Redirecting Code Blocks

Advanced Bash-Scripting Guide:
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# 20.2. Redirecting Code Blocks

Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of `stdin`. Even a function may use this form of redirection (see Example 24-11). The < operator at the end of the code block accomplishes this.

Example 20-5. Redirected while loop

    #!/bin/bash
    # redir2.sh

    if [ -z "$1" ]
    then
      Filename=names.data       # Default, if no filename specified.
    else
      Filename=$1
    fi
    #+ Filename=${1:-names.data}
    #  can replace the above test (parameter substitution).

    count=0

    echo

    while [ "$name" != Smith ]  # Why is variable $name in quotes?
    do
      read name                 # Reads from $Filename, rather than stdin.
      echo $name
      let "count += 1"
    done <"$Filename"           # Redirects stdin to file $Filename.
    #    ^^^^^^^^^^^^

    echo; echo "$count names read"; echo

    exit 0

    #  Note that in some older shell scripting languages,
    #+ the redirected loop would run as a subshell.
    #  Therefore, $count would return 0, the initialized value outside the loop.
    #  Bash and ksh avoid starting a subshell *whenever possible*,
    #+ so that this script, for example, runs correctly.
    #  (Thanks to Heiner Steven for pointing this out.)

    #  However . . .
    #  Bash *can* sometimes start a subshell in a PIPED "while-read" loop,
    #+ as distinct from a REDIRECTED "while" loop.

    abc=hi
    echo -e "1\n2\n3" | while read l
         do abc="$l"
            echo $abc
         done
    echo $abc

    #  Thanks, Bruno de Oliveira Schneider, for demonstrating this
    #+ with the above snippet of code.
    #  And, thanks, Brian Onn, for correcting an annotation error.

Example 20-6. Alternate form of redirected while loop

    #!/bin/bash

    # This is an alternate form of the preceding script.

    #  Suggested by Heiner Steven
    #+ as a workaround in those situations when a redirect loop
    #+ runs as a subshell, and therefore variables inside the loop
    # +do not keep their values upon loop termination.

    if [ -z "$1" ]
    then
      Filename=names.data     # Default, if no filename specified.
    else
      Filename=$1
    fi

    exec 3<&0                 # Save stdin to file descriptor 3.
    exec 0<"$Filename"        # Redirect standard input.

    count=0
    echo

    while [ "$name" != Smith ]
    do
      read name               # Reads from redirected stdin ($Filename).
      echo $name
      let "count += 1"
    done                      #  Loop reads from file $Filename
                              #+ because of line 20.

    #  The original version of this script terminated the "while" loop with
    #+      done <"$Filename"
    #  Exercise:
    #  Why is this unnecessary?

    exec 0<&3                 # Restore old stdin.
    exec 3<&-                 # Close temporary fd 3.

    echo; echo "$count names read"; echo

    exit 0

Example 20-7. Redirected until loop

    #!/bin/bash
    # Same as previous example, but with "until" loop.

    if [ -z "$1" ]
    then
      Filename=names.data         # Default, if no filename specified.
    else
      Filename=$1
    fi

    # while [ "$name" != Smith ]
    until [ "$name" = Smith ]     # Change  !=  to =.
    do
      read name                   # Reads from $Filename, rather than stdin.
      echo $name
    done <"$Filename"             # Redirects stdin to file $Filename.
    #    ^^^^^^^^^^^^

    # Same results as with "while" loop in previous example.

    exit 0

Example 20-8. Redirected for loop

    #!/bin/bash

    if [ -z "$1" ]
    then
      Filename=names.data          # Default, if no filename specified.
    else
      Filename=$1
    fi

    line_count=`wc $Filename | awk '{ print $1 }'`
    #           Number of lines in target file.
    #
    #  Very contrived and kludgy, nevertheless shows that
    #+ it's possible to redirect stdin within a "for" loop...
    #+ if you're clever enough.
    #
    # More concise is     line_count=$(wc -l < "$Filename")

    for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers.
    # while [ "$name" != Smith ]   --   more complicated than a "while" loop   --
    do
      read name                    # Reads from $Filename, rather than stdin.
      echo $name
      if [ "$name" = Smith ]       # Need all this extra baggage here.
      then
        break
      fi
    done <"$Filename"              # Redirects stdin to file $Filename.
    #    ^^^^^^^^^^^^

    exit 0

We can modify the previous example to also redirect the output of the loop.

Example 20-9. Redirected for loop (both `stdin` and `stdout` redirected)

    #!/bin/bash

    if [ -z "$1" ]
    then
      Filename=names.data          # Default, if no filename specified.
    else
      Filename=$1
    fi

    Savefile=$Filename.new         # Filename to save results in.
    FinalName=Jonah                # Name to terminate "read" on.

    line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.

    for name in `seq $line_count`
    do
      read name
      echo "$name"
      if [ "$name" = "$FinalName" ]
      then
        break
      fi
    done < "$Filename" > "$Savefile"     # Redirects stdin to file $Filename,
    #    ^^^^^^^^^^^^^^^^^^^^^^^^^^^       and saves it to backup file.

    exit 0

Example 20-10. Redirected if/then test

    #!/bin/bash

    if [ -z "$1" ]
    then
      Filename=names.data   # Default, if no filename specified.
    else
      Filename=$1
    fi

    TRUE=1

    if [ "$TRUE" ]          # if true    and   if :   also work.
    then
     read name
     echo $name
    fi <"$Filename"
    #  ^^^^^^^^^^^^

    # Reads only first line of file.
    # An "if/then" test has no way of iterating unless embedded in a loop.

    exit 0

Example 20-11. Data file names.data for above examples

    Aristotle
    Arrhenius
    Belisarius
    Capablanca
    Dickens
    Euler
    Goethe
    Hegel
    Jonah
    Laplace
    Maroczy
    Purcell
    Schmidt
    Schopenhauer
    Semmelweiss
    Smith
    Steinmetz
    Tukhashevsky
    Turing
    Venn
    Warshawski
    Znosko-Borowski

    #  This is a data file for
    #+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".

Redirecting the `stdout` of a code block has the effect of saving its output to a file. See Example 3-2.

Here documents are a special case of redirected code blocks. That being the case, it should be possible to feed the output of a here document into the `stdin` for a while loop.

    # This example by Albert Siersema
    # Used with permission (thanks!).

    function doesOutput()
     # Could be an external command too, of course.
     # Here we show you can use a function as well.
    {
      ls -al *.jpg | awk '{print $5,$9}'
    }

    nr=0          #  We want the while loop to be able to manipulate these and
    totalSize=0   #+ to be able to see the changes after the 'while' finished.

    while read fileSize fileName ; do
      echo "$fileName is $fileSize bytes"
      let nr++
      totalSize=$((totalSize+fileSize))   # Or: "let totalSize+=fileSize"
    done<<EOF
    $(doesOutput)
    EOF

    echo "$nr files totaling $totalSize bytes"

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