Chapter 2. Starting Off With a Sha-Bang

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.

# Cleanup# Run as root, of course.cd /var/logcat /dev/null > messagescat /dev/null > wtmpecho "Log files cleaned up."

There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one from the command-line on the console or in a terminal window. The advantages of placing the commands in a script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it can easily be modified or customized for a particular application.

#!/bin/bash# Proper header for a Bash script.# Cleanup, version 2# Run as root, of course.# Insert code here to print error message and exit if not root.LOG_DIR=/var/log# Variables are better than hard-coded values.cd $LOG_DIRcat /dev/null > messagescat /dev/null > wtmpecho "Logs cleaned up." exit #  The right and proper method of "exiting" from a script. #  A bare "exit" (no parameter) returns the exit status #+ of the preceding command. 

Now that's beginning to look like a real script. But we can go even farther . . .

#!/bin/bash# Cleanup, version 3#  Warning:#  -------#  This script uses quite a number of features that will be explained#+ later on.#  By the time you've finished the first half of the book,#+ there should be nothing mysterious about it.LOG_DIR=/var/logROOT_UID=0 # Only users with $UID 0 have root privileges.LINES=50   # Default number of lines saved.E_XCD=86   # Can't change directory?E_NOTROOT=87   # Non-root exit error.# Run as root, of course.if [ "$UID" -ne "$ROOT_UID" ]then  echo "Must be root to run this script."   exit $E_NOTROOTfi  if [ -n "$1" ]# Test whether command-line argument is present (non-empty).then  lines=$1else lines=$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=85  # Non-numerical argument (bad argument format).## case "$1" in# ""  ) lines=50;# *[!0-9]*) echo "Usage: `basename $0` lines-to-cleanup";# exit $E_WRONGARGS;# *   ) lines=$1;# esac##* Skip ahead to "Loops" chapter to decipher all this.cd $LOG_DIRif [ `pwd` != "$LOG_DIR" ]  # or   if [ "$PWD" != "$LOG_DIR" ] # Not in /var/log?then  echo "Can't change to $LOG_DIR."   exit $E_XCDfi  # Doublecheck if in right directory before messing with log file.# Far more efficient is:## cd /var/log || {#   echo "Cannot change to necessary directory." >&2#   exit $E_XCD;# }tail -n $lines messages > mesg.temp # Save last section of message log file.mv mesg.temp messages   # Rename it as system log file.#  cat /dev/null > messages#* No longer needed, as the above method is safer.cat /dev/null > wtmp  #  ': > wtmp' and '> wtmp'  have the same effect.echo "Log files cleaned up." #  Note that there are other log files in /var/log not affected#+ by this script.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 version of the script keeps the last section of the message log intact. You will constantly discover ways of fine-tuning previously written scripts for increased effectiveness.

* * *

The sha-bang ( #!) [1] 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 [2]magic number, a special marker thatdesignates a file type, or in this case an executable shellscript (type man magic for moredetails on this fascinating topic). Immediately followingthe sha-bang is a pathname. This is the path to the program that interpretsthe commands in the script, whether it be a shell, a programminglanguage, or a utility. This command interpreter then executesthe commands in the script, starting at the top (the linefollowing the sha-bang line), and ignoringcomments. [3]

#!/bin/sh#!/bin/bash#!/usr/bin/perl#!/usr/bin/tcl#!/bin/sed -f#!/bin/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. [4] Using #!/bin/sh, the default Bourne shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific features. The script will, however, conform to the POSIX [5] 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. [6]

#! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct.[7] Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine.

E_WRONG_ARGS=85script_parameters="-a -h -m -z" #  -a = all, -h = help, etc.if [ $# -ne $Number_of_expected_args ]then  echo "Usage: `basename $0` $script_parameters"   # `basename $0` is the script's filename.  exit $E_WRONG_ARGSfi