====== Bash Shell Tests ======

Quelle((http://tldp.org/LDP/abs/html/comparison-ops.html))
===== Other Comparison Operators =====

==== integer comparison ====

----
-eq

is equal to

<code>    if [ "$a" -eq "$b" ]</code>
----    
-ne

is not equal to

<code>    if [ "$a" -ne "$b" ]</code>
----
-gt

is greater than

  <code>  if [ "$a" -gt "$b" ]</code>
----  
-ge

is greater than or equal to

<code>if [ "$a" -ge "$b" ]</code>
----
-lt

is less than

<code>if [ "$a" -lt "$b" ]</code>
----
-le
is less than or equal to

<code>if [ "$a" -le "$b" ]</code>
----
<

is less than (within double parentheses)

<code>(("$a" < "$b"))</code>
----
//<=//

is less than or equal to (within double parentheses)

<code>(("$a" <= "$b"))</code>
----
>

is greater than (within double parentheses)

<code>(("$a" > "$b"))</code>
----
//>=//

is greater than or equal to (within double parentheses)

<code>(("$a" >= "$b"))</code>

==== string comparison ====


=

is equal to

<code>if [ "$a" = "$b" ]</code>
----
==

is equal to

<code>if [ "$a" == "$b" ]</code>

This is a synonym for =.

    Note	

The == comparison operator behaves differently within a double-brackets test than within single brackets.
<code>
    [[ $a == z* ]]   # True if $a starts with an "z" (pattern matching).
    [[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

    [ $a == z* ]     # File globbing and word splitting take place.
    [ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

</code>
----
!=

is not equal to

<code>if [ "$a" != "$b" ]</code>

This operator uses pattern matching within a [[ ... ]] construct.
----
<

is less than, in ASCII alphabetical order
<code>
    if [[ "$a" < "$b" ]]
    if [ "$a" \< "$b" ]
</code>
Note that the "<" needs to be escaped within a [ ] construct.
----
>

is greater than, in ASCII alphabetical order

<code>
    if [[ "$a" > "$b" ]]
    if [ "$a" \> "$b" ]
</code>

Note that the ">" needs to be escaped within a [ ] construct.

See Example 27-11 for an application of this comparison operator.
----
-z

string is null, that is, has zero length
<code>
String=''   # Zero-length ("null") string variable.

    if [ -z "$String" ]
    then
      echo "\$String is null."
    else
      echo "\$String is NOT null."
    fi     # $String is null.
</code>
----
-n

string is not null.

Caution	

The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string.

=== Example 7-5. Arithmetic and string comparisons ===

<code>
#!/bin/bash

a=4
b=5

#  Here "a" and "b" can be treated either as integers or strings.
#  There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.

#  Bash permits integer operations and comparisons on variables
#+ whose value consists of all-integer characters.
#  Caution advised, however.

echo

if [ "$a" -ne "$b" ]
then
  echo "$a is not equal to $b"
  echo "(arithmetic comparison)"
fi

echo

if [ "$a" != "$b" ]
then
  echo "$a is not equal to $b."
  echo "(string comparison)"
  #     "4"  != "5"
  # ASCII 52 != ASCII 53
fi

# In this particular instance, both "-ne" and "!=" work.

echo

exit 0</code>

=== Example 7-6. Testing whether a string is null ===

<code>
#!/bin/bash
#  str-test.sh: Testing null strings and unquoted strings,
#+ but not strings and sealing wax, not to mention cabbages and kings . . .

# Using   if [ ... ]

# If a string has not been initialized, it has no defined value.
# This state is called "null" (not the same as zero!).

if [ -n $string1 ]    # string1 has not been declared or initialized.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi                    # Wrong result.
# Shows $string1 as not null, although it was not initialized.

echo

# Let's try it again.

if [ -n "$string1" ]  # This time, $string1 is quoted.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi                    # Quote strings within test brackets!

echo

if [ $string1 ]       # This time, $string1 stands naked.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi                    # This works fine.
# The [ ... ] test operator alone detects whether the string is null.
# However it is good practice to quote it (if [ "$string1" ]).
#
# As Stephane Chazelas points out,
#    if [ $string1 ]    has one argument, "]"
#    if [ "$string1" ]  has two arguments, the empty "$string1" and "]" 


echo


string1=initialized

if [ $string1 ]       # Again, $string1 stands unquoted.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi                    # Again, gives correct result.
# Still, it is better to quote it ("$string1"), because . . .


string1="a = b"

if [ $string1 ]       # Again, $string1 stands unquoted.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi                    # Not quoting "$string1" now gives wrong result!

exit 0   
</code>

=== Example 7-7. zmore ===

<code>
#!/bin/bash
# zmore

# View gzipped files with 'more' filter.

E_NOARGS=65
E_NOTFOUND=66
E_NOTGZIP=67

if [ $# -eq 0 ] # same effect as:  if [ -z "$1" ]
# $1 can exist, but be empty:  zmore "" arg2 arg3
then
  echo "Usage: `basename $0` filename" >&2
  # Error message to stderr.
  exit $E_NOARGS
  # Returns 65 as exit status of script (error code).
fi  

filename=$1

if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces.
then
  echo "File $filename not found!" >&2   # Error message to stderr.
  exit $E_NOTFOUND
fi  

if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
  echo "File $1 is not a gzipped file!"
  exit $E_NOTGZIP
fi  

zcat $1 | more

# Uses the 'more' filter.
# May substitute 'less' if desired.

exit $?   # Script returns exit status of pipe.
#  Actually "exit $?" is unnecessary, as the script will, in any case,
#+ return the exit status of the last command executed.
</code>

==== compound comparison ====


-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.
----
-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 is true.

These are similar to the Bash comparison operators && and ||, used within double brackets.

<code>[[ condition1 && condition2 ]]</code>

The -o and -a operators work with the test command or occur within single test brackets.

<code>
if [ "$expr1" -a "$expr2" ]
then
  echo "Both expr1 and expr2 are true."
else
  echo "Either expr1 or expr2 is false."
fi
</code>

Caution	

But, as rihad points out:
<code>
[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ]   # true
[ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ]   # (no output)
# ^^^^^^^ False condition. So far, everything as expected.

# However ...
[ 1 -eq 2 -a -n "`echo true 1>&2`" ]       # true
# ^^^^^^^ False condition. So, why "true" output?

# Is it because both condition clauses within brackets evaluate?
[[ 1 -eq 2 && -n "`echo true 1>&2`" ]]     # (no output)
# No, that's not it.

# Apparently && and || "short-circuit" while -a and -o do not.
</code>