| Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash | ||
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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 |
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 |
Returns true if...
file exists
file is a regular file
file is not zero size
file is a directory
file is a block device (floppy, cdrom, etc.)
file is a character device (keyboard, modem, sound card, etc.)
file is a pipe
file is a symbolic link
file is a socket
file is readable (has read permission)
file has write permission
file has execute permission
group-id flag set on file
user-id flag set on file
"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)
you are owner of file
group-id of file same as yours
file descriptor n is open
This usually refers to stdin, stdout, and stderr (file descriptors 0 - 2).
file f1 is newer than f2
file f1 is older than 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 |
integer comparison
is equal to ($a -eq $b)
is not equal to ($a -ne $b)
is greater than ($a -gt $b)
is greater than or equal to ($a -ge $b)
is less than ($a -lt $b)
is less than or equal to ($a -le $b)
string comparison
is equal to ($a = $b)
is not equal to ($a != $b)
is less than, in ASCII alphabetical order ($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.
string is "null", that is, has zero length
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
logical and
exp1 -a exp2 returns true if both exp1 and exp2 are true.
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.