The if/then construct tests whether a condition is
true, and if so, executes one or more commands. Note that in this
context, 0 (zero) will evaluate as true, as will
a random string of alphanumerics. Puzzling out the logic of this is
left as an exercise for the reader.
Example 3-9. What is truth?
#!/bin/bash
if [ 0 ]
#zero
then
echo "0 is true."
else
echo "0 is false."
fi
if [ ]
#NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi
if [ xyz ]
#string
then
echo "Random string is true."
else
echo "Random string is false."
fi
if [ $xyz ]
#string
then
echo "Undeclared variable is true."
else
echo "Undeclared variable is false."
fi
exit 0
Exercise. Explain the behavior of Example 3-9, above.
if [ condition-true ]
then
command 1
command 2
...
else
# Optional (may be left out if not needed).
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi
Add a semicolon when 'if' and 'then' are on same line.
if [ -x filename ]; then
elif
This is a contraction for else if.
The effect is to nest an inner if/then construction
within an outer one.
if [ condition ]
then
command
command
command
elif
# Same as else if
then
command
command
else
default-command
fi
The test condition-true construct is the exact
equivalent of if [condition-true ]. The left bracket
[ is, in fact, an alias for test. (The
closing right bracket ] in a test should not therefore
be strictly necessary, however newer versions of bash detect it as a
syntax error and complain.)
Example 3-10. Equivalence of [ ] and test
#!/bin/bash
echo
if test -z $1
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
# Both code blocks are functionally identical.
if [ -z $1 ]
# if [ -z $1
# also works, but outputs an error message.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
exit 0
file is a character device (keyboard, modem, sound
card, etc.)
-p
file is a pipe
-L
file is a symbolic link
-S
file is a socket
-r
file is readable (has read permission)
-w
file has write permission
-x
file has execute permission
-g
group-id flag set on file
-u
user-id flag set on file
-k
"sticky bit" set (if user does not
own a directory that has the sticky bit set, she cannot delete
files in it, not even files she owns)
-O
you are owner of file
-G
group-id of file same as yours
-t n
file descriptor n is open
This usually refers to stdin,
stdout, and stderr
(file descriptors 0 - 2).
f1 -nt f2
file f1 is newer than
f2
f1 -ot f2
file f1 is older than
f2
f1 -ef f2
files f1 and
f2 are links to the same
file
!
"not" -- reverses the sense of the
tests above (returns true if condition absent).
Example 3-11. Tests, command chaining, redirection
#!/bin/bash
# This line is a comment.
filename=sys.log
if [ ! -f $filename ]
then
touch $filename; echo "Creating file."
else
cat /dev/null > $filename; echo "Cleaning out file."
fi
# Of course, /var/log/messages must have
# world read permission (644) for this to work.
tail /var/log/messages > $filename
echo "$filename contains tail end of system log."
exit 0
is less than, in ASCII alphabetical order ($a
\< $b)
Note that the "<" needs to be
escaped.
\>
is greater than, in ASCII alphabetical order
($a \> $b)
Note that the ">" needs to be
escaped.
See Example 3-91 for an application of this
comparison operator.
-z
string is "null", that is, has zero length
-n
string is not "null".
Caution
This test requires that the string be quoted
within the test brackets. You may use
! -z instead, or even just
the string itself, without a test operator (see Example 3-13).
Example 3-12. 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 integer comparisons.
# Be careful.
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)"
fi
echo
exit 0
Example 3-13. testing whether a string is null
#!/bin/bash
# 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
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.
echo
string1=initialized
if [ $string1 ] # This time, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Again, gives correct result.
exit 0
# Thanks to Florian Wisser for pointing this out.
Example 3-14. zmost
#!/bin/bash
#View gzipped files with 'most'
NOARGS=1
if [ $# = 0 ]
# same effect as: if [ -z $1 ]
then
echo "Usage: `basename $0` filename" >&2
# Error message to stderr.
exit $NOARGS
# Returns 1 as exit status of script
# (error code)
fi
filename=$1
if [ ! -f $filename ]
then
echo "File $filename not found!" >&2
# Error message to stderr.
exit 2
fi
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit 3
fi
zcat $1 | most
exit 0
# Uses the file viewer 'most'
# (similar to 'less')
compound comparison
-a
logical and
exp1 -a exp2 returns true if
both exp1 and exp2 are true.
-o
logical or
exp1 -o exp2 returns
true if either exp1 or exp2 are
true.
These are simpler forms of the comparison operators
&& and ||,
which require brackets to separate the target expressions.
Refer to Example 3-15 to see compound comparison operators
in action.
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