Read file line by line

Reading a file into a bash script on a per-line basis is pretty simple:

1while read -r line; do command "${line}"; done < src.file

here we read a line from stdin and as long as a line was read the line is passed to command as a single command argument.

the -r option prevents backslash escapes in the line being interpreted.

If you want to prevent leading or trailling whitespace from being removed from the line then add IFS= to the line:

1while IFS= read -r line; do command "${line}"; done < src.file

Within scripts

The above is useful when writing something quick on the command line, but in a script it's more customary to write this across multiple lines.

What I tend to do is wrap the while loop within ( ) with the input redirect after the parentheses. To me this makes it clearer on what the bounds of the script relates to the input.

For example, here we have a script which writes the content of a file to stdout but adds line numbers to the output:

#!/bin/bash
input="$1"
(
  lc=1
  while IFS= read -r line
  do
    printf "%03d %s\n" ${lc} "${line}"
    lc=$((lc+1))
  done
) < "${input}"

When run against itself:

$ ./morelines.sh morelines.sh
#!/bin/bash
input="$1"
(
 lc=1
 while IFS= read -r line
 do
   printf "%03d %s\n" ${lc} "${line}"
   lc=$((lc+1))
 done
) < "${input}"

Read file line by line

Table of Contents

Within scripts