CMD List [9]

1          Shell

1.1         variable ( $ )

System variable.

Option	Function
$abc ${abc}	abc = "blue" echo ${abc}
$(command)	e.g. v1 = "$(hostname)", v2 = "$(pwd)"
$?	Expands to the exit status of the most recent pipeline. (last status code)
$n	$1 first argument, $2 second …
$#	Count of argument of command line
$0	Program name.
$*	Store all argument with “arg1 arg2 …”
$@	Store all argument with “arg1” “arg2” …

Note: If using in “…”, add {}, e.g. “… ${0} …”

 

User variable

e.g. name="Winter" echo $name

 

Environment variable

e.g. PATH, LOGNAME (logon user name)…

 

{}

Concatenate variable and string. E.g. SAT=Satur echo ${SAT}day

 

Unset

Clean variable. E.g. unset a

 

readonly

Content of variable can’t be changed. E.g. readonly a=2;unset a;echo “a=$a”;

 

array

e.g. name[0]=”a”;name[1]=”b”; echo ${name[$i]};

 

1.2         typeset

typeset [options] [variable[=value ...]]

 

Korn shell only. Assign a type to each variable. -option enables the type and +option disables it.

 

argument	Function
i	Integer, e.g. typeset -i n1 n2 n3
(nil)	Character, e.g. typeset sh_name

e.g.

typeset +r PWD End read-only status of PWD.

 

1.3         let

let expressions

     or

((expressions))

 

Korn shell only. Specified by one or more integer expressions which consist of numbers, operators, shell variables. Expressions must be quoted if they contain spaces.

e.g.

i=`expr $i + 1`

let i=i+1

let "i = i + 1"

(( i = i + 1 ))

let $a + $b

 

1.4         Control Statement

while [condition]  (or [[ … ]]) do     … done (or done < InputFile)

 

for string in list do     … done

 

if / elif [condition] (or [[ … ]]) then     … (else)     … fi

 

Operation in control statement

Operator [[ ]]   -a file -d file -e file -f file -r file -s file -w file -x file   -O file -G file 1 -nt 2 1 -ot 2   -lt   !=, >, < -n str -z str   &&,||,-a,-o

True if... It is better integrated into the Korn shell, it provides many options to check different attributes of a file, returns an exit status. True if the file exists. file is a directory True if the file exists. file is a regular file (i.e., not a directory or other special type of file) You have read permission on file file exists and is not empty You have write permission on file You have execute permission on file, or directory search permission if it is a directory You own file Your group ID is the same as that of file file1 is newer than file2 file1 is older than file2   Less than (other like -le, -eq, -ge, -gt, -ne)   Normal compare. Str is not null (length greater than 0). Str is null (length is 0).   Seems same as and (&&) or (||).

 

Other

break	Exit from a while or for loop.
continue	Begin next iteration of while or for loop without reaching the bottom.
exit	Do not execute remaining instruction and read no new input. END procedures will be executed.
return [expr]	For nawk. Used at end of user-defined functions to exit function, returning value of expression.

 

e.g.

if [[ -f "$ldcmd" ]]

then

       while read LINE

       do

              …

       Done < ${ldcmd}

fi

rc=$?

return ${rc}

 

if command && [[ condition_1 || condition_2 ]]; then …

while [ $hour -le 0030 -o $hour -ge 2200 ]

 

1.5         Function

function  function_name () {commands; commands; }

unset -f function_name

 

1.6         Concatenate

Ø      Break line: ( backslash \ )

Ø      Concatenate string: ( blank )