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, 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

   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".

   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.