Chapter 25. List Constructs

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.