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?
1 #!/bin/bash 2 3 if [ 0 ] 4 #zero 5 then 6 echo "0 is true." 7 else 8 echo "0 is false." 9 fi 10 11 if [ ] 12 #NULL (empty condition) 13 then 14 echo "NULL is true." 15 else 16 echo "NULL is false." 17 fi 18 19 if [ xyz ] 20 #string 21 then 22 echo "Random string is true." 23 else 24 echo "Random string is false." 25 fi 26 27 if [ $xyz ] 28 #string 29 then 30 echo "Undeclared variable is true." 31 else 32 echo "Undeclared variable is false." 33 fi 34 35 exit 0 |
Exercise. Explain the behavior of Example 3-9, above.
1 if [ condition-true ] 2 then 3 command 1 4 command 2 5 ... 6 else 7 # Optional (may be left out if not needed). 8 # Adds default code block executing if original condition tests false. 9 command 3 10 command 4 11 ... 12 fi |
Add a semicolon when 'if' and 'then' are on same line.
1 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.
1 if [ condition ] 2 then 3 command 4 command 5 command 6 elif 7 # Same as else if 8 then 9 command 10 command 11 else 12 default-command 13 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
1 #!/bin/bash 2 3 echo 4 5 6 if test -z $1 7 then 8 echo "No command-line arguments." 9 else 10 echo "First command-line argument is $1." 11 fi 12 13 # Both code blocks are functionally identical. 14 15 if [ -z $1 ] 16 # if [ -z $1 17 # also works, but outputs an error message. 18 then 19 echo "No command-line arguments." 20 else 21 echo "First command-line argument is $1." 22 fi 23 24 25 echo 26 27 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
1 #!/bin/bash 2 3 # This line is a comment. 4 5 filename=sys.log 6 7 if [ ! -f $filename ] 8 then 9 touch $filename; echo "Creating file." 10 else 11 cat /dev/null > $filename; echo "Cleaning out file." 12 fi 13 14 # Of course, /var/log/messages must have 15 # world read permission (644) for this to work. 16 tail /var/log/messages > $filename 17 echo "$filename contains tail end of system log." 18 19 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".
![]() | 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
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 integer comparisons. 8 # Be careful. 9 10 if [ $a -ne $b ] 11 then 12 echo "$a is not equal to $b" 13 echo "(arithmetic comparison)" 14 fi 15 16 echo 17 18 if [ $a != $b ] 19 then 20 echo "$a is not equal to $b." 21 echo "(string comparison)" 22 fi 23 24 echo 25 26 exit 0 |
Example 3-13. testing whether a string is null
1 #!/bin/bash 2 3 # If a string has not been initialized, it has no defined value. 4 # This state is called "null" (not the same as zero). 5 6 7 if [ -n $string1 ] # $string1 has not been declared or initialized. 8 then 9 echo "String \"string1\" is not null." 10 else 11 echo "String \"string1\" is null." 12 fi 13 # Wrong result. 14 # Shows $string1 as not null, although it was not initialized. 15 16 echo 17 18 # Lets try it again. 19 20 if [ -n "$string1" ] # This time, $string1 is quoted. 21 then 22 echo "String \"string1\" is not null." 23 else 24 echo "String \"string1\" is null." 25 fi 26 27 echo 28 29 if [ $string1 ] # This time, $string1 stands naked. 30 then 31 echo "String \"string1\" is not null." 32 else 33 echo "String \"string1\" is null." 34 fi 35 # This works fine. 36 # The [ ] test operator alone detects whether the string is null. 37 38 echo 39 40 string1=initialized 41 42 if [ $string1 ] # This time, $string1 stands naked. 43 then 44 echo "String \"string1\" is not null." 45 else 46 echo "String \"string1\" is null." 47 fi 48 # Again, gives correct result. 49 50 51 exit 0 52 53 # Thanks to Florian Wisser for pointing this out. |
Example 3-14. zmost
1 #!/bin/bash 2 3 #View gzipped files with 'most' 4 5 NOARGS=1 6 7 if [ $# = 0 ] 8 # same effect as: if [ -z $1 ] 9 then 10 echo "Usage: `basename $0` filename" >&2 11 # Error message to stderr. 12 exit $NOARGS 13 # Returns 1 as exit status of script 14 # (error code) 15 fi 16 17 filename=$1 18 19 if [ ! -f $filename ] 20 then 21 echo "File $filename not found!" >&2 22 # Error message to stderr. 23 exit 2 24 fi 25 26 if [ ${filename##*.} != "gz" ] 27 # Using bracket in variable substitution. 28 then 29 echo "File $1 is not a gzipped file!" 30 exit 3 31 fi 32 33 zcat $1 | most 34 35 exit 0 36 37 # Uses the file viewer 'most' 38 # (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.