Chapter 25. List Constructs
The "and list" and "or list"
constructs provide a means of processing a number of commands
consecutively. These can effectively replace complex
nested if/then or even
case statements.
Chaining together commands
- and list
command-1 && command-2 && command-3 && ... command-n |
Each command executes in turn provided that
the previous command has given a return value of
true (zero). At the first
false (non-zero) return, the
command chain terminates (the first command returning
false is the last one to
execute).Example 25-1. Using an "and list" to test for command-line arguments
#!/bin/bash
# "and list"
if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && echo "Argument #2 = $2"
then
echo "At least 2 arguments passed to script."
# All the chained commands return true.
else
echo "Less than 2 arguments passed to script."
# At least one of the chained commands returns false.
fi
# Note that "if [ ! -z $1 ]" works, but its supposed equivalent,
# if [ -n $1 ] does not. However, quoting fixes this.
# if [ -n "$1" ] works. Careful!
# It is best to always quote tested variables.
# This accomplishes the same thing, using "pure" if/then statements.
if [ ! -z "$1" ]
then
echo "Argument #1 = $1"
fi
if [ ! -z "$2" ]
then
echo "Argument #2 = $2"
echo "At least 2 arguments passed to script."
else
echo "Less than 2 arguments passed to script."
fi
# It's longer and less elegant than using an "and list".
exit 0 |
Example 25-2. Another command-line arg test using an "and list"
#!/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 $arg1 to command line arguments, if any.
[ -z "$arg1" ] && arg1=DEFAULT
# Set to DEFAULT if not specified on command line. |
- or list
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.
# AND LIST executes on true, OR LIST on false.
exit 0 |
 | If the first command in an "or list"
returns true, it
will execute. |
 | The exit
status of an and list or an
or list is the exit status of the last
command executed. |
Clever combinations of "and" and "or"
lists are possible, but the logic may easily become convoluted and
require extensive debugging.
false && true || echo false # false
# Same result as
( false && true ) || echo false # false
# But *not*
false && ( true || echo false ) # (nothing echoed)
# Note left-to-right grouping and evaluation of statements,
#+ since the logic operators "&&" and "||" have equal precedence.
# It's best to avoid such complexities, unless you know what you're doing.
# Thanks, S.C. |
See Example A-8 and Example 7-4 for illustrations of using an and
/ or list to test variables.
