How can I read a file (data stream, variable) line-by-line (and/or field-by-field)?

Don't try to use "for". Use a while loop and the read command:

    while IFS= read -r line; do
      printf '%s\n' "$line"
    done < "$file"

The -r option to read prevents backslash interpretation (usually used as a backslash newline pair, to continue over multiple lines). Without this option, any backslashes in the input will be discarded. You should almost always use the -r option with read.

The most common exception to this rule is when -e is used, which uses Readline to obtain the line from an interactive shell. In that case, tab completion will add backslashes to escape spaces and such, and you do not want them to be literally included in the variable. This would never be used when reading anything line-by-line, though, and -r should always be used when doing so.

In the scenario above IFS= prevents trimming of leading and trailing whitespace. Remove it if you want this effect.

line is a variable name, chosen by you. You can use any valid shell variable name there.

The redirection < "$file" tells the while loop to read from the file whose name is in the variable file. If you would prefer to use a literal pathname instead of a variable, you may do that as well. If your input source is the script's standard input, then you don't need any redirection at all.

If your input source is the contents of a variable/parameter, BASH can iterate over its lines using a "here string":

    while read -r line; do
      printf '%s\n' "$line"
    done <<< "$var"

The same can be done in any Bourne-type shell by using a "here document" (although read -r is POSIX, not Bourne):

    while read -r line; do
      printf '%s\n' "$line"
    done <<EOF
$var
EOF

If avoiding comments starting with # is desired, you can simply skip them inside the loop:

    # Bash
    while read -r line; do
      [[ $line = \#* ]] && continue
      printf '%s\n' "$line"
    done < "$file"

If you want to operate on individual fields within each line, you may supply additional variables to read:

    # Input file has 3 columns separated by white space.
    while read -r first_name last_name phone; do
      # Only print the last name (second column)
      printf '%s\n' "$last_name"
    done < "$file"

If the field delimiters are not whitespace, you can set IFS (internal field separator):

    # Extract the username and its shell from /etc/passwd:
    while IFS=: read -r user pass uid gid gecos home shell; do
      printf '%s: %s\n' "$user" "$shell"
    done < /etc/passwd

For tab-delimited files, use IFS=$'\t'.

You do not necessarily need to know how many fields each line of input contains. If you supply more variables than there are fields, the extra variables will be empty. If you supply fewer, the last variable gets "all the rest" of the fields after the preceding ones are satisfied. For example,

    read -r first last junk <<< 'Bob Smith 123 Main Street Elk Grove Iowa 123-555-6789'

    # first will contain "Bob", and last will contain "Smith".
    # junk holds everything else.

Some people use the throwaway variable _ as a "junk variable" to ignore fields. It (or indeed any variable) can also be used more than once in a single read command, if we don't care what goes into it:

    read -r _ _ first middle last _ <<< "$record"

    # We skip the first two fields, then read the next three.
    # Remember, the final _ can absorb any number of fields.
    # It doesn't need to be repeated there.

The read command modifies each line read; by default it removes all leading and trailing whitespace characters (spaces and tabs, or any whitespace characters present in IFS). If that is not desired, the IFS variable has to be cleared:

    # Exact lines, no trimming
    while IFS= read -r line; do
      printf '%s\n' "$line"
    done < "$file"

One may also read from a command instead of a regular file:

    some command | while read -r line; do
      printf '%s\n' "$line"
    done

This method is especially useful for processing the output of find with a block of commands:

    find . -type f -print0 | while IFS= read -r -d '' file; do
        mv "$file" "${file// /_}"
    done

This reads one filename at a time from the find command and renames the file, replacing spaces with underscores.

Note the usage of -print0 in the find command, which uses NUL bytes as filename delimiters; and -d '' in the read command to instruct it to read all text into the file variable until it finds a NUL byte. By default, find and read delimit their input with newlines; however, since filenames can potentially contain newlines themselves, this default behaviour will split up those filenames at the newlines and cause the loop body to fail. Additionally it is necessary to set IFS to an empty string, because otherwise read would still strip leading and trailing whitespace. See FAQ #20 for more details.

Using a pipe to send find's output into a while loop places the loop in a SubShell and may therefore cause problems later on if the commands inside the body of the loop attempt to set variables which need to be used after the loop; in that case, see FAQ 24, or use a ProcessSubstitution like:

    while read -r line; do
      printf '%s\n' "$line"
    done < <(some command)

If you want to read lines from a file into an array, see FAQ 5.

My text files are broken! They lack their final newlines!

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 || [[ $line ]]; do
      printf '%s\n' "$line"
    done < "$file"

    printf 'line 1\ntruncated line 2' | while read -r line || [[ $line ]]; do echo "$line"; done

How to keep other commands from "eating" the input

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 read -r -u 9 line; do
      cat > ignoredfile
      printf '%s\n' "$line"
    done 9< "$file"

    # Note that read -u is not portable, and pretty much useless when you can use simple redirections instead:
    while read -r line <&9; do
      cat > ignoredfile
      printf '%s\n' "$line"
    done 9< "$file"

Or:

    # Bourne
    exec 9< "$file"
    while read -r line <&9; do
      cat > ignoredfile
      printf '%s\n' "$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.


CategoryShell

BashFAQ/001 (last edited 2013-08-29 16:15:19 by e36freak)