7.3. Other Comparison Operators

   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.
  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

   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 
   8 # If a string has not been initialized, it has no defined value.
   9 # This state is called "null" (not the same as zero).
  10 
  11 if [ -n $string1 ]    # $string1 has not been declared or initialized.
  12 then
  13   echo "String \"string1\" is not null."
  14 else  
  15   echo "String \"string1\" is null."
  16 fi  
  17 # Wrong result.
  18 # Shows $string1 as not null, although it was not initialized.
  19 
  20 
  21 echo
  22 
  23 
  24 # Lets try it again.
  25 
  26 if [ -n "$string1" ]  # This time, $string1 is quoted.
  27 then
  28   echo "String \"string1\" is not null."
  29 else  
  30   echo "String \"string1\" is null."
  31 fi                    # Quote strings within test brackets!
  32 
  33 
  34 echo
  35 
  36 
  37 if [ $string1 ]       # This time, $string1 stands naked.
  38 then
  39   echo "String \"string1\" is not null."
  40 else  
  41   echo "String \"string1\" is null."
  42 fi  
  43 # This works fine.
  44 # The [ ] test operator alone detects whether the string is null.
  45 # However it is good practice to quote it ("$string1").
  46 #
  47 # As Stephane Chazelas points out,
  48 #    if [ $string1 ]    has one argument, "]"
  49 #    if [ "$string1" ]  has two arguments, the empty "$string1" and "]" 
  50 
  51 
  52 
  53 echo
  54 
  55 
  56 
  57 string1=initialized
  58 
  59 if [ $string1 ]       # Again, $string1 stands naked.
  60 then
  61   echo "String \"string1\" is not null."
  62 else  
  63   echo "String \"string1\" is null."
  64 fi  
  65 # Again, gives correct result.
  66 # Still, it is better to quote it ("$string1"), because . . .
  67 
  68 
  69 string1="a = b"
  70 
  71 if [ $string1 ]       # Again, $string1 stands naked.
  72 then
  73   echo "String \"string1\" is not null."
  74 else  
  75   echo "String \"string1\" is null."
  76 fi  
  77 # Not quoting "$string1" now gives wrong result!
  78 
  79 exit 0
  80 # Thank you, also, Florian Wisser, for the "heads-up".

   1 #!/bin/bash
   2 
   3 #View gzipped files with 'most'
   4 
   5 NOARGS=65
   6 NOTFOUND=66
   7 NOTGZIP=67
   8 
   9 if [ $# -eq 0 ] # same effect as:  if [ -z "$1" ]
  10 # $1 can exist, but be empty:  zmost "" arg2 arg3
  11 then
  12   echo "Usage: `basename $0` filename" >&2
  13   # Error message to stderr.
  14   exit $NOARGS
  15   # Returns 65 as exit status of script (error code).
  16 fi  
  17 
  18 filename=$1
  19 
  20 if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces.
  21 then
  22   echo "File $filename not found!" >&2
  23   # Error message to stderr.
  24   exit $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 $NOTGZIP
  32 fi  
  33 
  34 zcat $1 | most
  35 
  36 # Uses the file viewer 'most' (similar to 'less').
  37 # Later versions of 'most' have file decompression capabilities.
  38 # May substitute 'more' or 'less', if desired.
  39 
  40 
  41 exit $?   # Script returns exit status of pipe.
  42 # Actually "exit $?" is unnecessary, as the script will, in any case,
  43 # return the exit status of the last command executed.