| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 7. Tests | Next | 
A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.
is equal to
if [ "$a" -eq "$b" ]
is not equal to
if [ "$a" -ne "$b" ]
is greater than
if [ "$a" -gt "$b" ]
is greater than or equal to
if [ "$a" -ge "$b" ]
is less than
if [ "$a" -lt "$b" ]
is less than or equal to
if [ "$a" -le "$b" ]
is less than (within double parentheses)
(("$a" < "$b"))
is less than or equal to (within double parentheses)
(("$a" <= "$b"))
is greater than (within double parentheses)
(("$a" > "$b"))
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
is equal to
if [ "$a" = "$b" ]
|  | Note the whitespace framing the =. if [ "$a"="$b" ] is not equivalent to the above. | 
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
|  | The == comparison operator behaves differently within a double-brackets test than within single brackets. 
 | 
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 27-11 for an application of this comparison operator.
string is null, that is, has zero length
|    1  String=''   # Zero-length ("null") string variable.
   2 
   3 if [ -z "$String" ]
   4 then
   5   echo "\$String is null."
   6 else
   7   echo "\$String is NOT null."
   8 fi     # $String is null. | 
string is not null.
|  | The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1] | 
Example 7-5. Arithmetic and string comparisons
| 1 #!/bin/bash 2 3 a=4 4 b=5 5 6 # Here "a" and "b" can be treated either as integers or strings. 7 # There is some blurring between the arithmetic and string comparisons, 8 #+ since Bash variables are not strongly typed. 9 10 # Bash permits integer operations and comparisons on variables 11 #+ whose value consists of all-integer characters. 12 # Caution advised, however. 13 14 echo 15 16 if [ "$a" -ne "$b" ] 17 then 18 echo "$a is not equal to $b" 19 echo "(arithmetic comparison)" 20 fi 21 22 echo 23 24 if [ "$a" != "$b" ] 25 then 26 echo "$a is not equal to $b." 27 echo "(string comparison)" 28 # "4" != "5" 29 # ASCII 52 != ASCII 53 30 fi 31 32 # In this particular instance, both "-ne" and "!=" work. 33 34 echo 35 36 exit 0 | 
Example 7-6. Testing whether a string is null
|    1 #!/bin/bash
   2 #  str-test.sh: Testing null strings and unquoted strings,
   3 #+ but not strings and sealing wax, not to mention cabbages and kings . . .
   4 
   5 # Using   if [ ... ]
   6 
   7 # If a string has not been initialized, it has no defined value.
   8 # This state is called "null" (not the same as zero!).
   9 
  10 if [ -n $string1 ]    # string1 has not been declared or initialized.
  11 then
  12   echo "String \"string1\" is not null."
  13 else  
  14   echo "String \"string1\" is null."
  15 fi                    # Wrong result.
  16 # Shows $string1 as not null, although it was not initialized.
  17 
  18 echo
  19 
  20 # Let's try it again.
  21 
  22 if [ -n "$string1" ]  # This time, $string1 is quoted.
  23 then
  24   echo "String \"string1\" is not null."
  25 else  
  26   echo "String \"string1\" is null."
  27 fi                    # Quote strings within test brackets!
  28 
  29 echo
  30 
  31 if [ $string1 ]       # This time, $string1 stands naked.
  32 then
  33   echo "String \"string1\" is not null."
  34 else  
  35   echo "String \"string1\" is null."
  36 fi                    # This works fine.
  37 # The [ ... ] test operator alone detects whether the string is null.
  38 # However it is good practice to quote it (if [ "$string1" ]).
  39 #
  40 # As Stephane Chazelas points out,
  41 #    if [ $string1 ]    has one argument, "]"
  42 #    if [ "$string1" ]  has two arguments, the empty "$string1" and "]" 
  43 
  44 
  45 echo
  46 
  47 
  48 string1=initialized
  49 
  50 if [ $string1 ]       # Again, $string1 stands unquoted.
  51 then
  52   echo "String \"string1\" is not null."
  53 else  
  54   echo "String \"string1\" is null."
  55 fi                    # Again, gives correct result.
  56 # Still, it is better to quote it ("$string1"), because . . .
  57 
  58 
  59 string1="a = b"
  60 
  61 if [ $string1 ]       # Again, $string1 stands unquoted.
  62 then
  63   echo "String \"string1\" is not null."
  64 else  
  65   echo "String \"string1\" is null."
  66 fi                    # Not quoting "$string1" now gives wrong result!
  67 
  68 exit 0   # Thank you, also, Florian Wisser, for the "heads-up". | 
Example 7-7. zmore
|    1 #!/bin/bash
   2 # zmore
   3 
   4 # View gzipped files with 'more' filter.
   5 
   6 E_NOARGS=85
   7 E_NOTFOUND=86
   8 E_NOTGZIP=87
   9 
  10 if [ $# -eq 0 ] # same effect as:  if [ -z "$1" ]
  11 # $1 can exist, but be empty:  zmore "" arg2 arg3
  12 then
  13   echo "Usage: `basename $0` filename" >&2
  14   # Error message to stderr.
  15   exit $E_NOARGS
  16   # Returns 85 as exit status of script (error code).
  17 fi  
  18 
  19 filename=$1
  20 
  21 if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces.
  22 then
  23   echo "File $filename not found!" >&2   # Error message to stderr.
  24   exit $E_NOTFOUND
  25 fi  
  26 
  27 if [ ${filename##*.} != "gz" ]
  28 # Using bracket in variable substitution.
  29 then
  30   echo "File $1 is not a gzipped file!"
  31   exit $E_NOTGZIP
  32 fi  
  33 
  34 zcat $1 | more
  35 
  36 # Uses the 'more' filter.
  37 # May substitute 'less' if desired.
  38 
  39 exit $?   # Script returns exit status of pipe.
  40 #  Actually "exit $?" is unnecessary, as the script will, in any case,
  41 #+ return the exit status of the last command executed. | 
These are similar to the Bash comparison operators && and ||, used within double brackets.
| 1 [[ condition1 && condition2 ]] | 
The -o and -a operators work with the test command or occur within single test brackets.
| 1 if [ "$expr1" -a "$expr2" ] 2 then 3 echo "Both expr1 and expr2 are true." 4 else 5 echo "Either expr1 or expr2 is false." 6 fi | 
|  | But, as rihad points out: 
 | 
Refer to Example 8-3, Example 27-17, and Example A-29 to see compound comparison operators in action.
| [1] | As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x -o "x$a" = "x$b" ] (the "x's" cancel out). |