Tests constructs

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 return an exit status, according to whether the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic-expansion constructs may therefore be used to perform arithmetic comparisons.

(( 0 && 1 ))                 # Logical AND
echo $?     # 1     ***
# And so ...
let "num = (( 0 && 1 ))"
echo $num   # 0
# But ...
let "num = (( 0 && 1 ))"
echo $?     # 1     ***
(( 200 || 11 ))              # Logical OR
echo $?     # 0     ***
# ...
let "num = (( 200 || 11 ))"
echo $num   # 1
let "num = (( 200 || 11 ))"
echo $?     # 0     ***
(( 200 | 11 ))               # Bitwise OR
echo $?                      # 0     ***
# ...
let "num = (( 200 | 11 ))"
echo $num                    # 203
let "num = (( 200 | 11 ))"
echo $?                      # 0     ***
# The "let" construct returns the same exit status
#+ as the double-parentheses arithmetic expansion.

Caution: Again, note that the exit status of an arithmetic expression is not an error value.

var=-2 && (( var+=2 ))
echo $?                   # 1
var=-2 && (( var+=2 )) && echo $var
                          # Will not echo $var!

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

What is truth?

#!/bin/bash

# Tip: # If you're unsure how a certain condition might evaluate, #+ test it in an if-test.

echo

echo "Testing \"0\"" if [ 0 ] # zero then echo "0 is true." else # Or 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

Exercise: Explain the behavior of example above.

if [ condition-true ]
then
   command 1
   command 2
   ...
else  # Or else ...
      # Adds default code block executing if original condition tests false.
   command 3
   command 4
   ...
fi

Note: 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 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 [1] 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.

Note: 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

If, for some reason, you wish to use /usr/bin/test in a Bash script, then specify it by full pathname.

Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[

#!/bin/bash

echo

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

echo

if /usr/bin/test -z "$1" # Equivalent to "test" builtin. # ^^^^^^^^^^^^^ # Specifying full pathname. then echo "No command-line arguments." else echo "First command-line argument is $1." fi

echo

if [ -z "$1" ] # Functionally identical to above code blocks. # 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

if /usr/bin/[ -z "$1" ] # Again, functionally identical to above. # if /usr/bin/[ -z "$1" # Works, but gives an error message. # # Note: # This has been fixed in Bash, version 3.x. then echo "No command-line arguments." else echo "First command-line argument is $1." fi

echo

exit 0

The [[ ]] construct is the more versatile Bash version 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.

Arithmetic evaluation of octal / hexadecimal constants takes place automatically within a [[ ... ]] construct.

# [[ Octal and hexadecimal evaluation ]]
# Thank you, Moritz Gronbach, for pointing this out.
decimal=15
octal=017   # = 15 (decimal)
hex=0x0f    # = 15 (decimal)
if [ "$decimal" -eq "$octal" ]
then
  echo "$decimal equals $octal"
else
  echo "$decimal is not equal to $octal"       # 15 is not equal to 017
fi      # Doesn't evaluate within [ single brackets ]!
if [[ "$decimal" -eq "$octal" ]]
then
  echo "$decimal equals $octal"                # 15 equals 017
else
  echo "$decimal is not equal to $octal"
fi      # Evaluates within [[ double brackets ]]!
if [[ "$decimal" -eq "$hex" ]]
then
  echo "$decimal equals $hex"                  # 15 equals 0x0f
else
  echo "$decimal is not equal to $hex"
fi      # [[ $hexadecimal ]] also evaluates!

Note: 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.

Arithmetic Tests using (( ))

#!/bin/bash
# arith-tests.sh
# 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 "Exit status of \"(( 5 > 4 ))\" is $?."     # 0
(( 5 > 9 ))                                      # false
echo "Exit status of \"(( 5 > 9 ))\" is $?."     # 1
(( 5 == 5 ))                                     # true
echo "Exit status of \"(( 5 == 5 ))\" is $?."    # 0
# (( 5 = 5 ))  gives an error message.
(( 5 - 5 ))                                      # 0
echo "Exit status of \"(( 5 - 5 ))\" is $?."     # 1
(( 5 / 4 ))                                      # Division o.k.
echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0
(( 1 / 2 ))                                      # Division result < 1.
echo "Exit status of \"(( 1 / 2 ))\" is $?."     # Rounded off to 0.
                                                 # 1
(( 1 / 0 )) 2>/dev/null                          # Illegal division by 0.
#           ^^^^^^^^^^^
echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1
# What effect does the "2>/dev/null" have?
# What would happen if it were removed?
# Try removing it, then rerunning the script.
# ======================================= #
# (( ... )) also useful in an if-then test.
var1=5
var2=4
if (( var1 > var2 ))
then #^      ^      Note: Not $var1, $var2. Why?
  echo "$var1 is greater than $var2"
fi     # 5 is greater than 4
exit 0

[1]A token is a symbol or short string with a special meaning attached to it (a meta-meaning). In Bash, certain tokens, such as [ and . (dot-command), may expand to keywords and commands.