kugarocks

Loop

For Statement

for var in list
do
    ...
done

The var variable can be normally used outside the loop body.

#!/usr/bin/env bash

for var in apple banana
do
    echo "hello $var"
done
echo "outside $var"
hello apple
hello banana
outside var

Storing a list in a variable.

#!/usr/bin/env bash

list="apple banana"
list=$list" orange"
for var in $list

Command substitution.

for var in $(cat file)

Wildcard iteration over a directory.

for var in /home/soda/*

IFS - Internal Field Separator

The environment variable IFS (Internal Field Separator) specifies the field separator.

echo $IFS

The default value is empty, indicating that the following characters are separated.

  • Space
  • Tab
  • Newline

Separating by Newline

IFS=$'\n'

Separating by Multiple Symbols

IFS=$'\n:;"'

Indicates that newline, colon, semicolon, and double quotes are used as separators.

For Loop in C Language

The i variable can be normally accessed outside.

#!/usr/bin/env bash

for (( i = 1; i < 3; i++ ))
do
    echo "index $i"
done
echo "last index $i"
index 1
index 2
last index 3

While Statement

while test command
do
    ...
done
#!/usr/bin/env bash

i=3
while [ $i -gt 0 ]
do
    echo "index $i"
    i=$[ $i - 1 ]
done
index 3
index 2
index 1

Multiple while conditions are judged by the exit code of the last condition.

Until Statement

until test command
do
    ...
done
#!/usr/bin/env bash

i=3
until [ $i -le 0 ]
do
    echo "index $i"
    i=$[ $i - 1 ]
done
index 3
index 2
index 1

Break Statement

Supports specifying the number of loop levels to exit.

#!/usr/bin/env bash

for (( i = 0; i < 3; i++ )); do
    echo "i=$i"
    for (( j = 0; j < 3; j++ )); do
        echo "  j=$j"
        if [ $j -eq 1 ]; then
            break 2
        fi
    done
done
i=0
  j=0
  j=1

The break statement defaults to 1, indicating exit from 1 level of loop.

Continue Statement

#!/usr/bin/env bash

for (( i = 0; i < 3; i++ )); do
    echo "i=$i"
    for (( j = 0; j < 3; j++ )); do
        echo "  j=$j"
        if [ $j -eq 1 ]; then
            continue 2
        fi
    done
done
i=0
  j=0
  j=1
i=1
  j=0
  j=1
i=2
  j=0
  j=1

Like break, the default is also 1.

Loop Output Redirection

The standard output of the loop can be redirected.

#!/usr/bin/env bash

for var in apple banana
do
    echo "hello $var"
done > output.txt

The output of the loop body will be redirected to output.txt.

Loop Output Pipe

The output of the loop body can also be piped.

#!/usr/bin/env bash

for var in apple banana
do
    echo "hello $var"
done | sort

Summary

I surprisingly like the syntax sugar of break.

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