Home » How to loop over the lines of a file?

How to loop over the lines of a file?

Solutons:


With for and IFS:

#!/bin/bash

IFS=$'n'       # make newlines the only separator
set -f          # disable globbing
for i in $(cat < "$1"); do
  echo "tester: $i"
done

Note however that it will skip empty lines as newline being an IFS-white-space character, sequences of it count as 1 and the leading and trailing ones are ignored. With zsh and ksh93 (not bash), you can change it to IFS=$'nn' for newline not to be treated specially, however note that all trailing newline characters (so that includes trailing empty lines) will always be removed by the command substitution.

Or with read (no more cat):

#!/bin/bash

while IFS= read -r line; do
  echo "tester: $line"
done < "$1"

There, empty lines are preserved, but note that it would skip the last line if it was not properly delimited by a newline character.

(9 years later:)
Both provided answers would fail on files without a newline at the end, this will effectively skip the last line, produce no errors, would lead to disaster (learned hard way:).

The best concise solution I found so far that “Just Works” (in both bash and sh):

while IFS='' read -r LINE || [ -n "${LINE}" ]; do
    echo "processing line: ${LINE}"
done < /path/to/input/file.txt

For more in-depth discussion see this StackOverflow discussion: How to use “while read” (Bash) to read the last line in a file if there’s no newline at the end of the file?

Beware: this approach adds an additional newline to the last line if there is none already.

If you can avoid it, don’t especially if it’s to process text.

Most text utilities are already designed to process text one line at a time, and, at least for the GNU implementations, do it efficiently, correctly and handle error conditions nicely. Piping one to another which runs them in parallel also means you can leverage more than one processor to do the job.

Here:

<input.txt sed 's/^/tester /' > output.txt

Or:

<input.txt awk '{print "tester", $0}' > output.txt

More on that at: Why is using a shell loop to process text considered bad practice?

If it’s not about text processing and you do need to run some command per line of a file, also note GNU xargs where you can do:

xargs -rd'n' -I@ -a input.txt cp -- @ @.back

for instance.

With the bash shell, you can get each line of a file into an array with the readarray builtin:

readarray -t lines < input.txt &&
  for line in "${lines[@]}"; do
    do-some-non-text-processing-you-cannot-easily-do-with-xargs "$line" || break
  done

POSIXly, you can use IFS= read -r line to read one line off some input, but beware that if you redirect the whole while read loop with the input file on stdin, then commands inside the loop will also have their stdin redirected to the file, so best is to use a different fd which you close inside the loop:

while
  IFS= read -r line <&3 ||
    [ -n "$line" ] # to cover for an unterminated last line.
do
  {
    do-some-non-text-processing-you-cannot-easily-do-with-xargs "$line" ||
      break # abort upon failure if relevant
  } 3<&-
done 3< input.txt > output.txt

read -r line removes leading and trailing whitespace characters from the line that it reads provided they are in the $IFS variable, though only the yash shell honours that POSIX requirement. With most shells, that’s limited to space and tab. ksh93 and recent versions of bash do it for all single-byte characters considered as whitespace in the locale.

So to read a line and also strip leading and trailing blanks, you can do: IFS=$' t' read -r line. With ksh93, yash¹ or recent versions of bash. IFS=$' tr' would also strip the trailing CR character found in text files from the Microsoft world.


¹ though yash doesn’t support the $'...' syntax yet, you’d need IFS=$(printf ' tr') there.

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