Advanced Bash-Scripting Guide
#+ and usage info.
################## for directory in $directorys; do
if [ -d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi done exit $?
Example 31-1, Example 11-8, Example 11-3, Example 31-3, and Example A-1 also illustrate uses of the file
test operators.
-lt
-le
<
7.3. Other Comparison Operators
A binary comparison operator compares two variables or quantities. Note that integer and string comparison
use a different set of operators.
integer comparison
-eq is equal to
if [ "$a" -eq "$b" ]
-ne is not equal to
if [ "$a" -ne "$b" ]
-gt is greater than
if [ "$a" -gt "$b" ]
-ge is greater than or equal to
<=
if [ "$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" < "$b"))
is less than or equal to (within double parentheses)
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>
>=
Advanced Bash-Scripting Guide
(("$a" <= "$b"))
is greater than (within double parentheses)
(("$a" > "$b"))
is greater than or equal to (within double parentheses)
(("$a" >= "$b")) string comparison
= is equal to
if [ "$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.
[[ $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 $a is equal to z* (literal matching).
# Thanks, Stéphane Chazelas
!=
< is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ]
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>
-z
-n
Advanced Bash-Scripting Guide
Note that the "<" needs to be escaped within a
[ ]
construct.
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a
[ ]
construct.
See Example 27-11 for an application of this comparison operator.
string is null, that is, has zero length
String='' # Zero-length ("null") string variable.
if [ -z "$String" ] then
echo "\$String is null." else
echo "\$String is NOT null." fi # $String is null.
string is not null.
The
-n
test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets
(see Example 7-6) normally works, however, this is an unsafe practice. Always quote a
tested string. [37]
Example 7-5. Arithmetic and string comparisons
#!/bin/bash a=4 b=5
# Here "a" and "b" can be treated either as integers or strings.
# There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.
# Bash permits integer operations and comparisons on variables
#+ whose value consists of all-integer characters.
# Caution advised, however.
echo if [ "$a" -ne "$b" ] then
echo "$a is not equal to $b"
echo "(arithmetic comparison)" fi echo
Chapter 7. Tests 67
if [ "$a" != "$b" ] then
echo "$a is not equal to $b."
echo "(string comparison)"
# "4" != "5"
# ASCII 52 != ASCII 53 fi
Advanced Bash-Scripting Guide
# In this particular instance, both "-ne" and "!=" work.
echo exit 0
Example 7-6. Testing whether a string is null
#!/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
Chapter 7. Tests 68
Advanced Bash-Scripting Guide 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 # Thank you, also, Florian Wisser, for the "heads-up".
Example 7-7. zmore
#!/bin/bash
# zmore
# View gzipped files with 'more' filter.
E_NOARGS=85
E_NOTFOUND=86
E_NOTGZIP=87 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 85 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
Chapter 7. Tests 69
