Read a file (data stream, variable) line-by-line (and/or field-by-field).

Basic read

Input source selection

Read fields from a file

Read from a command instead of a regular file

Read from an interactive shell

Skip Reading Comments

Troubleshooting

If there are some characters after the last line in the file (or to put it differently, if the last line is not terminated by a newline character), then read will read it but return false, leaving the broken partial line in the read variable(s). You can process this after the loop:

# Emulate cat
while IFS= read -r line; do
  printf '%s\n' "$line"
done < "$file"
[[ -n $line ]] && printf %s "$line"

or:

# This does not work:
printf 'line 1\ntruncated line 2' | while read -r line; do echo $line; done
 
# This does not work either:
printf 'line 1\ntruncated line 2' | while read -r line; do echo "$line"; done; [[ $line ]] && echo -n "$line"
 
# This works:
printf 'line 1\ntruncated line 2' | { while read -r line; do echo "$line"; done; [[ $line ]] && echo "$line"; }

The first example, beyond missing the after-loop test, is also missing quotes. See Quotes or Arguments for an explanation why. The Arguments page is an especially important read.

For a discussion of why the second example above does not work as expected, see FAQ #24.

Alternatively, you can simply add a logical OR to the while test:

while IFS= read -r line || [[ -n $line ]]; do
  printf '%s\n' "$line"
done < "$file"
 
printf 'line 1\ntruncated line 2' | while read -r line || [[ -n $line ]]; do echo "$line"; done

Some commands greedily eat up all available data on standard input. The examples above do not take precautions against such programs. For example,

while read -r line; do
  cat > ignoredfile
  printf '%s\n' "$line"
done < "$file"

will only print the contents of the first line, with the remaining contents going to “ignoredfile”, as cat slurps up all available input.

One workaround is to use a numeric FileDescriptor rather than standard input:

# Bash
while IFS= read -r -u 9 line; do
  cat > ignoredfile
  printf '%s\n' "$line"
done 9< "$file"
 
# Note that read -u is not portable to every shell. Use a redirect to ensure it works in any POSIX compliant shell:
while IFS= read -r line <&9; do
  cat > ignoredfile
  printf '%s\n' "$line"
done 9< "$file"

or:

exec 9< "$file"
while IFS= read -r line <&9; do
  cat > ignoredfile
  printf '%s\n' "$line"
done
exec 9<&-

This example will wait for the user to type something into the file ignoredfile at each iteration instead of eating up the loop input.

You might need this, for example, with mencoder which will accept user input if there is any, but will continue silently if there isn't. Other commands that act this way include ssh and ffmpeg. Additional workarounds for this are discussed in FAQ #89.