linux bash中while循环结构

The syntax is as follows:

           while [ condition ]
           do
                 command1
                 command2
                 ..
                 ....
                 commandN
           done

Command1..commandN will execute while a condition is true. For example following while loop will print welcome 5 times on screen:

#!/bin/bash
# set n to 1
n=1

# continue until $n equals 5
while [ $n -le 5 ]
do
    echo "Welcome $n times."
    n=$(( n+1 ))     # increments $n
done

To read a text file line-by-line, use the following syntax:

           while IFS= read -r line
           do
                 command1 on $line
                 command2 on $line
                 ..
                 ....
                 commandN
           done < "/path/to/filename"

OR

while IFS= read -r field1 filed2 field3 ... fieldN
           do
                 command1 on $field1
                 command2 on $field1 and $field3
                 ..
                 ....
                 commandN on $field1 ... $fieldN
           done < "/path/to dir/file name with space"

IFS is used to set field separator (default is while space). The -r option to read command disables backslash escaping (e.g., \n, \t). This is failsafe while read loop for reading text files.