John Backus famously observed about "changeable parts" and "framework":

"Let us distinguish two parts of a programming language. First, its framework which gives the overall rules of the system, and second, its changeable parts, whose existence is anticipated by the framework but whose particular behavior is not specified by it. For example, the for statement, and almost all other statements, are part of Algol's framework but library functions and user- defined procedures are changeable parts." (Link)

Frameworks in Bash

Control structures are part of the framework of Bash
Control structures express either alternation or iteration

Alternation

if/then/fi, if/then/else/fi, if/then/elif/else/fi
case

Iteration

for/in/do/done, for((;;))/do/done
while/do/done
until/do/done
break/continue

if/then/fi

Argument for "if" is a program to execute, usually "test" or its synonym "[", but it can be anything that returns a 0 for true, and something else for false.

WRONG!

$ if -f /etc/hosts 
> then
>  echo /etc/hosts
> fi
-f: command not found

if/then/fi

RIGHT!

$ if [ -f /etc/hosts ]
> then
>   echo /etc/hosts
> fi
/etc/hosts

Something other than test

$ if env > /dev/null
> then
>    echo true
> fi

if/then/else/fi

$ if touch /werwer 2> /dev/null
> then
>    echo created /werwer
> else
>    echo did not create /werwer
> fi
did not create /werwer

if/then/elif/then/else/fi

$ if touch /werwer 2> /dev/null
> then
>   echo created /werwer
> elif touch /tmp/werwer 2> /dev/null
> then
>   echo created /tmp/werwer
> else
>   echo cannot create any werwer
> fi
created /tmp/werwer

test syntax

The program test can take quite a bit of syntax

Syntax	Meaning
( EXP )	Expression is true
! EXP	Expression is false
EXP1 -a EXP2	Logical and
EXP1 -o EXP2	Logical or
-n STR	String has length > 0
-z STR	String has length = 0
STR1 = STR2	Strings are equal
STR1 != STR2	Strings are not equal

test syntax

The program test can take quite a bit of syntax

Syntax		Meaning
INT1 -eq INT2		Integers are equal
INT1 -ge INT2		INT1 is greater or equal to INT2
INT1 -gt INT2		INT1 is greater than INT2
INT1 -le INT2		INT1 is less than or equal to INT2
INT1 -lt INT2		INT1 is less than INT2
INT1 -ne INT2		INT1 is not equal to INT2

test syntax

The program test can take quite a bit of syntax

Syntax	Meaning
FILE1 -ef FILE2	files have same device and inode number
FILE1 -nt FILE2	FILE1 is newer than FILE2
FILE1 -ot FILE2	FILE1 is older than FILE2
-d DIR	DIR is an existing directory

test syntax

The program test can take quite a bit of syntax

Syntax	Meaning
-e FILE	FILE exists
-f FILE	FILE exists and is a regular file
-h FILE, -L FILE	FILE exists and is a soft link
-r FILE	FILE exists and is readable
-s FILE	FILE exists and has a size > 0
-w FILE	FILE exists and is writable
-x FILE	FILE exists and is executable
-t FD FD r	efers to a terminal device

for in

One of the most useful control structures when dealing with files

$ for name in !(*~)
> do
> echo $name
> done
01-introduction.tex
1.aux
1.html
1.log
1.nav
1.out
1.pdf

for without "in"

One special form of for syntax saves us from shifting:

#!/bin/bash -x

# prints each command line argument

for name
do
  echo I found $name
done

Arithmetic with for (( ))

Like the classic C

$ for((i=0; i<10; i++))
> do
>   echo $i
> done
0
1
2
3
4
5
6
7
8
9

while

quite common iteration

$ while true
> do
>  echo 1
> done
1
1
1 [...]

until

uncommon iteration

until false
> do
> echo 1
> done
1
1
1

case

case is useful alternation

$ x=yellow
$ case $x in
>    green)
>      echo this is green
>      ;;
>    yellow)
>      echo this is yellow
>      ;;
>    esac
this is yellow

Positional parameters

$0 → program name
$1 - $n → positional parameters

Changing the arguments

set → replace the argument list

$ cat ./simple-set.sh 
#!/bin/bash -x
echo Current params: $*
set `hostname`
echo Current params: $*
$ ./simple-set.sh a b c d e f
+ echo Current params: a b c d e f
Current params: a b c d e f
++ hostname
+ set linprog3
+ echo Current params: linprog3
Current params: linprog3

Shifting

Removes the first argument ($1) and promotes the rest one rank; thus, $2 becomes $1, and so forth

$ set a b c d e f
$ echo $*
a b c d e f
$ shift
$ echo $*
b c d e f

"$*" versus "$@"

"$*" is a single string
"$@" is still separate strings

Counting parameters

For instance, using the parameter count variable:

$ set a b c d e f
$ echo $#
6
$ set "$*"
$ echo $#
1
$ set a b c d e f
$ set "$@"
$ echo $#
6

Other special variables

$? → Exit value of last pipeline
$$ → Process id

Environment variables

Use "printenv" to see environment (or even just "env")

$ cat /proc/self/environ | tr '\0' '\n'

Environmental variables

For instance

$ cat /proc/self/environ  | tr '\0' '\n' | grep ALI
$ export ALIBABA=HOCUSPOCUS
$ cat /proc/self/environ  | tr '\0' '\n' | grep ALI
ALIBABA=HOCUSPOCUS

(For more fun with environments, grab environ.c)

Array variables

One very useful thing to do with Bash array variables is to work with lines of a file

$ readarray hosts < /etc/hosts
$ for name in "${hosts[@]}"
> do
>   echo $name
> done
127.0.0.1 localhost
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Chapter 10

John Backus famously observed about "changeable parts" and "framework":

Frameworks in Bash

Alternation

Iteration

if/then/fi

if/then/fi

Something other than test

if/then/else/fi

if/then/elif/then/else/fi

test syntax

test syntax

test syntax

test syntax

for in

for without "in"

Arithmetic with for (( ))

while

until

case

Positional parameters

Changing the arguments

Shifting

"$*" versus "$@"

Counting parameters

Other special variables

Environment variables

Environmental variables

Array variables