12 Compound commands

Question 1

Lists

Answer 1

A list is a sequence of one or more pipelines separated by “;”, “&”, “&&” or “||”, and optionally terminated by one of “;”, “&” or end-of-line. The return value of a list is that of the last command executed in it.

Question 2

Execute list in a subshell to contain state changes

Answer 2

( ⟨list⟩ ) executes list in a subshell (e.g., to contain state changes)

Question 3

Group a list to override operator priorities

Answer 3

{ ⟨list⟩ ; } groups a list (to override operator priorities)

Question 4

For loop

Answer 4

for ⟨variable ⟩ in ⟨words ⟩ ; do ⟨list ⟩ ; done

for f in *.txt ; do cp $f $f.bak ; done

Question 5

Conditionals

Answer 5

if ⟨list⟩ ; then ⟨list⟩ ; elif ⟨list⟩

then ⟨list⟩ ; else ⟨list⟩ ; fi

Question 6

While

Answer 6

while ⟨list⟩ ; do ⟨list⟩ ; done until ⟨list⟩ ; do ⟨list⟩ ; done

Question 7

Ignore next linefeed

Answer 7

single backslash at end of line

Question 8

case

Answer 8

case ⟨word ⟩ in ⟨pattern⟩|⟨pattern⟩|. . . )
⟨list⟩ ;; …
esac

Matches expanded ⟨word⟩ against each ⟨pattern⟩ in turn (same matching rules as pathname expansion) and executes the corresponding ⟨list⟩ when first match is
found

case "$command" in
      start)
app_server &
        processid=$! ;;
      stop)
        kill $processid ;;
      *)
        echo 'unknown command' ;;
    esac

Question 9

Boolean tests – inverse logic

Answer 9

[  ] 
e.g. 
-e  = file exists
-d ... = exists + directory 
...

Question 10

String comparisons lexico

Answer 10

⟨string1⟩ == ⟨string2⟩

⟨string1⟩ != ⟨string2⟩

⟨string1⟩ < ⟨string2⟩

⟨string1⟩ > ⟨string2⟩

Question 11

Aliases allow a string to be substituted for the first word of a command:

Answer 11

$ alias dir=’ls -la’

$ dir

Question 12

Shell functions

Answer 12

Shell functions are defined with “⟨name⟩ () { ⟨list⟩ ; }”. In the function body, the command-line arguments are available as $1, $2, $3, etc. The variable $* contains all arguments and $# their number.
[unalias dir]
$ dir () { ls -la $* ; }

Question 13

Function variable access

Answer 13

Outside the body of a function definition, the variables $*, $#, $1, $2, $3, . . . can be used to access the command-line arguments passed to a shell script.