At the time of writing of Bash scripts, sometimes you need to read a file line by line. In this tutorial, we will show you how to read file line by line in Bash.
Syntax for Read File Line by Line
The following is the common syntax for reading a file line-by-line:
while IFS= read -r line do echo "$line" done < filename
Single line version:
while IFS= read -r line; do echo $line; done < filename
filename is the input file which you want to be open and read with
read command. It will read the file line by line, it will assign a each line to the line variable. While loop will be terminated once the all lines are processed. The internal field separator (
IFS) is set to the null string to preserve leading and trailing whitespace which is the default behavior of the
Examples for Read a File Line By Line
For example, a file name with
cities.txt which contains the list of cities along with the state names separated by comma (
Loa Angeles,California Houston,Texas San Antonio,Texas Columbus,Ohio Las Vegas,Nevada
You would use the following code to read the file line by line:
while IFS= read -r line do echo "$line" done < cities.txt
while IFS= read -r line do if [[ "$line" == *"Texas"* ]]; then echo "$line" fi done < cities.txt
Houston,Texas San Antonio,Texas
The read command allows you to pass multiple variable which will split the line into fields based on the
IFS. It will assign first field to the first variable and second to the second and so on. If there are more fields than variables, the leftover fields are assigned to the last variable.
In the below example, IFS is set to a comma (
,) and passing two variables
state to the read command. So it will assign starting from the line until the first comma to the first variable (
city) and the remaining part of the line will be assigned to the second variable (
while IFS=, read -r city state do echo "$city" belongs to "$state" done < cities.txt
Loa Angeles belongs to California Houston belongs to Texas San Antonio belongs to Texas Columbus belongs to Ohio Las Vegas belongs to Nevada
File Reading Alternate Methods
Process substitution allows you to pass output from command as a filename:
while IFS= read -r line do echo "$line" done < <(cat filename )
Using a Here String
Here String is a variant of Here document . The string (
cat filename ) will keep the newlines:
while IFS= read -r line do echo "$line" done <<< $(cat filename )
Using File descriptor
Using file descriptor you can give the input to the loop:
while IFS= read -r -u9 line do echo "$line" done 9< filename
When working with file descriptor use the number between 4 to 9 to avoid conflict with shell internal file descriptors.
In Bash, we can read a file line-by-line by providing the filename as an input to a while read loop.
If you have any questions or suggestion, please leave a comment below.