ICS 54: Shell Programming I: Brief notes for Chapter 15

Introduction to Shell Programming

For this aspect of UNIX shells, we switch our focus from the C shell, csh, to the Bourne shell, sh.

Shell programming is easy.

• Put the commands you want to execute into a file.
• Make the file executable.
• Use the file's name as a command.

Example:

% cat > dated.list << EOD
# Comments are nice.  This is one
date   # and so is the rest of this line
pwd
ls -l 
EOD
% chmod +x dated.list
./dated.list

Notes

• This assumes we want to use the Bourne shell.
  To use any other shell, make the first line of the file
     #!full path name of shell and options
  Examples:
     #!/usr/bin/csh -f
     #!/usr/bin/sh
• It's standard practice and a very good idea to always always have the first line indicate the shell.
     #!/usr/bin/sh
• We could have skipped step 2 above (making the file executable) and explicitly told the shell to start a new shell to run the commands in the file:
     sh dated.list
• To run short sequences of commands in their own subshell, use parentheses. Example:
     (date; pwd; ls -l) > my.listing
• The Bourne shell also uses "{...}" for grouping commands in the current shell.
• For the Bourne shell, "." (that's right, just dot) is equivalent to the C shell's "source"
• A superb and very important collection of shell programs can be found at /etc/rc?.d
• After a bit of UNIX System Administration, you will not only appreciate these individually, but find special meaning in
     ls -i /etc/rc?.d/* | sort +n

Shell Variables, Revisited

somename=somev

Defines the variable somename to have the value somev

$somename

Replace by value of the variable somename:

${somename:-somev}

Replace by value of the variable somename if it is set; otherwise, use somev

${somename:=somev}

Replace by value of the variable somename if it is set; otherwise, set it to somev and then use that value.

${somename:?message}

Continue of the value of the variable somename is set; otherwise, send (the string) message to standard error, and stop the execution of the shell, returning a value of false (1) to the parent process. If message is the empty string (i.e., ${somename:}), use a standard default error message string.

Example:

#!/bin/sh -x   # Show lines as they are executed
# echo.vars -- Basic sh variables
echo "Inititally, X=\"$X\" and Y=\"$Y\""
X=Alpha
echo "X=\"$X\""
echo '${Y:-"Beta"}'=${Y:-"Beta"}
echo "Y=\"$Y\""
echo '${Y:="Gamma"}'=${Y:="Gamma"}
echo "Y=\"$Y\""

Shell Script Parameters

Examples:

#!/bin/sh
# SN = Switch the Names of 2 files
mv $1 0funnyname.$$
mv $2 $1
mv 0funnyname.$$ $2

########################################################

#!/bin/sh
# echo.args -- sh script echoing its arguments
echo "\n$0 reporting that it has \$# = $# argument(s)"
echo "Arguments (\$*) = $* ====="
shift
echo "After a shift, arguments (\$*) = $* ====="
echo "This has PID (\$\$) = $$\n"

##### Sample Run:
  % ./echo.args this is nice

  ./echo.args reporting that it has $# = 3 argument(s)
  Arguments ($*) = one two three =====
  After a shift, arguments ($*) = two three =====
  This has PID ($$) = 9596

  %

Shell Output and Input

echo  string

The basic output mechanism: Echo/print string.
Special escape sequences in string have special meanings, such as \c for do not end this line and \n for start a newline.

read  vars

The basic input mechanism: Read user input into var

set  string

Set the arguments $1, $2, $3, ..., etc. from string.

Examples:

#!/bin/sh
# read.set -- read in new arguments
echo "$# arguments = $* ==="
echo "New arguments please: \c"
read in
echo "Input = $in ==="
set $in
echo "$# new argument(s) = $* ==="

##### Sample Run:
  % ./read.set what will happen
  3 arguments = what will happen ===
  New arguments please: let us see    what will happen
  Input = let us see    what will happen ===
  6 new argument(s) = let us see what will happen ===

########################################################

#!/bin/sh
# read2in -- read 2 variables
echo "Input please: \c"
read in1 in2
echo "Input1 = $in1 ===\nInput2 = $in2 ==="

##### Sample Run:
  % ./read2in
  Input please: what do you want
  Input1 = what ===
  Input2 = do you want ===

Debugging

Use
   #!/usr/bin/sh -x
or
   sh -x  command arguments