7.3. Other Comparison Operators

#!/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

#!/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 $string1 as not null, although it was not initialized.
echo
# Lets try it again.
if [ -n "$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, $string1 stands naked.
then
 echo "String \"string1\" is not null."
else 
 echo "String \"string1\" is null."
fi 
# This works fine.
# The [ ] test operator alone detects whether the string is null.
# However it is good practice to quote it ("$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, $string1 stands naked.
then
 echo "String \"string1\" is not null."
else 
 echo "String \"string1\" is null."
fi 
# Again, gives correct result.
# Still, it is better to quote it ("$string1"), because . . .
string1="a = b"
if [ $string1 ] # Again, $string1 stands naked.
then
 echo "String \"string1\" is not null."
else 
 echo "String \"string1\" is null."
fi 
# Not quoting "$string1" now gives wrong result!
exit 0
# Thank you, also, Florian Wisser, for the "heads-up".

#!/bin/bash
# zmore
#View gzipped files with 'more'
NOARGS=65
NOTFOUND=66
NOTGZIP=67
if [ $# -eq 0 ] # same effect as: if [ -z "1ドル" ]
# 1ドル can exist, but be empty: zmore "" arg2 arg3
then
 echo "Usage: `basename 0ドル` filename" >&2
 # Error message to stderr.
 exit $NOARGS
 # Returns 65 as exit status of script (error code).
fi 
filename=1ドル
if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.
then
 echo "File $filename not found!" >&2
 # Error message to stderr.
 exit $NOTFOUND
fi 
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
 echo "File 1ドル is not a gzipped file!"
 exit $NOTGZIP
fi 
zcat 1ドル | more
# Uses the filter 'more.'
# 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.