1. Copyright © 2016-2017 – Curt Hill
Bash
Flow of Control
Copyright © – Curt Hill

2. Copyright © 2016-2017 – Curt Hill
Introduction
We have seen some of the basics of scripting now onto flow of control
Everyone in this class knows C++ and one or more additional programming languages
Covering flow of control should be comparatively easy
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Aside
As scripting languages go the shells are comparatively primitive
They were originally conceived to make running systems of programs
They were very early
Almost every construct is an executable
Thus the syntax is command syntax
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Exit Condition
Recall the form of main function: int main(int argc, char * argv[])
Every program returns an integer that indicates its action
In general zero means success and anything else indicates some issues
Each program determines its results
It may also give an error message
We may see the return code by echoing the expression $?
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Example
ls -l
total 4
-rw-rw-r-- 1 curthill curthill 4 Nov 20 11:44 test.sh
cd nowhere
-bash: cd: nowhere: No such file or directory
echo $? 1
ls -l -a
total 12
drwxrwxr-x 2 curthill curthill 4096 Nov 20 11:44 .
drwxr-xr-x 12 curthill curthill 4096 Nov 20 11:44 ..
-rw-rw-r-- 1 curthill curthill 4 Nov 20 11:44 test.sh
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Exit
The exit statement may be used in a script to return a value
Such as: exit 0
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Test
This will often be the condition of an if statement
It is merely an executable with parameters
When executed it will produce a 0 or 1 that is used by an if
It may be executed outside of an if
The parameters determine what it tests
First we will determine if a file exists
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File Exists
$ test -f test.sh
$ echo $?
$ test -f test.x 1
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The If
The form of the if: if condition then statements fi
Or if condition then statements else statements fi
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Notes
The condition is often a test statement but can be other things
The statements then else fi are just markers
There is no need for a block since the else and/or fi show the boundaries
You may put multiple things on a line if you separate with semicolons
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Example
$ cat test.sh
#!/bin/bash
ls -a
if test -f test.sh
then
echo "All good"
else
echo "where did it go?" fi
if test -f missing.file ; then
echo "I found it but I don't believe it"
echo "As I thought - it is not here"
$ ./test.sh
test.sh test.sh~
All good
As I thought - it is not here
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12. Copyright © 2016-2017 – Curt Hill
Tracing – Apply -x
$ ./test.sh
+ ls -a
junk.sh test.sh test.sh~
+ test -f test.sh
+ echo 'All good'
All good
+ test -f missing.file
+ echo 'As I thought - it is not here'
As I thought - it is not here
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Back to Test
Test comes in two forms test parms [ parms ]
Thus the if test -f test.sh could have been if [ -f test.sh ]
Many find this second form easier to read
There must be blanks around things
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Test Parameters
DOS only gives us exist but test many more options
-d file
True if file is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-L file
True if file is a symbolic link.
-r file
True if file is a file readable by you.
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More Options
-w file
True if file is a file writable by you.
-x file
True if file is a file executable by you.
file1 -nt file2
True if file1 is newer than (according to modification time) file2
file1 -ot file2
True if file1 is older than file2
-z string
True if string is empty.
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Last Set of Options
-n string
True if string is not empty.
string1 = string2
True if string1 equals string2.
string1 != string2
True if string1 does not equal string2.
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Null String
Variables that are not defined have the empty string as a value
Thus if [$v = $q]
This could give a syntax error if q is empty if [ $v = ]
Compare variables enclosed in quotes to avoid a syntax error
Which is a run-time error in script languages
Use if [ “$v” = “$q” ]
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Elif
Each if need a fi to end
In the case of nested ifs this can be rather tedious and error prone
In the case where we have an else if we can replace with an elif and not need another fi
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Example
if [ "$character" = "1" ]; then
echo "You entered one.“
elif [ "$character" = "2" ]; then
echo "You entered two."
elif [ "$character" = "3" ]; then
echo "You entered three."
else echo “Oops." fi
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The For
Similar to the for all of Java
Form: for arg in [list] do statements done
List is a blank separated list
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For Example
$ ./c3.sh 1 3 5 7 9 11
$ cat c3.sh
#!/bin/bash
#count
count=1
for count in ; do
echo $count
done
exit 0
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The While
Form is: while condition do statements done
The while, do and done are reserved
The condition is the same as in an if
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Example
$ ./counter.sh 1 2 3 4 5 6 7 8 9
$ cat counter.sh
#!/bin/bash
#count
count=1
while [ $count -lt 10 ] ; do
echo $count
let count=$count+1
done
exit 0
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Finally
There are a more things to know that have to wait for a subsequent presentation
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