Bash

http://www.tldp.org/LDP/Bash-Beginners-Guide/html/

четене от стандартния вход (най-често терминала)

read from stdin

#!/bin/bash
echo "write something!"
  read line 
echo "tova e: $line"

for

How do I use bash for loop to repeat certain task under Linux / UNIX operating system? How do I set infinite loops using for statement? How do I use three-parameter for loop control expression?

A 'for loop' is a bash programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement i.e. it is the repetition of a process within a bash script.

For example, you can run UNIX command or task 5 times or read and process list of files using a for loop. A for loop can be used at a shell prompt or within a shell script itself. for loop syntax

Numeric ranges for syntax is as follows:

for VARIABLE in 1 2 3 4 5 .. N
do
	command1
	command2
	commandN
done

#!/bin/bash
 for i in 1 2 3
 do 
   echo "write something!"
   read line 
   echo "$i. $line"
done

for VARIABLE in file1 file2 file3
do
  command1 on $VARIABLE
  command2
  commandN
done

for OUTPUT in $(Linux-Or-Unix-Command-Here)
do
  command1 on $OUTPUT
  command2 on $OUTPUT
  ...
  commandN

done

Примери: This type of for loop is characterized by counting. The range is specified by a beginning (#1) and ending number (#5). The for loop executes a sequence of commands for each member in a list of items. A representative example in BASH is as follows to display welcome message 5 times with for loop:

#!/bin/bash
for i in 1 2 3 4 5
do
  echo "Welcome $i times"
done

Sometimes you may need to set a step value (allowing one to count by two's or to count backwards for instance). Latest bash version 3.0+ has inbuilt support for setting up ranges:

#!/bin/bash
for i in {1..5}
 do
 echo "Welcome $i times"
 done

Bash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax:

#!/bin/bash
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
 do
    echo "Welcome $i times"
 done

Sample outputs:

Bash version 4.0.33(0)-release...
Welcome 0 times
Welcome 2 times
Welcome 4 times
Welcome 6 times
Welcome 8 times
Welcome 10 times

Three-expression bash for loops syntax

This type of for loop share a common heritage with the C programming language. It is characterized by a three-parameter loop control expression; consisting of an initializer (EXP1), a loop-test or condition (EXP2), and a counting expression (EXP3).

for (( EXP1; EXP2; EXP3 ))
do
	command1
	command2
	command3
done

A representative three-expression example in bash as follows:

Три-условен for

 #!/bin/bash
 for (( c=1; c<=5; c++ ))
 do
   echo "Welcome $c times"
 done

//Sample output:

Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times

Три-условен for с масив

#! /bin/bash
Unix[0]='Debian'
Unix[1]='Red hat'
Unix[2]='Ubuntu'
Unix[3]='Suse'
for ((i=0; i<4; i++))
do
  echo ${Unix[$i]}
done

//Резултат
Debian
Red hat
Ubuntu
Suse

Безкраен цикъл

#!/bin/bash
for (( ; ; ))
do
  echo "infinite loops [ hit CTRL+C to stop]"
done

Изход с break

You can do early exit with break statement inside the for loop. You can exit from within a FOR, WHILE or UNTIL loop using break. General break statement inside the for loop:

for I in 1 2 3 4 5
do
 statements1      #Executed for all values of I, up to a disaster-condition if any.
 statements2
 if (disaster-condition)
 then

break #Abandon the loop.

 fi
 statements3          #While good and, no disaster-condition.
done

Following shell script will go though all files stored in /etc directory. The for loop will be abandon when /etc/resolv.conf file found.

#!/bin/bash
for file in /etc/*
  do
  if [ "${file}" == "/etc/resolv.conf" ]
  then
    countNameservers=$(grep -c nameserver /etc/resolv.conf)
    echo "Total  ${countNameservers} nameservers defined in ${file}"
    break
  fi
done

Early continuation with continue statement. To resume the next iteration of the enclosing FOR, WHILE or UNTIL loop use continue statement.

for I in 1 2 3 4 5
do
 statements1      #Executed for all values of I, up to a disaster-condition if any.
 statements2
 if (condition)
 then

continue #Go to next iteration of I in the loop and skip statements3

 fi
 statements3
done

===Пример продължение This script make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.

#!/bin/bash
FILES="$@"
for f in $FILES
do
# if .bak backup file exists, read next file
  if [ -f ${f}.bak ]
  then
    echo "Skiping $f file..."
    continue  # read next file and skip cp command
  fi
# we are hear means no backup file exists, just use cp command to copy file
 /bin/cp $f $f.bak
done

Масиви

#!/bin/bash

declare abc=()

for((i=0;i<100;i++))
do 
# echo "write something!"
# read line 

sleep 1
 ping 10.3.0.1  >/dev/null &
 abc+=($!)

 echo "Process: $i. s PID ${abc[$i]}"
 echo "start pinging"
done

sleep 5

for((i=0;i<100;i++))
do 
 sleep 1
 kill ${abc[$i]}
 echo "killed:  PID ${abc[$i]}"
 echo "start pinging"
done