Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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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 if grep -q Bash file then echo "File contains at least one occurrence of Bash." 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 Stephane Chazelas.

Example 7-1. What is truth?

#!/bin/bash

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.


echo

exit 0

Exercise. Explain the behavior of Example 7-1, above.

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

When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate.

if [ -x "$filename" ]; then

Else if and elif

elif

elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.

if [ condition1 ] then command1 command2 command3 elif [ condition2 ] # Same as else if then command4 command5 else default-command fi

The if test condition-true construct is the exact equivalent of if [ condition-true ]. As it happens, the left bracket, [ , is a token which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it.

The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test.

bash$ type test test is a shell builtin bash$ type '[' [ is a shell builtin bash$ type '[[' [[ is a shell keyword bash$ type ']]' ]] is a shell keyword bash$ type ']' bash: type: ]: not found

Example 7-2. Equivalence of [ ] and test

#!/bin/bash

echo

if test -z "$1"
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi


if [ -z "$1" ]    # Functionally identical to above code block.
#   if [ -z "$1"   should work, but...
#+  Bash responds to a missing close bracket with an error message.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

exit 0

The [[ ]] construct is the shell equivalent of [ ]. This is the extended test command, adopted from ksh88.

No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution.

file=/etc/passwd if [[ -e $file ]] then echo "Password file exists." fi

Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, The &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary.
dir=/home/bozo if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message. echo "Now in $dir." else echo "Can't change to $dir." fi
The "if COMMAND" construct returns the exit status of COMMAND.

Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct.
var1=20 var2=22 [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2" home=/home/bozo [ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or "false". A non-zero expression returns an exit status of 0, or "true". This is in marked contrast to using the test and [ ] constructs previously discussed.

Example 7-3. Arithmetic Tests using (( ))

#!/bin/bash
# Arithmetic tests.

# The (( ... )) construct evaluates and tests numerical expressions.
# Exit status opposite from [ ... ] construct!

(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?."   # 1

(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?."   # 0

(( 5 > 4 ))                                # true
echo $?                                    # 0

(( 5 > 9 ))                                # false
echo $?                                    # 1

exit 0