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Comment: added section on how to avoid a program in the loop eating input
Revision 39 as of 2010-11-17 19:07:03
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Comment: that's already in FAQ 89, but I'll let it stand....
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The classic example listed above will not work if some of the commands take input. For example, Some commands greedily eat up all available data on standard input. The examples above do not take precautions against such programs. For example,
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will only print the contents of the first line, with the remaining contents going to "ignoredfile" will only print the contents of the first line, with the remaining contents going to "ignoredfile", as `cat` slurps up all available input.
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One workaround is to use a numeric file descriptor. One workaround is to use a numeric file descriptor rather than standard input:
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    # Bash
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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. 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 [[BashFAQ/089|FAQ #89]].

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

Use a while loop and the read command:

    while read -r line
    do
        echo "$line"
    done < "$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 always use the -r option with read.

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

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
        echo "$line"
    done <<< "$var"

The same can be done in any Bourne-type shell by using a "here document":

    while read -r line; do
        echo "$line"
    done <<EOF
$var
EOF

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
      ...
    done < "$file"

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

    while IFS=: read -r user pass uid gid gecos home shell; do
      ...
    done < /etc/passwd

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

Also, please note that 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,

    while read -r first_name last_name junk; do
      ...
    done <<< 'Bob Smith 123 Main Street Elk Grove Iowa 123-555-6789'

    # Inside the loop, first_name will contain "Bob", and
    # last_name will contain "Smith".  The variable "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"

    # Remember, the final _ can absorb any number of fields.
    # It doesn't need to be repeated there.

The read command modifies each line read, e.g. by default it removes all leading whitespace characters (blanks, tab characters, ... -- any whitespace characters present in IFS). If that is not desired, the IFS variable has to be cleared:

    while IFS= read -r line
    do
        echo "$line"
    done < "$file"

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

    some command | while read -r line; do
       other commands
    done

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

    find . -print0 | while IFS= read -r -d $'\0' file; do
        mv "$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 $'\0' 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 with newlines and cause the loop body to fail. Additionally it is necessary to set IFS to an empty string, because otherwise read would strip 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 outside the loop; in that case, see FAQ 24, or use ProcessSubstitution like:

    while read -r line; do
        other commands
    done < <(some command)

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

If the the input source lacks a trailing newline (such as /proc/$$/cmdline on Linux), the line will not be processed by a while read ... loop, as read returns a failure that aborts the while loop, thus failing to process the last line. It does, however, store the contents of the partial line in the variable, so you can test whether there was such an unterminated line by checking whether the variable is non-empty at the end of the loop:

    # 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")

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

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
        echo "$line"
    done < "$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 file descriptor rather than standard input:

    # Bash
    exec 9< "$file"
    while read -r -u9 line
    do
        cat > ignoredfile
        echo "$line"
    done 

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.

Why you don't use "for" for this

Using for instead of while read ... is generally less efficient and suffers a number of unexpected side-effects.

    $ cat afile
    ef gh
    *

    $ while read -r i ; do echo "$i" ; done < afile
    ef gh
    *

    $ for i in $(<afile) ; do echo "$i" ; done
    ef
    gh
    afile
    # the glob was expanded, and it looped per word.

    #workaround:
    $ oIFS=$IFS IFS=$'\n' ;set -f ; for i in $(<afile) ; do echo "$i" ; done; IFS=$oIFS
    ef gh
    *

Notice that the syntax to get this "right" is more verbose. All-in-all, (ab)using for this way is a more dangerous, less intuitive (you don't get what you expect out of a normal for!) and not any more useful method.

Also, as discussed in FAQ #5, the use of IFS=$'\n' (or any other "whitespace" in IFS) causes the shell to consolidate all consecutive instances of the whitespace delimiter into one. In other words, it skips over blank lines. Did you see the blank line in the input file, and how it was missing in the output? It's easier to spot if it's not at the end:

    ~$ cat foo
    line one

    line three
    ~$ IFS=$'\n'; set -f; for line in $(<foo); do echo "$line"; done; unset IFS; set +f
    line one
    line three
    ~$ 

All that setting and unsetting, and we still couldn't even mimic a simple cat. If preservation of blank lines is important, just go back to using while read.


CategoryShell

BashFAQ/001 (last edited 2020-04-14 15:53:25 by phy1729)