Advanced Bash−Scripting Guide

34.7. Assorted Tips
Advanced Bash−Scripting Guide
• To keep a record of which user scripts have run during a particular sesssion or over a number of
sessions, add the following lines to each script you want to keep track of. This will keep a continuing
file record of the script names and invocation times.
# Append (>>) following to end of each script tracked.
date>> $SAVE_FILE
echo $0>> $SAVE_FILE
echo>> $SAVE_FILE
#Date and time.
#Script name.
#Blank line as separator.
# Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
# (something like ~/.scripts−run)
•
The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is,
to paste it in at the beginning?
file=data.txt
title="***This is the title line of data text file***"
echo $title | cat − $file >$file.new
# "cat −" concatenates stdout to $file.
# End result is
#+ to write a new file with $title appended at *beginning*.
Of course, sed can also do this.
• A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or
even a Makefile. It can be invoked as an external shell command in a C program using the
system() call, i.e., system("script_name");.
• Put together files containing your favorite and most useful definitions and functions. As necessary,
"include" one or more of these "library files" in scripts with either the dot (.) or source command.
# SCRIPT LIBRARY
# −−−−−− −−−−−−−
# Note:
# No "#!" here.
# No "live code" either.
# Useful variable definitions
ROOT_UID=0 # Root has $UID 0.
E_NOTROOT=101
# Not root user error.
MAXRETVAL=256
# Maximum (positive) return value of a function.
SUCCESS=0
FAILURE=−1
# Functions
Usage ()
{
if [ −z "$1" ]
then
# "Usage:" message.
# No arg passed.
Chapter 34. Miscellany 347