Red Hat Enterprise Linux Implementation

Using Bash Special Variables

Evaluating Exit Codes

Evaluating Exit Codes

Testing Script Inputs

Using Conditional Structures

Predefined variables that store value of command-line arguments

Named numerically:

Values referenced with syntax $1, $2, etc.

Special variables refer to positional parameters: $* and $@

$# represents number of command-line arguments passed to a script

Can be used to verify whether any arguments, or correct number of arguments, are passed to script

[student@server1 bin]$ cat countargs
#!/bin/bash
echo "There are $# arguments."

[student@server1 bin]$ ./countargs
There are 0 arguments.

[student@server1 bin]$ ./countargs 1 2 3
There are 3 arguments.

Every command returns exit status

Successful command exits with status 0

Unsuccessful commands exit with nonzero status

Exit status is passed to parent process and stored in ? variable

Display value of $? to retrieve exit status of executed command

Script exits when it has processed its contents

May want script to exit part way through, such as when error occurs

When script encounters exit, it immediately stops processing and exits

Can execute`exit` with optional integer argument between 0 and 255 (exit code)

Exit code is passed back to parent process and stored in ? variable

To retrieve value, use $?

If exit is used without argument, script exits and passes exit status of last command executed

[student@server1 bin]$ cat hello
#!/bin/bash
echo "Hello, world"
exit 0

[student@server1 bin]$ ./hello
Hello, world

[student@server1 bin]$ echo $?
0

[student@server1 bin]$ cat hello
#!/bin/bash
echo "Hello, world"
exit 1

[student@server1 bin]$ ./hello
Hello, world

[student@server1 bin]$ echo $?
1

To perform integrity checking, use test

Upon completion, test produces exit code stored as $?

Exit status 0 indicates test passed, and nonzero values indicate failure

Comparison test expressions use binary operators

Operators expect object on each side of operator and evaluate objects for equality and inequality

[ <ITEM1> <BINARY COMPARISON OPERATOR> <ITEM2> ]

Bash uses different operators for string and numeric comparisons

Binary numeric comparison is limited to integers

String comparison uses following binary operators

Examples of Bash’s string comparison operators

[student@server1 ~]$ [ abc = abc ]; echo $?
0
[student@server1 ~]$ [ abc == def ]; echo $?
1
[student@server1 ~]$ [ abc != def ]; echo $?
0

Bash also has unary operators for string evaluation

Unary operators evaluate just one item

[ <UNARY OPERATOR> <ITEM> ]

Unary operators for string evaluation

Examples of string unary operators

[student@server1 ~]$ STRING=''; [ -z "$STRING" ]; echo $?
0
[student@server1 ~]$ STRING='abc'; [ -n "$STRING" ]; echo $?
0

Test script interactions with external entities, such as files and directories

Space character is required in these locations:

Use logical AND operator && to test if both of two conditions are true

Use logical OR operator || to test if either of two conditions is true

[student@server1 ~]$ [ 2 -gt 1 ] && [ 1 -gt 0 ]; echo $?
0
[student@server1 ~]$ [ 2 -gt 1 ] && [ 1 -gt 2 ]; echo $?
1
[student@server1 ~]$ [ 2 -gt 1 ] || [ 1 -gt 2 ]; echo $?
0
[student@server1 ~]$ [ 0 -gt 1 ] || [ 1 -gt 2 ]; echo $?
1

Simple shell scripts are a series of commands that execute from beginning to end

Adding conditional structures allows scripts to incorporate decision-making

Simplest conditional structure

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
    fi

If condition is met, action is taken

Numeric, string, and file tests are frequently used as conditions in if/then statements

Different actions are taken based on whether condition is met

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
    else
      <STATEMENT>
      ...
      <STATEMENT>
fi

To test multiple conditions, use elif (ELSE IF)

Bash tests conditions in order

Upon finding true condition:

If no conditions is true, Bash executes else actions

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
    elif <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
    else
      <STATEMENT>
      ...
      <STATEMENT>
    fi

Example: Use if/then/elif/then/else statement to:

if/then/elif/then/else statement can have as many elif clauses as needed

Statement and logic become harder to read and understand

For complex situations, use case statements

case <VALUE> in
   <PATTERN1>)
       <STATEMENT>
       ...
       <STATEMENT>
       ;;
   <PATTERN2>)
       <STATEMENT>
       ...
       <STATEMENT>
       ;;
esac

case statement attempts to match <VALUE> to each <PATTERN> in order, one by one

To mimic behavior of else in if/then/elif/then/else, use * as final pattern

Because * matches anything, it executes a set of commands if no other patterns were matched

case statements are widely used in init scripts

case "$1" in
   start)
       start
       ;;
   stop)
       rm -f $lockfile
       stop
       ;;
   restart)
       restart
       ;;
   reload)
       reload
       ;;
   status)
       status
       ;;
   *)
       echo "Usage: $0 (start|stop|restart|reload|status)"
       ;;
esac

case statements are widely used in init scripts

case "$1" in
   start)
       start
       ;;
   stop)
       rm -f $lockfile
       stop
       ;;
   restart)
       restart
       ;;
   reload)
       reload
       ;;
   status)
       status
       ;;
   *)
       echo "Usage: $0 (start|stop|restart|reload|status)"
       ;;
esac

If action is same for multiple patterns, can combine patterns to share action block

To separate multiple patterns, use pipe character, |

case "$1" in
   ...
   reload|restart)
       restart
       ;;
   ...
esac

Man pages: bash(1) and test(1)

Using Bash Special Variables

Evaluating Exit Codes

Evaluating Exit Codes

Testing Script Inputs

Using Conditional Structures