has met with universal approval. The
convention used by ash and all the programs included in the ash
distribution is as follows:
0 Success.
1 Alternate success.
2 Failure.
129-... Command terminated by a signal.
The alternate success return is used by commands to indicate various
conditions which are not errors but which can, with a little imagination,
be conceived of as less successful than plain success. For example, test
returns 1 when the tested condition is false and getopts returns 1 when
there are no more options. Because this convention is not used
universally, the -e option of ash causes the shell to exit when a command
returns 1 even though that contradicts the convention described here.
When a command is terminated by a signal, the uses 128 plus the signal
number as the exit code for the command.
Builtin Commands
This concluding section lists the builtin commands which are builtin
because they need to perform some operation that can't be performed by a
separate process. In addition to these, there are several other commands
(catf, echo, expr, line, nlecho, test, ``:'', and true) which can
optionally be compiled into the shell. The builtin commands described
below that accept options use the System V Release 2 getopt(3) syntax.
bg [ job ] ...
Continue the specified jobs (or the current job if no jobs are given) in
the background. This command is only available on systems with Bekeley
job control.
command command arg...
Execute the specified builtin command. (This is useful when you have a
shell function with the same name as a builtin command.)
cd [ directory ]
Switch to the specified directory (default $HOME). If the an entry for
CDPATH appears in the environment of the cd command or the shell variable
CDPATH is set and the directory name does not begin with a slash, then
the directories listed in CDPATH will be searched for the specified
directory. The format of CDPATH is the same as that of PATH. In an
interactive shell, the cd command will print out the name of the
directory that it actually switched to if this is different from the name
that the user gave. These may be different either because the CDPATH
mechanism was used or because a symbolic link was crossed.
. file
The commands in the specified file are read and executed by the shell. A
path search is not done to find the file because the directories in PATH
generally contain files that are intended to be executed, not read.
eval string...
The strings are parsed as shell commands and executed. (This differs
from the System V shell, which concatenates the arguments (separated by
spaces) and parses the result as a single command.)
exec [ command arg... ]
Unless command is omitted, the shell process is replaced with the
specified program (which must be a real program, not a shell builtin or
function). Any redirections on the exec command are marked as permanent,
so that they are not undone when the exec command finishes. If the
command is not found, the exec command causes the shell to exit.
exit [ exitstatus ]
Terminate the shell process. If exitstatus is given it is used as the
exit status of the shell; otherwise the exit status of the preceding
command is used.
export name...
The specified names are exported so that they will appear in the
environment of subsequent commands. The only way to un-export a variable
is to unset it. Ash allows the value of a variable to be set at the same
time it is exported by writing
export name=value
With no arguments the export command lists the names of all exported
variables.
fg [ job ]
Move the specified job or the current job to the foreground. This
command is only available on systems with Bekeley job control.
getopts optstring var
The System V getopts command.
hash -rv command...
The shell maintains a hash table which remembers the locations of
commands. With no arguments whatsoever, the hash command prints out the
contents of this table. Entries which have not been looked at since the
last cd command are marked with an asterisk; it is possible for these
entries to be invalid.
With arguments, the hash command removes the specified commands from the
hash table (unless they are functions) and then locates them. With the
-v option, hash prints the locations of the commands as it finds them.
The -r option causes the hash command to delete all the entries in the
hash table except for functions.
jobid [ job ]
Print the process id's of the processes in the job. If the job argument
is omitted, use the current job.
jobs
This command lists out all the background processes which are children of
the current shell process.
pwd
Print the current directory. The builtin command may differ from the
program of the same name because the builtin command remembers what the
current directory is rather than recomputing it each time. This makes it
faster. However, if the current directory is renamed, the builtin
version of pwd will continue to print the old name for the directory.
read [ -p prompt ] [ -e ] variable...
The prompt is printed if the -p option is specified and the standard
input is a terminal. Then a line is read from the standard input. The
trailing newline is deleted from the line and the line is split as
described in the section on word splitting above, and the pieces are
assigned to the variables in order. If there are more pieces than
variables, the remaining pieces (along with the characters in IFS that
separated them) are assigned to the last variable. If there are more
variables than pieces, the remaining variables are assigned the null
string.
The -e option causes any backslashes in the input to be treated
specially. If a backslash is followed by a newline, the backslash and
the newline will be deleted. If a backslash is followed by any other
character, the backslash will be deleted and the following character will
be treated as though it were not in IFS, even if it is.
readonly name...
The specified names are marked as read only, so that they cannot be
subsequently modified or unset. Ash allows the value of a variable to be
set at the same time it is marked read only by writing
readonly name=value
With no arguments the readonly command lists the names of all read only
variables.
set [ { -options | +options | -- } ] arg...
The set command performs three different functions.
With no arguments, it lists the values of all shell variables.
If options are given, it sets the specified option flags, or clears them
if the option flags are introduced with a + rather than a -. Only the
first argument to set can contain options. The possible options are:
-e Causes the shell to exit when a command terminates with a nonzero
exit status, except when the exit status of the command is explicitly
tested. The exit status of a command is considered to be explicitly
tested if the command is used to control an if, elif, while, or
until; or if the command is the left hand operand of an ``&&'' or
``||'' operator.
-f Turn off file name generation.
-I Cause the shell to ignore end of file conditions. (This doesn't
apply when the shell a script sourced using the ``.'' command.) The
shell will in fact exit if it gets 50 eof's in a row.
-i Make the shell interactive. This causes the shell to prompt for
input, to trap interrupts, to ignore quit and terminate signals, and
to return to the main command loop rather than exiting on error.
-j Turns on Berkeley job control, on systems that support it. When the
shell starts up, the -j is set by default if the -i flag is set.
-n Causes the shell to read commands but not execute them. (This is
marginally useful for checking the syntax of scripts.)
-s If this flag is set when the shell starts up, the shell reads
commands from its standard input. The shell doesn't examine the
value of this flag any other time.
-x If this flag is set, the shell will print out each command before
executing it.
-z If this flag is set, the file name generation process may generate
zero files. If it is not set, then a pattern which does not match
any files will be replaced by a quoted version of the pattern.
The third use of the set command is to set the values of the shell's
positional parameters to the specified args. To change the positional
parameters without changing any options, use ``--'' as the first argument
to set. If no args are present, the set command will leave the value of
the positional parameters unchanged, so to set the positional parameters
to set of values that may be empty, execute the command
shift $#
first to clear out the old values of the positional parameters.
setvar variable value
Assigns value to variable. (In general it is better to write
variable=value rather than using setvar. Setvar is intended to be used
in functions that assign values to variables whose names are passed as
parameters.)
shift [ n ]
Shift the positional parameters n times. A shift sets the value of $1 to
the value of $2, the value of $2 to the value of $3, and so on,
decreasing the value of $# by one. If there are zero positional
parameters, shifting doesn't do anything.
trap [ action ] signal...
Cause the shell to parse and execute action when any of the specified
signals are received. The signals are specified by signal number.
Action may be null or omitted; the former causes the specified signal to
be ignored and the latter causes the default action to be taken. When
the shell forks off a subshell, it resets trapped (but not ignored)
signals to the default action. The trap command has no effect on signals
that were ignored on entry to the shell.
umask [ mask ]
Set the value of umask (see umask(2)) to the specified octal value. If
the argument is omitted, the umask value is printed.
unset name...
The specified variables and functions are unset and unexported. If a
given name corresponds to both a variable and a function, both the
variable and the function are unset.
wait [ job ]
Wait for the specified job to complete and return the exit status of the
last process in the job. If the argument is omitted, wait for all jobs
to complete and the return an exit status of zero.
EXAMPLES
The following function redefines the cd command:
cd() {
if command cd "$@"
then if test -f .enter
then . .enter
else return 0
fi
fi
}
This function causes the file ``.enter'' to be read when you enter a
directory, if it exists. The command command is used to access the real
cd command. The ``return 0'' ensures that the function will return an
exit status of zero if it successfully changes to a directory that does
not contain a ``.enter'' file. Redefining existing commands is not
always a good idea, but this example shows that you can do it if you want
to.
The suspend function distributed with ash looks like
# Copyright (C) 1989 by Kenneth Almquist. All rights reserved.
# This file is part of ash, which is distributed under the terms
# specified by the Ash General Public License.
suspend() {
local -
set +j
kill -TSTP 0
}
This turns off job control and then sends a stop signal to the current
process group, which suspends the shell. (When job control is turned on,
the shell ignores the TSTP signal.) Job control will be turned back on
when the function returns because ``-'' is local to the function. As an
example of what not to do, consider an earlier version of suspend:
suspend() {
suspend_flag=$-
set +j
kill -TSTP 0
set -$suspend_flag
}
There are two problems with this. First, suspend_flag is a global
variable rather than a local one, which will cause problems in the
(unlikely) circumstance that the user is using that variable for some
other purpose. Second, consider what happens if shell received an
interrupt signal after it executes the first set command but before it
executes the second one. The interrupt signal will abort the shell
function, so that the second set command will never be executed and job
control will be left off. The first version of suspend avoids this
problem by turning job control off only in a local copy of the shell
options. The local copy of the shell options is discarded when the
function is terminated, no matter how it is terminated.
HINTS
Shell variables can be used to provide abbreviations for things which you
type frequently. For example, I set
export h=$HOME
in my .profile so that I can type the name of my home directory simply by
typing ``$h''.
When writing shell procedures, try not to make assumptions about what is
imported from the environment. Explicitly unset or initialize all
variables, rather than assuming they will be unset. If you use cd, it is
a good idea to unset CDPATH.
People sometimes use ``<&-'' or ``>&-'' to provide no input to a command
or to discard the output of a command. A better way to do this is to
redirect the input or output of the command to /dev/null.
Word splitting and file name generation are performed by default, and you
have to explicitly use double quotes to suppress it. This is backwards,
but you can learn to live with it. Just get in the habit of writing
double quotes around variable and command substitutions, and omit them
only when you really want word splitting and file name generation. If
you want word splitting but not file name generation, use the -f option.
AUTHORS
Kenneth Almquist
SEE ALSO
echo(1), expr(1), line(1), pwd(1), true(1).
BUGS
When command substitution occurs inside a here document, the commands
inside the here document are run with their standard input closed. For
example, the following will not word because the standard input of the
line command will be closed when the command is run:
cat <<-!
Line 1: $(line)
Line 2: $(line)
!
Unsetting a function which is currently being executed may cause strange
behavior.
The shell syntax allows a here document to be terminated by an end of
file as well as by a line containing the terminator word which follows
the ``<<''. What this means is that if you mistype the terminator line,
the shell will silently swallow up the rest of your shell script and
stick it in the here document.