Bash Read File Line Into Multiple Variables
This article is all nearly how to read files in bash scripts using a while loop. Reading a file is a common operation in programming. You should exist familiar with different methods and which method is more efficient to use. In bash, a single job can be achieved in many ways but there is e'er an optimal mode to get the job washed and we should follow it.
Before seeing how to read file contents using while loop, a quick primer on how while loop works. While loop evaluates a condition and iterates over a given set of codes when the condition is true.
while [ Condition ] do code block done
Let'due south break downwards while loop syntax.
- while loop should outset with a while keyword followed by a condition.
- A condition should be enclosed inside [ ] or [[ ]]. The condition should always return truthful for the loop to be executed.
- The actual block of code will be placed betwixt do and done.
NUMBER=0 while [[ $NUMBER -le 10 ]] do repeat " Welcome ${NUMBER} times " (( NUMBER++ )) done 
This is a very simple instance, where the loop executes until NUMBER is not greater than x and prints the repeat argument.
Forth with while nosotros will utilise the read command to read the contents of a file line by line. Below is the syntax of how while and read commands are combined. At present at that place are unlike ways to pass the file as input and we will see them all.
# SYNTAX while read VARIABLE do code done
Pipe in Linux
Normally we volition use the true cat command to view the contents of the file from the last. Also, we will pipe the output of the cat control to other commands like grep, sort, etc.
Similarly, we will use the cat control here to read the content of the file and pipage it to a while loop. For demonstration, I am using /etc/passwd file merely it is non appropriate to mess with this file so take a backup copy of this file and play with it if y'all want so.
cat /etc/passwd | while read LREAD do repeat ${LREAD} done 
Allow's break downwardly what will happen when the above code is submitted.
- cat /etc/passwd volition read the contents of the file and pass it as input through the pipe.
- read command reads each line passed as input from true cat command and stores it in the LREAD variable.
- read command volition read file contents until EOL is interpreted.
You lot tin can too use other commands like head, tail, and pipage it to while loop.
head -n 5 /etc/passwd | while read LREAD do repeat ${LREAD} washed 
Input Redirection in Linux
Nosotros tin redirect the content of the file to while loop using the Input redirection operator (<).
while read LREAD do echo ${LREAD} done < /etc/passwd | head -n v 
You can also store the file proper noun to a variable and pass it through a redirection operator.
FILENAME="/etc/passwd" while read LREAD do echo ${LREAD} washed < ${FILENAME} 
Yous tin also pass file names every bit an argument to your script.
while read LREAD do echo ${LREAD} done < $1 | caput -n 5 
Internal Field Separator
You may work with different types of file formats (CSV, TXT, JSON) and you may want to separate the contents of the file based on a custom delimiter. In this example, y'all can use "Internal field separator (IFS)" to divide the content of the file and store it in variables.
Let me demonstrate how information technology works. Take a look at the /etc/passwd file which has a colon (:) equally the delimiter. You tin now dissever each discussion from a line and store it in a separate variable.
In the below example, I am splitting /etc/passwd file with a colon every bit my separator and storing each divide into different variables.
while IFS=":" read A B C D Eastward F G practice echo ${A} echo ${B} repeat ${C} echo ${D} echo ${E} echo ${F} echo ${G} done < /etc/passwd 
I displayed just 1 line split in the above screenshot considering screenshot size.
Empty Lines in Linux
Empty lines are non ignored when y'all loop through the file content. To demonstrate this I accept created a sample file with the below content. In that location are 4 lines and few empty lines, leading whitespace, trailing white space, tab characters in line ii, and some escape characters (\n and \t).
while read LREAD do echo ${LREAD} done < testfile 
Come across the outcome, bare line is non ignored. Also, an interesting matter to note is how white spaces are trimmed by the read command. A simple way to ignore blank lines when reading the file content is to use the exam operator with the -z flag which checks if the string length is zero. At present let's repeat the same case but this time with a exam operator.
while read LREAD do if [[ ! -z $LREAD ]] and so echo ${LREAD} fi done < testfile 
Now from the output, you lot can see empty lines are ignored.
Escape Characters
Escape characters like \n, \t, \c will non be printed when reading a file. To demonstrate this I am using the same sample file which has few escape characters.
while read LREAD practise echo ${LREAD} washed < testfile 
You can see from the output escape characters have lost their significant and only n and t are printed instead of \n and \t. You tin can employ -r to prevent backslash estimation.
while read -r LREAD do echo ${LREAD} done < testfile 
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