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Sequences of commands in Unix shell



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Conditional execution in shell is used not only in if statements and while loops. It also can be used for emulating if statements themselves like in

(( $a>0 )) && print $a

See Short-circuit if statements

Conditional Execution && and ||

All Unix shells allows you to combine exit statuses logically, so that you can test more than one thing at a time:

In both cases it's useful to think about them "short-circuit and" and "short-circuit or," respectively:

They can be used in if statement too:

if statement1 && statement2


if statement1 || statement2

Typical usage is "cd directory && rm -rf *" (not "cd directory; rm -rf *": If the directory in question does not exist, this command can result in really unintended consequences)

The operator "||" executes the second command only if the first one failed, but it is used is shell much less. It is more often used is Perl to check if file opened successfully:

open(SYSIN, "$myfile") || die (" cannot open $myfile\n");

You can group commands that are affected by && or || by enclosing them in braces. You have to type a semicolon after the last command of the group. This feature is used relatively rarely, it is needed mainly for putting a sequence of commands in the background (i.e. letting it be executed without blocking the terminal). An example is the UNIX way of reminding yourself when your tea is ready:

{ cd directory && rm -rf * } > ~/Logs/deletion.log


Old News ;-)

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. Note that the exit status of an "and list" or an "or list" is the exit status of the last command executed.

and list
   1 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. At the first false return, the command chain terminates (the first command returning false is the last one to execute).

Example 3-87. Using an "and list" to test for command-line arguments

   1 #!/bin/bash
   3 # "and list"
   5 if [ ! -z $1 ] && echo "Argument #1 = $1" && [ ! -z $2 ] && echo "Argument #2 = $2"
   6 then
   7   echo "At least 2 arguments to script."
   8   # All the chained commands return true.
   9 else
  10   echo "Less than 2 arguments to script."
  11   # At least one of the chained commands returns false.
  12 fi  
  13 # Note that "if [ ! -z $1 ]" works, but its supposed equivalent,
  14 # "if [ -n $1 ]" does not. This is a bug, not a feature.
  17 # This accomplishes the same thing, coded using "pure" if/then statements.
  18 if [ ! -z $1 ]
  19 then
  20   echo "Argument #1 = $1"
  21 fi
  22 if [ ! -z $2 ]
  23 then
  24   echo "Argument #2 = $2"
  25   echo "At least 2 arguments to script."
  26 else
  27   echo "Less than 2 arguments to script."
  28 fi
  29 # It's longer and less elegant than using an "and list".
  32 exit 0

or list
   1 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 3-88. Using "or lists" in combination with an "and list"

   1 #!/bin/bash
   3 # "Delete", not-so-cunning file deletion utility.
   4 # Usage: delete filename
   6 if [ -z $1 ]
   7 then
   8   file=nothing
   9 else
  10   file=$1
  11 fi  
  12 # Fetch file name (or "nothing") for deletion message.
  15 [ ! -f $1 ] && echo "$1 not found. Can't delete a nonexistent file."
  16 # AND LIST, to give error message if file not present.
  18 [ ! -f $1 ] || ( rm -f $1; echo "$file deleted." )
  19 # OR LIST, to delete file if present.
  20 # ( command1 ; command2 ) is, in effect, an AND LIST variant.
  22 # Note logic inversion above.
  23 # AND LIST executes on true, OR LIST on false.
  25 [ ! -z $1 ] ||  echo "Usage: `basename $0` filename"
  26 # OR LIST, to give error message if no command line arg (file name).
  28 exit 0

Clever combinations of "and" and "or" lists are possible, but the logic may easily become convoluted and require extensive debugging.



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