NOTE & BLOG

Assignment and Substitution

a=375
hello=$a

Variables

1. Built-in Variables
   For example $HOME $PWD ..., for more info, see environ(7)
2. Positional Parameters
   echo $para1$para2 $para3$para4 $0$1 $2$3 $4$@
3. Special Parameters
   $? # exit status of a command, function, or the script itself

Branches

if [ condition1 ];then
    command_series1
elif [ condition2 ];then
    command_series2
else
    default_command_series3
fi

Loops

range for

for arg in `seq 10`; do
  echo $arg
done

for in C-like syntax

LIMIT=10
for ((a=1; a<=LIMIT; a++)); do
  echo "$a "
done

while

LIMIT=10
a=1
while ((a<=LIMIT)); do
    echo "$a "
    ((a += 1))
done

IO

command < input-file > output-file    # rewrite
command >> output-file             # appending

Function

# define a function
function fun_name(){
    command...
}
## or
fun_name(){ # arg1 arg2 arg3
    command...
}


# apply a function
fun_name $arg1 $arg2 $arg3


# dereference
fun_name(){ # arg1
    eval "$1=hello"
}
fun_name arg1
## the above code block is equivalent to
arg1=hello

Debugging

1. take good use of sh(1)
   for example:
   sh -n script: checks for syntax
   sh -v script: echo each command before executing it
   sh -x script: echo the result of each command in an abbreviated manner
2. use echo
3. use trap

Parallel

use GNU parallel

Script with Style

1. Comment your code
2. Avoid using magic number
3. Use exit codes in a systematic and meaningful way
4. Use standardized parameter flags for script invocation