The for loop

In general, for loop can be used in the below scenarios.

Executes a command or set of commands many times
Iterate through files
Iterate through lines within the file
Iterate through output of the command

How does it work?

This loop allows for specification of a list of values. A list of commands is executed for each value in the list.

Syntax

for <variable> in [ LIST ] 
do 
    COMMANDS/statements
done

whereas, LIST can be any list of words, strings or numbers or generated by any command.

The first time through the loop, "variable" is set to the first item in LIST. The second time, its value is set to the second item in the list, and so on.

The loop terminates when no items are left in LIST. The return status is the exit status of the last command that executes in the for loop. If no commands are executed, the return status is zero.

With Strings

Example

#!/bin/bash
for name in Sachin Rahul Saurav Viru Anil Javagal Kapil Zaheer Harbhajan
do
  echo "${name}"
done

NOTE:

When [ LIST ] is empty and number of arguments are passed through runtime as environmental variable, then [ LIST] is replaced with in $@

Example

Assume below code snippet saved as shell script and while executing this shell script, we need to pass the input list as an arguments.

for name in $@
do
  echo "${name}"
done

We can pass the list from the file as well. We can use backquotes to pass the input file as shown below. This is one way of passing the input list.
```
for name in `cat names.txt`
do
  echo "${name}"
done
```

Output

Another way of passing the input list is within parenthesis. This is the best practice to follow. The reason is that you can pass multiple commands within parenthesis.
```
for name in $(cat names.txt | grep -i il)
do
  echo "${name}"
done
```

Output

REMEMBER:

You can pass multiple commands within backquotes too (like below). But that is not a suggestible way.
```
for name in `cat names.txt | grep -i il`
do
  echo "${name}"
done
```

With Numbers

for number in 1 2 3 4 5
do
  echo "${number}"
done

The same code can be little modified with {starting_number..ending_number} instead of individual number.

for number in {1..5}
do
  echo "${number}"
done

Note

There is no space within the curly braces. If there is any space, then that will be considered as separate string
for number in { 1..5 }
do
 echo "${number}"
done
Output will be
{
1..5
}

C style for loop with numbers

for((index=1; index<=5; index++))
do
  touch file${index}.txt
  echo "${index}" > file"${index}.txt"
  echo "Number of lines in file${index}:$(cat file${index}.txt |wc -l)"
done

Use of break command

Break command is used to exit of the immediate loop.

Syntax

for <variable> in [ LIST ]
do
  statements/COMMANDS
  if [[ condition ]]
  then
    break # In general, break can be used in loops to check if certain condition is met. If the condition is met, then exit of the loop
  fi
done

Example

Let us assume, if the file contains many names. We would like to search a particular name if it exists in the file. If found, we can stop the search. To do this,

#!/bin/bash
# This script is to just print the given number if it is less than 4.
for name in $(cat names.txt)
do
  echo "${name}"
  if [[ "${name}" == "Kapil" ]]
  then
    echo "Name ${name} found"
    break
  fi
done

Another example with nested for loop along with break command

#!/bin/bash
## This script is to just print the given number and also names for each number

for number in {1..5} # OUTER FOR LOOP
do
  echo ${number}
  for name in Satheesh Jeganathan Adhira Praba # INNER FOR LOOP
  do
    echo ${number}. ${name}
    if [[ "${name}" == "Jeganathan" ]]; then
      echo ${name} is found and breaking my immediate loop
      break # BREAK ONLY INNER FOR LOOP
    fi
  done

  if [[ ${number} -eq 3 ]]; then
    break # BREAK ONLY OUTER FOR LOOP
  fi
done

Use of continue command

Continue command is used to skip the current iteration and move to next iteration in the loop.

Syntax

for <variable> in [ LIST ]
do
  statements/COMMANDS
  if [[ condition ]]
  then
    continue # In general, continue can be used in loops to check if certain condition is met. If the condition is met, then skip that iteration 
  fi
done

Example

#!/bin/bash
## This script is to just print the player who's name is not Saurav.
for name in $(cat names.txt)
do
  if [[ "${name}" == "Saurav" ]]
  then
    continue # Skip the rest of the statements in for loop (immediate)
  fi
  echo "${name} is selected for the tournament"
done