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Incomplete sheet

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Korn shell

https://en.wikipedia.org/wiki/KornShell

Details Notes:

  • Start script with: #!/bin/ksh
  • Enable executation for file by running chmod u+x script.sh

Command-line argument parsing

Action Code Details
Program name, including the path if started from another directory 
$0
Number of arguments 
$#
Get the _n_th argument 
$n
Get the _n_th argument, with default value 
${n-"Default value here"}
Expand all arguments to a single word 
$*
Expand all arguments to single string 
$@
Expand arguments to separate words 
"$@"

Test

Action Code Details
Check for zero arguments 
if [[ $# -eq 0 ]];then
    print "No Arguments"
    exit
fi

Special variables

Note that the positional parameters are special variables too.

Action Code Details
Exit status of last command 
$?
Process id of current program 
$$
Process id of last background job 
$!

Constants

Action Code Details
Declare constant 
declare -r PASSWD_FILE=/etc/passwd

Variables

  • Don't put dots in variable names!
  • By default, all variables are global! Even within functions!
Action Code Details
Get variable value 
$var
Get variable value with default value value 
${var:-value}
Get variable value but throw error if not set 
${var:?"Error! var not set!"}
Get variable and set it with default value if it is not set 
${var:=value}
Check if variable is set 
${var:+1}
Returns 1 if set, else nothing
Set value for variable 
var=value
Set value from user input 
read var
Set command output for variable 
var=command args
Declare local variable 
typeset var
Set local variable 
typeset var = value

Functions

Use the local statement to define local variables.

Action Code Details
Define function 
function get_name {
    return $name
}
Define function with arguments 
function increment {
    local x = $1
    return (( $x + 1 ))
}
Call function 
foo
Call function with arguments 
foo arg1 arg2

Control flows

See http://www.bolthole.com/solaris/ksh-basics.html

Action Code Details
Chain (pipe) commands 
command1 | command2 | command3
If 
if [[ $value -eq 2 ]];then
    print "$value is 2"
fi
If-else 
if [[ $value -eq 2 ]];then
    print "$value is 2"
else
    print "$value is not 2"
fi
If-elseif 
if [[ $status -eq 200 ]];then
    print "OK (status = 200)"
elif [[ $value -gt 400 ]];then
    print "Error (status > 400)"
else
    print "Not 200 but < 400"
fi
Switch 
case $food in
    apple|pear) print "fruit";;
    onion)  print "veggy";;
    *)  print "unknown";;
esac
Cases don't fall through, unless ;& is used instead of ;; Strings can be matched using wildcards.
For 
for f in $(ls);do
    (( count += 1 ))
done
print "Total: $count"
Use continue to skip the loop. Use break to exit the loop.
Until 
until [[ $stopnow -eq 1 ]] ; do
    echo just run this once
    stopnow=1;
    echo we should not be here again.
done
Pause for n seconds 
sleep n

Error handling

Action Code Details
Throw error and stop execution, with unspecified error 
exit 1
Throw error with message 
echo "Error!" 1>&2
exit 2
Run command only if the former command succeeds 
command1 && command2
Run command only if the former command fails 
command1 || command2
Run (cleanup) function on error 
trap cleanup ERR
For a previously defined cleanup function
Run code on error 
trap "echo Oh no!" ERR
Run code on interupt 
trap "echo Oh no!" INT
e.g., when user presses Ctrl+C
Run code on error or interrupt 
trap "echo Oh no!" ERR INT
Run code on process termination 
trap "echo Oh no!" TERM

Number handling

Action Code Details
Increment variable 
(( count += 1))
Increment variable 
let count += 1
Sum variables 
sum = $(( a + b ))
Equals number num 
$count -eq num
Not equals number num 
$count -ne num
Greater than number num 
$value -gt num
Less than number num 
$value -lt num

String handling

Action Code Details
Equals string str 
$name = "str"
Not equals string str 
$name != "str"

Path handling

Action Code Details
Get parent path 
${name%/*}
Get filename from path 
${name##*/}

Output

Action Code Details
Print message 
echo "Hello world!"
Print variable 
echo $var
Formatted printing 
printf "counting %d days" $days

Data redirection

Action Code Details
Write to new file or overwrite file 
command > file
Append file 
command >> file
Redirect error output 
command 2> file
Discard errors 
command 2>/dev/null
Redirect to normal output 
command 2>&1
Discard all output 
command > /dev/null 2>&1
File as input to command 
command < file

Filesystem handling

Action Code Details
Check if directory exists 
[ -d "$DIRPATH" ]
Check if directory is missing 
[ ! -d "$DIRPATH" ]
Create directory 
mkdir "$DIRPATH"
Check if file exists 
[ -f "$FILEPATH" ]
Count the number of files in a directory 
count=find $dir -maxdepth 1 -name "*.txt" -type f | wc -l