Chapter 2. Starting Off With a Sha-Bang

Table of Contents
2.1. Invoking the script
2.2. Preliminary Exercises

In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked.

Example 2-1. cleanup: A script to clean up the log files in /var/log

# cleanup
# Run as root, of course.

cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."

There is nothing unusual here, just a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script can easily be modified, customized, or generalized for a particular application.

Example 2-2. cleanup: An enhanced and generalized version of above script.

#!/bin/bash
# cleanup, version 2
# Run as root, of course.

LOG_DIR=/var/log
ROOT_UID=0     # Only users with $UID 0 have root privileges.
LINES=50       # Default number of lines saved.
E_XCD=66       # Can't change directory?
E_NOTROOT=67   # Non-root exit error.


if [ "$UID" -ne "$ROOT_UID" ]
then
  echo "Must be root to run this script."
  exit $E_NOTROOT
fi  

if [ -n "$1" ]
# Test if command line argument present (non-empty).
then
  lines=$1
else  
  lines=$LINES # Default, if not specified on command line.
fi  


#  Stephane Chazelas suggests the following,
#+ as a better way of checking command line arguments,
#+ but this is still a bit advanced for this stage of the tutorial.
#
#    E_WRONGARGS=65  # Non-numerical argument (bad arg format)
#
#    case "$1" in
#    ""      ) lines=50;;
#    *[!0-9]*) echo "Usage: `basename $0` file-to-cleanup"; exit $E_WRONGARGS;;
#    *       ) lines=$1;;
#    esac
#
#* Skip ahead to "Loops" to understand this.


cd $LOG_DIR

if [ `pwd` != "$LOG_DIR" ]  # or   if [ "$PWD" != "LOG_DIR" ]
                            # Not in /var/log?
then
  echo "Can't change to $LOG_DIR."
  exit $E_XCD
fi  # Doublecheck if in right directory, before messing with log file.

# far better is:
# ---
# cd /var/log || {
#   echo "Cannot change to necessary directory." >&2
#   exit $E_XCD;
# }




tail -$lines messages > mesg.temp # Saves last section of message log file.
mv mesg.temp messages             # Becomes new log directory.


# cat /dev/null > messages
#* No longer needed, as the above method is safer.

cat /dev/null > wtmp  # > wtemp   has the same effect.
echo "Logs cleaned up."

exit 0
#  A zero return value from the script upon exit
#+ indicates success to the shell.

Since you may not wish to wipe out the entire system log, this variant of the first script keeps the last section of the message log intact. You will constantly discover ways of refining previously written scripts for increased effectiveness.

The sha-bang ( #!) at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte [1] "magic number", a special marker that designates a file type, or in this case an executable shell script (see man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (line 1 of the script), ignoring comments. [2]

#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/usr/awk -f

Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. [3] Using #!/bin/sh, the default Bourne Shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you may have to sacrifice a few Bash-specific features (the script will conform to the POSIX [4] sh standard).

Note that the path given at the "sha-bang" must be correct, otherwise an error message, usually "Command not found" will be the only result of running the script.

#! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. Example 2, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct. Note that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine.

Important

This tutorial encourages a modular approach to constructing a script. Make note of and collect "boilerplate" code snippets that might be useful in future scripts. Eventually you can build a quite extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters.
if [ $# -ne Number_of_expected args ]
then
  echo "Usage: `basename $0` whatever"
  exit $WRONG_ARGS
fi

Notes

[1]

Some flavors of UNIX (those based on 4.2BSD) take a four-byte magic number, requiring a blank after the !, #! /bin/sh.

[2]

The #! line in a shell script will be the first thing the command interpreter (sh or bash) sees. Since this line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally executes the script. The line has already served its purpose - calling the command interpreter.

[3]

This allows some cute tricks.

#!/bin/rm
# Self-deleting script.

# Nothing much seems to happen when you run this... except that the file disappears.

WHATEVER=65

echo "This line will never print (betcha!)."

exit $WHATEVER  # Doesn't matter. The script will not exit here.

Also, try starting a README file with a #!/bin/more, and making it executable. The result is a self-listing documentation file.

[4]

Portable Operating System Interface, an attempt to standardize UNIX-like OSes.