Differences between revisions 20 and 21
Revision 20 as of 2008-10-06 23:04:39
Size: 6014
Editor: GreyCat
Comment: link to ProcessSubstitution and SubShell
Revision 21 as of 2008-11-22 14:08:34
Size: 6020
Editor: localhost
Comment: converted to 1.6 markup
Deletions are marked like this. Additions are marked like this.
Line 1: Line 1:
[[Anchor(faq1)]] <<Anchor(faq1)>>
Line 47: Line 47:
As a feature, the {{{read}}} command concatenates lines that end with a backslash '\' character to one single line. To disable this feature, KornShell and ["BASH"], as well as the POSIX standard for the Bourne shell, have {{{read -r}}}: As a feature, the {{{read}}} command concatenates lines that end with a backslash '\' character to one single line. To disable this feature, KornShell and [[BASH]], as well as the POSIX standard for the Bourne shell, have {{{read -r}}}:
Line 56: Line 56:
'''Note that reading a file line by line this way is ''very slow'' for large files. Consider using e.g. ["AWK"] instead if you get performance problems.''' '''Note that reading a file line by line this way is ''very slow'' for large files. Consider using e.g. [[AWK]] instead if you get performance problems.'''
Line 77: Line 77:
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. See [:BashFAQ/020:FAQ #20] for more details. 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. See [[BashFAQ/020|FAQ #20]] for more details.
Line 79: Line 79:
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 [:BashFAQ/024:FAQ 24], or use ProcessSubstitution like: 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 [[BashFAQ/024|FAQ 24]], or use ProcessSubstitution like:
Line 87: Line 87:
Sometimes it's useful to read a file into an [:BashFAQ/005:array], one array element per line. You can do that with the following example: Sometimes it's useful to read a file into an [[BashFAQ/005|array]], one array element per line. You can do that with the following example:
Line 103: Line 103:
Of course, this will blow up in your face if the filenames contain newlines; see [:BashFAQ/020:FAQ 20] for hints on dealing with such filenames. Of course, this will blow up in your face if the filenames contain newlines; see [[BashFAQ/020|FAQ 20]] for hints on dealing with such filenames.
Line 105: Line 105:
Both of these array-stuffing examples fail if the shell encounters a [:glob:] that matches files in the current directory as one of the input lines. Glob expansion can be disabled with `set -f` and then re-enabled afterward with `set +f` if needed. For more details on arrays, see [:BashFAQ/005:FAQ 5]. Moreover, since bash will treat sequences of IFS whitespace as a single character, if the input has empty lines (meaning that groups of two or more consecutive \n characters appear in the file), they will be lost. So, for example: Both of these array-stuffing examples fail if the shell encounters a [[glob]] that matches files in the current directory as one of the input lines. Glob expansion can be disabled with `set -f` and then re-enabled afterward with `set +f` if needed. For more details on arrays, see [[BashFAQ/005|FAQ 5]]. Moreover, since bash will treat sequences of IFS whitespace as a single character, if the input has empty lines (meaning that groups of two or more consecutive \n characters appear in the file), they will be lost. So, for example:
Line 138: Line 138:
For a discussion of why the second example above does not work as expected, see [:BashFAQ/024:FAQ #24]. For a discussion of why the second example above does not work as expected, see [[BashFAQ/024|FAQ #24]].

How can I read a file line-by-line?

    while read line
    do
        echo "$line"
    done < "$file"          # or   <<< "$var"    to iterate over a variable 

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 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 user pass uid gid gecos home shell; do
      ...
    done < /etc/passwd

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 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.

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

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

As a feature, the read command concatenates lines that end with a backslash '\' character to one single line. To disable this feature, KornShell and BASH, as well as the POSIX standard for the Bourne shell, have read -r:

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

Note that reading a file line by line this way is very slow for large files. Consider using e.g. AWK instead if you get performance problems.

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

    some command | while read line; do
       other commands
    done

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

    find . -print0 | while read -d $'\0' file; do
        mv "$file" "${file// /_}"
    done

This command reads one filename at a time from the file command and renames the file so that its spaces are replaced by 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. 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 line; do
        other commands
    done < <(some command)

Sometimes it's useful to read a file into an array, one array element per line. You can do that with the following example:

    O=$IFS IFS=$'\n' arr=($(< myfile)) IFS=$O
    # Warning: breaks if lines contain "*" or similar

This temporarily changes the Input Field Separator to a newline, so that each line will be considered one field by read. Then it populates the array arr with the fields. Then it sets the IFS back to what it was before.

This same trick works on a stream of data as well as a file:

    O=$IFS IFS=$'\n' arr=($(find . -type f)) IFS=$O
    # Same warning as the previous example

Of course, this will blow up in your face if the filenames contain newlines; see FAQ 20 for hints on dealing with such filenames.

Both of these array-stuffing examples fail if the shell encounters a glob that matches files in the current directory as one of the input lines. Glob expansion can be disabled with set -f and then re-enabled afterward with set +f if needed. For more details on arrays, see FAQ 5. Moreover, since bash will treat sequences of IFS whitespace as a single character, if the input has empty lines (meaning that groups of two or more consecutive \n characters appear in the file), they will be lost. So, for example:

    $ cat myfile
    line1
    
    line2
    line3
    $ O=$IFS IFS=$'\n' arr=($(< myfile)) IFS=$O
    $ declare -p arr
    declare -a arr='([0]="line1" [1]="line2" [2]="line3")'

In the end, the safest way to read a file into an array is still to use a loop:

    i=0
    while IFS= read -r arr[i++]; do :;done < "$file"
    # or  <<< "$var"  to iterate over a variable 

On the other hand, if the file lacks a trailing newline (such as /proc/$$/cmdline on Linux), the line will not be printed by a while read ... loop, as read returns a failure that aborts the while loop, thus failing to print the ultimate line:

    # This does not work:
    echo -en 'line 1\ntruncated line 2' | while read line; do echo $line; done

    # This does not work either:
    echo -en 'line 1\ntruncated line 2' | while read line; do echo "$line"; done; echo "$line"

    # This works:
    echo -en 'line 1\ntruncated line 2' | (while read line; do echo "$line"; done; echo "$line")

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

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