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SH ST-UNIX User's Manual SH
COMMAND
sh - comamnd line interpreter
FORMAT
sh [ -enstuvx ] [ file ] ...
DESCRIPTION
_✓S_✓h is a command line interpreter nterface to GEMDOS. It per-
mits the typing of commands from the keyboard and replaces
the GEM desktop.
The shell provides features found in programming languages
such as varaibles and loops.
OPTIONS
-e if the shell is not in interactive mode (ie. commands
are being read from a file) the shell exits if a com-
mand fails.
-n commands are read, and variable substitution takes
place, but the commands are not executed.
-t shell exits after reading and executing one command.
-u treat unset variables as an error when substituting.
-v print input lines as they are read.
-x print the command line before it is executed.
- turn off -x and -v options
COMMANDS
Command consist of a sequence of words. The first word is
the command name and the tail is the arguments to the com-
mand. More than one command can be placed on a line, a ';'
separates the commands.
Conditional execution of commands can be formed be separat-
ing the command with the operators '&&' and '||', for 'and'
and 'or' resepctively. Logical 'and', if the execution of
the first command is successful the second one proceeds,
else the rest of the command line is terminated. Logical
'or', the second command is executed only if the first one
fails.
REPETITION AND CONDITIONAL EXECUTION OF COMMANDS
A number of inbuilt commands are provided for performing
loops and conditional execution. Loops
Three types of loop control are provided.
Printed 10/April/1987 1st April 1987 1
SH ST-UNIX User's Manual SH
for name [in word ...] do command-list done
For every iteration of the loop, name is setto the next
word in the word list. If the 'in' command and word
list are ommited the variable '$@' (see later) is used.
Execution of the command list terminates when ther are
no more words in the list.
This example displays the names of all files with the
extension '.c':
for i in *.c
do
echo File: $i
done
while command-list1 [ do command-list2 ] done
The while command executes command-list1 and if the
last command in the list returns true command-list2 is
executed. Execution continues until command-list1
fails.
The following example asks the user for a name of a
file to edit and then invokes the editor. When the edit
is complete the user is asked for another file name.
This continues indefinitly.
while true
do
echo -n File to edit
read filename
edit $filename
done
if command-list1 then list1 [ elif command-list2 then list2
] ... [ else list ] fi
If executes command-list1, if the last command in the
list returns true then list1 is executed. If the com-
mand fails control passes to elif, then, or if returns.
The example shows the printing of a filename is the
file exists.
Printed 10/April/1987 1st April 1987 2
SH ST-UNIX User's Manual SH
echo -n File /?
read filename
if test -f $filename
then
echo $filename
done
case word in [ pattern ) command-list ;; ] ... esac
The case command executes the command-list associated
withthe matching of word with pattern
The example shows how to change the name of a file.
echo -n File /?
read filename
case $filename in
a.c) filename=b.c;;
c.c) filename=b.c;;
esac
VARIABLES
The shell provides variables as in conventional languages.
Variables are assigned a value with the '=' operator. The
varaible name is composed of a sequence of letters, digits,
or characters * @ # ? $ !.
var-name=string
Variable substitution on the command line is performed using
the '$' operator.
$variable=a long string
$echo ${variable}
a long string
$
The '{' and '}' braces in the example may be ommited if spe-
cial characters are not contained in the variable name. The
special characters are discussed.
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SH ST-UNIX User's Manual SH
${varname-word} If the variable is set (has been
assigned a value) it's value is substituted, else the
word is substituted.
${varname=word} If the variable is not set, it is set
to word, and it's value substituted.
${varname?word} If the variable is not set the word is
printed and the shell exits.
${varname+word} If the variable is set, word is substi-
tuted, otherwise nothing is substituted.
There are a number of internal parameters to the shell.
These can not be changed by assignment.
number The shell argument corresponding to number. For
example $0 is the command name 'sh'.
# The number of parameters to the shell.
- The options supplied to the shell.
? The value of the error returned by the last command
executed.
The shell has defined some variables which are accessable to
the user.
HOME The default directory used by 'cd' when no argu-
ment is supplied.
PATH The search path used in lokating a command to be
executed.
PS1 PS2 The prompt string, PS2 is used when a partialy
completed command is typed.
QUOTING OF STRINGS ON THE COMMAND
The interpretation of characters in the command line can be
changed by the use of quotes. The quotes ' and " are passed
unaltered to the command for it's interpretation. The '
quote passes everything unaltered, but the " quote inter-
prets $ as variable expansion. be not interpreted. This is
useful for inserting special characters, syuch as '$', into
the command line, and continueing very long lines (as the
newline character is ignored). A string in the backquotes ``
is executed as a command and the output of that command
inserted into the command line. For example:
pathname=`pwd`
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SH ST-UNIX User's Manual SH
set the variable 'pathname' to the current directory.
INPUT-OUTPUT REDIRECTION
The input and output of a command can be redirected using
the operators
<filename makes the command take it's input from the
file 'filename'.
>filename will cause the command to send its output to
the file 'filename' rather than a file.
command1 | command2 pipelines of commands can be formed
using the '|' operator. A pipline allows the output of
one command to form the input of another. For example,
command1 will send it's output to form the input for
command2.
COMMAND EXECTION
When the command line has be processed (variable substitu-
tion has taken place) then command is executed. The variable
PATH is used in locating the position of the command on the
disc. The pathnames in the variable list are searched for
the command, and if it is found the command is executed. The
separator ':' is used to separate pathnames in the list.
Before the command is executed it's type is checked. If the
file is a binary then the command, arguments, and environ-
ment are passed to GEMDOS for execution. If the file is
text, the shell takes the lines as input and executes them.
The environment is made up of the 'exported' variables in
the shell. The format of the environment passed to the com-
mand is a list of null terminated strings which is ter-
minated by a null.
COMMAND HISTORY AND COMMAND LINE EDITING
The history mechanism allows previously typed commands to be
used without having to retype the whole line again. If the
UP-ARROW key is pressed once the previous command type is
displayed. The key may be pressed upto the first command
typed. Pressing the DOWN-ARROW key lets you step forward
from the current command typed.
Command lines may be edited using the left and right arrow
keys. Moving the cursor over a previously typed character
allows text to be inserted before the cursor.
There are a number of editing keys available:
^I - TAB inserts spaces upto the next tab position in
the line. Tabs are placed at every eight characters.
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SH ST-UNIX User's Manual SH
^U deletes the whole line.
^W deletes the previous word.
BS - backspace deletes the character before the cursor.
DEL deletes the character under the cursor.
INBUILT SHELL COMMANDS
The following comamnds a special to the shell and are exe-
cuted internaly.
: This command does nothing except evaluate it's argu-
ments. It can be used as a comment.
break Will cause termination of a for or while loop.
clear Clears the screen.
continue Execution continues on the next iteration of
the for or while loop.
cd pathname The working directory is changed to path-
name.
eval command Command is executed
exec command Same as eval.
exit [error] The shell is terminated. Error is a number
which is returned if defined, default is zero. The
shell can also be terminated by typing ^D to the
prompt.
export [varname ] The variable is marked for exporta-
tion to an executing command. If no variable is sup-
plied a list of the exportable variables is displayed.
history Displays the history of command lines for the
last 20 commands executed.
read name ... A line is read from the keyboard and
it's value assigned to the variable. If more than one
variable is specified each word is assigned to each
variable, with any remaining words assigned to the last
varibale.
readonly [ varname ] The variable is marked as
readonly. It's value cannot be changed by subsequent
assignment. If no variable is supplied a list of
readonly variables is displayed.
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SH ST-UNIX User's Manual SH
set [-entuvx] Allows the setting of argument flags.
These are defined at the beginning of the document. If
no argument is supplied the values of the variables are
displayed.
shift The shell arguments are renamed. For example
argument $2 becomes $1
times command The execution time for command is
displayed.
Printed 10/April/1987 1st April 1987 7
