7.1. Test Constructs

• An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means "success" by UNIX convention), and if so, executes one or more commands.

• There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false).

• With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command.

  Bash sees [[ $a -lt $b ]] as a single element, which returns an exit status.

  The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons.

  let "1<2" returns 0 (as "1<2" expands to "1")
  (( 0 && 1 )) returns 1 (as "0 && 1" expands to "0")

• An if can test any command, not just conditions enclosed within brackets.

  if cmp a b &> /dev/null # Suppress output.
  then echo "Files a and b are identical."
  else echo "Files a and b differ."
  fi
  # The very useful "if-grep" construct:
  # ----------------------------------- 
  if grep -q Bash file
  then echo "File contains at least one occurrence of Bash."
  fi
  word=Linux
  letter_sequence=inu
  if echo "$word" | grep -q "$letter_sequence"
  # The "-q" option to grep suppresses output.
  then
   echo "$letter_sequence found in $word"
  else
   echo "$letter_sequence not found in $word"
  fi
  if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
  then echo "Command succeeded."
  else echo "Command failed."
  fi

• An if/then construct can contain nested comparisons and tests.

  if echo "Next *if* is part of the comparison for the first *if*."
   if [[ $comparison = "integer" ]]
   then (( a < b ))
   else
   [[ $a < $b ]]
   fi
  then
   echo '$a is less than $b'
  fi

  This detailed "if-test" explanation courtesy of St�phane Chazelas.

#!/bin/bash
# Tip:
# If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.
echo
echo "Testing \"0\""
if [ 0 ] # zero
then
 echo "0 is true."
else
 echo "0 is false."
fi # 0 is true.
echo
echo "Testing \"1\""
if [ 1 ] # one
then
 echo "1 is true."
else
 echo "1 is false."
fi # 1 is true.
echo
echo "Testing \"-1\""
if [ -1 ] # minus one
then
 echo "-1 is true."
else
 echo "-1 is false."
fi # -1 is true.
echo
echo "Testing \"NULL\""
if [ ] # NULL (empty condition)
then
 echo "NULL is true."
else
 echo "NULL is false."
fi # NULL is false.
echo
echo "Testing \"xyz\""
if [ xyz ] # string
then
 echo "Random string is true."
else
 echo "Random string is false."
fi # Random string is true.
echo
echo "Testing \"\$xyz\""
if [ $xyz ] # Tests if $xyz is null, but...
 # it's only an uninitialized variable.
then
 echo "Uninitialized variable is true."
else
 echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ] # More pedantically correct.
then
 echo "Uninitialized variable is true."
else
 echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
xyz= # Initialized, but set to null value.
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
 echo "Null variable is true."
else
 echo "Null variable is false."
fi # Null variable is false.
echo
# When is "false" true?
echo "Testing \"false\""
if [ "false" ] # It seems that "false" is just a string.
then
 echo "\"false\" is true." #+ and it tests true.
else
 echo "\"false\" is false."
fi # "false" is true.
echo
echo "Testing \"\$false\"" # Again, uninitialized variable.
if [ "$false" ]
then
 echo "\"\$false\" is true."
else
 echo "\"\$false\" is false."
fi # "$false" is false.
 # Now, we get the expected result.
# What would happen if we tested the uninitialized variable "$true"?
echo
exit 0

if [ condition-true ]
then
 command 1
 command 2
 ...
else
 # Optional (may be left out if not needed).
 # Adds default code block executing if original condition tests false.
 command 3
 command 4
 ...
fi

if [ -x "$filename" ]; then