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.
command-1 && command-2 && command-3 && ... command-n |
An interesting use of a two-condition and list from an ealy version of YongYe's Tetris game script:
equation() { # core algorithm used for doubling and halving the coordinates [[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2)) eval ${1}+=\"${x} ${y} \" } |
Example 26-1. 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 "Fewer than 2 arguments passed to script." # At least one of the chained commands returns false. fi # Note that "if [ ! -z $1 ]" works, but its alleged equivalent, # "if [ -n $1 ]" does not. # However, quoting fixes this. # if "[ -n "$1" ]" works. # ^ ^ Careful! # It is always best to QUOTE the variables being tested. # 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 "Fewer than 2 arguments passed to script." fi # It's longer and more ponderous than using an "and list". exit $? |
Example 26-2. Another command-line arg test using an and list
#!/bin/bash ARGS=1 # Number of arguments expected. E_BADARGS=85 # Exit value if incorrect number of args passed. test $# -ne $ARGS && \ # ^^^^^^^^^^^^ condition #1 echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS # ^^ # If condition #1 tests 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 $?" after script termination. |
Of course, an and list can also set variables to a default value.
arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT # Set $arg1 to command-line arguments, if any. # But . . . set to DEFAULT if not specified on command-line. |
command-1 || command-2 || command-3 || ... command-n |
Example 26-3. Using or lists in combination with an and list
#!/bin/bash # delete.sh, a not-so-cunning file deletion utility. # Usage: delete filename E_BADARGS=85 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 \"$file\" not found. \ Cowardly refusing to delete a nonexistent file." # AND LIST, to give error message if file not present. # Note echo message continuing on to a second line after an escape. [ ! -f "$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 $? |
If the first command in an or list returns true, it will execute. |
# ==> The following snippets from the /etc/rc.d/init.d/single #+==> script by Miquel van Smoorenburg #+==> illustrate use of "and" and "or" lists. # ==> "Arrowed" comments added by document author. [ -x /usr/bin/clear ] && /usr/bin/clear # ==> If /usr/bin/clear exists, then invoke it. # ==> Checking for the existence of a command before calling it #+==> avoids error messages and other awkward consequences. # ==> . . . # If they want to run something in single user mode, might as well run it... for i in /etc/rc1.d/S[0-9][0-9]* ; do # Check if the script is there. [ -x "$i" ] || continue # ==> If corresponding file in $PWD *not* found, #+==> then "continue" by jumping to the top of the loop. # Reject backup files and files generated by rpm. case "$1" in *.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig) continue;; esac [ "$i" = "/etc/rc1.d/S00single" ] && continue # ==> Set script name, but don't execute it yet. $i start done # ==> . . . |
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 close attention to operator precedence rules, and possibly 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 usually best to avoid such complexities. # Thanks, S.C. |
See Example A-7 and Example 7-4 for illustrations of using and / or list constructs to test variables.