Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
#!/bin/bash
ARGS=1 # Number of arguments expected.
E_BADARGS=65 # Exit value if incorrect number of args passed.
test $# −ne $ARGS && echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS
# If condition−1 true (wrong number of args passed to script),
# then the rest of the line executes, and script terminates.
# Line below executes only if the above test fails.
echo "Correct number of arguments passed to this script."
exit 0
# To check exit value, do a "echo $?" after script termination.
Of course, an and list can also set variables to a default value.
arg1=$@
# Set $arg1 to command line arguments, if any.
or list
[ −z "$arg1" ] && arg1=DEFAULT
# Set to DEFAULT if not specified on command line.
command−1 || command−2 || command−3 || ... command−n
Each command executes in turn for as long as the previous command returns false. At the first true
return, the command chain terminates (the first command returning true is the last one to execute).
This is obviously the inverse of the "and list".
Example 25−3. Using "or lists" in combination with an "and list"
#!/bin/bash
# delete.sh, not−so−cunning file deletion utility.
# Usage: delete filename
E_BADARGS=65
if [ −z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS # No arg? Bail out.
else
file=$1
# Set filename.
fi
[ ! −f "$file" ] && echo "File \"$file\" not found. \
Cowardly refusing to delete a nonexistent file."
# AND LIST, to give error message if file not present.
# Note echo message continued on to a second line with an escape.
[ ! −f "$file" ] || (rm −f $file; echo "File \"$file\" deleted.")
# OR LIST, to delete file if present.
# Note logic inversion above.
Chapter 25. List Constructs 290