• create helloworld.sh

  cat > helloworld.sh
  #!/bin/bash
  strhello="Hello World"
  #print variable on a screen
  echo $strhello

• chmod to excute and run:

  chmod +x helloworld.sh
  ./helloworld.sh

• syntax:

  sh -x script [arg1 ...]
  bash -x script [arg1 ...]

• turn on/off debug mode:

  set +x#turn on
  set -x#turn off

• example

  bash -x helloworld.sh
  + strhello='Hello World'
  + echo Hello World
  Hello World

• syntax:

  sh -v script [arg1 ...]
  bash -v script [arg1 ...]

• turn on/off debug mode:

  set +v#turn on
  set -v#turn off
    * example<code bash>
   bash -v helloworld.sh
  #!/bin/bash
  strhello="Hello World"
  #print variable on a screen
  echo $strhello
  Hello World

Declare simple bash variable and print it on the screen:

#!/bin/bash
strhello="Hello World"
#print variable on a screen
echo $strhello

#!/bin/bash
# BASH FUNCTIONS CAN BE DECLARED IN ANY ORDER
function function_0 {
  echo "Function 0"
}
function function_1 {
  echo "Function 1: $1"
}
function function_2 {
  echo "Function 2: $1,$2"
}
function function_3 {
  echo "Function 3: $1,$2,$3"
}
function_0
function_1 1
function_2 1 2
function_3 "hello" 1 2

Output:

Function 0
Function 1: 1
Function 2: 1,2
Function 3: hello,1,2

• create script test.sh with content below:

  #!/bin/bash
  #Define bash global variable
  #This variable is global and can be used anywhere in this bash script
  var_test="global variable"
  function test {
  #This variable is local to bash function only
    local var_test="local variable"
    echo $var_test
  }
  echo $var_test
  test
  # Note the bash global variable did not change
  # "local" is bash reserved word
  echo $var_test

• run script:

  bash test.sh

  Output:

  global variable
  local variable
  global variable

• create script arguments.sh:

  #!/bin/bash
  echo "use predefined variables to access passed arguments"
  echo $1 $2 $3
  echo "We can also store arguments from bash command line in special array"
  args=("$@")
  echo ${args[0]} ${args[1]} ${args[2]}
  echo "use \$@ to print out all arguments at once"
  echo $@
  echo "Number of arguments passed: $#"

• run script:

  ./function.sh param1 param2 param3

  output:

  use predefined variables to access passed arguments
  param1 param2 param3
  We can also store arguments from bash command line in special array
  param1 param2 param3
  use $@ to print out all arguments at once
  param1 param2 param3
  Number of arguments passed: 3

! NOT

if [ ! -f $FILENAME ]
then
  ...

&& AND

if [ $condition1 ] && [ $condition2 ]
#  Same as:  if [ $condition1 -a $condition2 ]
#  Returns true if both condition1 and condition2 hold true...
 
if [[ $condition1 && $condition2 ]]    # Also works.
#  Note that && operator not permitted inside brackets

OR

if [ $condition1 ] || [ $condition2 ]
# Same as:  if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...
 
if [[ $condition1 || $condition2 ]]    # Also works.
#  Note that || operator not permitted inside brackets
#+ of a [ ... ] construct.

-eq : is equal to

if [ "$a" -eq "$b" ]

-ne : is not equal to

if [ "$a" -ne "$b" ]

-gt : is greater than

if [ "$a" -gt "$b" ]

-ge : is greater than or equal to

if [ "$a" -ge "$b" ]

-lt : is less than

if [ "$a" -lt "$b" ]

-le : is less than or equal to

if [ "$a" -le "$b" ]

< : is less than (within double parentheses)

(("$a" < "$b"))

⇐: is less than or equal to (within double parentheses)

(("$a" <= "$b"))

is greater than (within double parentheses)

(("$a" > "$b"))

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

String Comparisions:

• =: is equal to

  if [ "$a" = "$b" ]

  Caution: Note the whitespace framing the =.

  if [ "$a"="$b" ] is not equivalent to the above.

• ==: is equal to

  if [ "$a" == "$b" ]

  This is a synonym for =

• !=: is not equal to

  if [ "$a" != "$b" ]

  This operator uses pattern matching within a ... construct.

• <: is less than, in ASCII alphabetical order

  if [[ "$a" < "$b" ]]
  if [ "$a" \< "$b" ]

  Note that the “<” needs to be escaped within a [ ] construct.

• >: is greater than, in ASCII alphabetical order

  if [[ "$a" > "$b" ]]
  if [ "$a" \> "$b" ]

  Note that the “>” needs to be escaped within a [ ] construct.

• -z: string is null, that is, has zero length

  String=''   # Zero-length ("null") string variable.
  if [ -z "$String" ]
  then
    echo "\$String is null."
  else
    echo "\$String is NOT null."
  fi     # $String is null.

• -n: string is not null

#!/bin/bash
if [ "aa$1" = "aa" ]; then
   date=`date +%Y_%m_%d`
else
   date=$1
fi
echo $date

loop with while:

<code bash>
while read line; do 
  username=`echo $line | awk '{print $1}'`
  serverid=`echo $line | awk '{print $2}'`
  echo $username,$serverid
  mysql -uzf_9thien -p'.7#:qyJ&$&>bsU83r/#j[=yC' zf_9thien <<< "update fastreg set server_id='$serverid' where username='$username' limit 1;"
done < /tmp/a1.txt

• Init the array and access the elements of array:

  arr=( zero one two three four five )
  echo ${arr[0]}
  echo ${arr[1]}
  echo $arr[0]
  echo $arr[1]

  output:

  zero
  one
  zero[0]
  zero[1]

• Assign the value for array:

  arr=( zero one two three four five )
  echo "before update:"
  echo ${arr[0]}
  arr[0]="new zero"
  echo "after update:"
  echo ${arr[0]}

  output:

  before update:
  zero
  after update:
  new zero

• Access all elements of array with while:

  arr=( zero one two three four five )
  element_count=${#arr[@]}
  index=0
  while [ "$index" -lt "$element_count" ]
  do
    echo ${arr[$index]}
    let "index = $index + 1"
    # Or: ((index++))
  done

• Access all elements of array with for:

  arr=( zero one two three four five )
  element_count=${#arr[@]}
  let "element_count = $element_count - 1"
  for index in `seq 0 $element_count`
  do
    echo ${arr[$index]}
  done

• Access all elements of array with for:

  arr=( zero one two three four five )
  element_count=${#arr[@]}
  for (( i = 0 ; i < $element_count ; i++ ))
  do
      echo "Element [$i]: ${arr[$i]}"
  done

  output:

  Element [0]: zero
  Element [1]: one
  Element [2]: two
  Element [3]: three
  Element [4]: four
  Element [5]: five

if [  -f filename ]; then
   echo "file exists"
else
   echo "file not exists"
fi;

• Simple

  grep -rnw . -e "Begin Facebook Code"

• Custom find with regular expression

  grep -rnw . -e 'mongo.*'

for i in `ls -1`;do sub_i=(`echo $i|cut -d'-' -f 1`);lensub_i=${#sub_i}; if [ $lensub_i -ge 29 ]; then echo $sub_i;echo $i; fi  done

refer: http://tldp.org/LDP/abs/html/string-manipulation.html

• String length: ${#string}:

  str="First String"; echo ${#str}

  output:

  12

• Replace String:

  ${string//substring/replacement}

  Example:

  str="First String"; echo ${str//First/Second}

  output:

  Second String

Below is array configs.txt:

c1 = 0,4,5,8,9,12,13,16,17,19,22,21,45,2,3,6,7,10,11,14,15,20,1,23,44,50,26,27,28,29,30,31,32,33,34,35,38,49,47,18,36,37,24,39,40,41,42,43,46,48,25,51
c2 = 51,4,5,8,9,12,13,16,17,25,22,21,45,2,3,6,7,10,11,14,15,20,1,23,44,50,26,27,28,29,30,31,32,33,34,35,38,49,47,18,36,37,24,39,40,0,42,43,46,48,19,41
c3 = 26,27,33,34,8,9,12,13,40,41,0,30,31,2,3,6,7,10,11,14,15,20,21,23,44,50,45,1,28,29,32,19,22,4,5,35,38,49,47,18,36,37,24,39,16,17,42,43,46,48,25,51

update array to files:

index=0;for i in `cat configs.txt | awk '{print $3}'`; do let "index = $index + 1";echo $i > c$index.txt; done