Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "You will not see this message, because script terminated above."
HERE=0
exit $HERE
# Will *not* exit here.
Example 9−14. Parameter substitution and "usage" messages
#!/bin/bash
# usage−message.sh
: ${1?"Usage: $0 ARGUMENT"}
# Script exits here if command−line parameter absent,
#+ with following error message.
# usage−message.sh: 1: Usage: usage−message.sh ARGUMENT
echo "These two lines echo only if command−line parameter given."
echo "command line parameter = \"$1\""
exit 0 # Will exit here only if command−line parameter present.
# Check the exit status, both with and without command−line parameter.
# If command−line parameter present, then "$?" is 0.
# If not, then "$?" is 1.
Parameter substitution and/or expansion. The following expressions are the complement to the match in
expr string operations (see Example 12−6). These particular ones are used mostly in parsing file path names.
Variable length / Substring removal
${#var}
String length (number of characters in $var). For an array, ${#array} is the length of the first
element in the array.
Exceptions:
◊ ${#*} and ${#@} give the number of positional parameters.
◊ For an array, ${#array[*]} and ${#array[@]} give the number of elements in
the array.
Example 9−15. Length of a variable
#!/bin/bash
# length.sh
E_NO_ARGS=65
if [ $# −eq 0 ] # Must have command−line args to demo script.
then
echo "Invoke this script with one or more command−line arguments."
exit $E_NO_ARGS
fi
Chapter 9. Variables Revisited 87