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

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.