Looping in

About Looping

Bash has two forms of looping:

• Repeat some commands for each item in a list of items,
• Repeat some commands while some condition is true (or false).

For Loops

To iterate over a list of items, we use the for loop.

for varname in words; do COMMANDS; done

words is expanded using word splitting and filename expansion (that we learned about in the Quoting concept). Each word is assigned to the varname in turn, and the COMMANDS are executed.

Here are some examples that show when the for loop can be useful

• Perform some commands on a group of files.

  # source all the bash library files in the current directory
  for file in ./*.bash; do
      source "$file"
  done
  

• Perform some command on each of the parameters to a script or function.

  for arg in "$@"; do
      echo "argument: ${arg}"
  done
  

• Perform some command for each whitespace-separated word in the output of a command.

  for i in $(seq 10); do
      echo "${i} squared is $((i * i))"
  done
  

  We'll see some warnings about this style later on.

Arithmetic For Loop

Bash does have an arithmetic loop. This will look somewhat familiar to other programming languages.

for ((INITIALIZATION; CONDITION; INCREMENT)); do COMMANDS; done

The double-parentheses in bash represents an arithmetic context (we'll see more about bash arithmetic in a later concept). The above example using seq can be written like this

for ((i = 1; i <= 10; i++)); do
    echo "${i} squared is $((i * i))"
done

When Not To Use For

Don't read lines with for.

This is an anti-pattern you'll often see: iterating over the output of cat.

for line in $(cat some.file); do 
    do_something_with "$line"
done

This is wrong on 2 counts.

1. The command substitution is unquoted, so it is subject to word splitting. Word splitting, by default, splits on whitespace not just newlines. This for loop is iterating over the words in the file, not the lines.
2. The command substitution is unquoted, so it is subject to filename expansion. Every word in the file will be matched as a glob pattern.

As we learned in the Quoting concept, word splitting can be controlled with the IFS variable, and filename expansion can be turned off. But this tends to be a fragile solution. for is the wrong method to iterate over the lines of a file. The idiomatic way to read a file is with a while loop.

While Loops

Use while to repeat a sequence of commands while some condition is true.

while CONDITION_COMMANDS; do COMMANDS; done

As with if (as we learned in the Conditionals concept, there is no special syntax for CONDITION_COMMANDS. The exit status of the command list will determine "true" or "false".

Controlling Loops

The break command jumps out of the loop. Control resumes with the command following the loop's done terminator.

The continue command jumps to the next iteration of the loop.

Reading the Lines of a File

As mentioned, a while loop is the idiomatic way to read a file.

while IFS= read -r line; do
    do_something_with "$line"
done < some.file

• The content of the file is provided as input to the loop with the < redirection.

• The read command returns "true" if it can read a full line from its input. When the input is consumed, or if the last line ends without a trailing newline, then read returns "false".

  • The truly idiomatic way to read the lines of a file, even if the last line does not end with a newline, is

    while IFS= read -r line || [[ -n "$line" ]]; do ...
    

• The -r option tells read to not substitute backslash sequences: a backslash is just a plain character.

• IFS= assigns the empty string to IFS only for the duration of the read command. This temporarily turns off word splitting so that any leading or trailing whitespace in the incoming line of text is preserved.

Until

The until construct repeats a sequence of commands while some condition is false. The loop repeats until the condition becomes true.

until CONDITION_COMMANDS; do COMMANDS; done

It is rare to use until. It is more common to use a "while not" loop.

while ! CONDITION_COMANDS; do ...

Infinite Loops

Sometimes you want to loop forever. Here are two ways to do it.

1. a while loop with a condition that always has a success exit status

   while true; do ...
   

2. an arithmetic for loop with empty expressions

   for ((;;)); do ...
   

Do-While

There is no explicit do-while construct, but you can achieve the same effect:

while true; do
    COMMANDS

    if END_CONDITION; then
        break
    fi
done