Differences between revisions 30 and 42 (spanning 12 versions)
Revision 30 as of 2013-06-07 05:10:14
Size: 6511
Editor: JariAalto
Comment: (Use lastpipe): Add comment to explain "set +m"
Revision 42 as of 2022-03-10 13:02:31
Size: 6587
Editor: 46
Comment: POSIX read requires a var param, usually I would pick _ to indicate that the var isn't going to be used, but in accordance with the other examples on this page, I used "line"
Deletions are marked like this. Additions are marked like this.
Line 6: Line 6:
    # Works only in ksh88/ksh93, or bash 4.2 with lastpipe enabled
    # In other shells, this will print 0
    linecount=0
# Works only in ksh88/ksh93, or zsh or bash 4.2 with lastpipe enabled
# In other shells, this will print 0
linecount=0
Line 10: Line 10:
    printf '%s\n' foo bar |
    while read -r line
    do
     linecount=$((linecount + 1))
    done
printf '%s\n' foo bar |
while IFS= read -r line
do
    linecount=$((linecount + 1))
done
Line 16: Line 16:
    echo "total number of lines: $linecount" echo "total number of lines: $linecount"
Line 19: Line 19:
The reason for this potentially surprising behaviour, as described above, is that each SubShell introduces a new variable context and environment. The {{{while}}} loop above is executed in a new subshell with its own copy of the variable {{{linecount}}} created with the initial value of '0' taken from the parent shell. This copy then is used for counting. When the {{{while}}} loop is finished, the subshell copy is discarded, and the original variable {{{linecount}}} of the parent (whose value hasn't changed) is used in the {{{echo}}} command. The reason for this potentially surprising behaviour, as described above, is that each SubShell introduces a new variable context and environment. The `while` loop above is executed in a new subshell with its own copy of the variable `linecount` created with the initial value of '0' taken from the parent shell. This copy then is used for counting. When the `while` loop is finished, the subshell copy is discarded, and the original variable `linecount` of the parent (whose value hasn't changed) is used in the `echo` command.
Line 23: Line 23:
 * [[BASH]] creates a new process only if the loop is part of a pipeline.
 * KornShell creates it only if the loop is part of a pipeline, but ''not'' if the loop is the last part of it. The read example above actually ''works'' in ksh88 and ksh93! (but not mksh)
 * [[BASH]], Yash and PDKsh-derived shells create a new process only if the loop is part of a pipeline.
 * KornShell and Zsh creates it only if the loop is part of a pipeline, but ''not'' if the loop is the last part of it. The read example above actually ''works'' in ksh88, ksh93, zsh! (but not MKsh or other PDKsh-derived shells)
Line 29: Line 29:
    # Bash 4
    # The problem also occurs without a loop
    printf '%s\n' foo bar | mapfile -t line
    printf 'total number of lines: %s\n' "${#line[@]}" # prints 0
# Bash 4
# The problem also occurs without a loop
printf '%s\n' foo bar | mapfile -t line
printf 'total number of lines: %s\n' "${#line[@]}" # prints 0
Line 36: Line 36:
    f() {
     if [[ -t 0 ]]; then
            echo "$1"
        else
     read -r var
        fi
    };
f() {
    if [[ -t 0 ]]; then
        echo "$1"
    else
        read -r var
    fi
}
Line 44: Line 44:
    f 'hello' | f
    echo "$var" # prints nothing
f 'hello' | f
echo "$var" # prints nothing
Line 48: Line 48:
Again, in both cases the pipeline causes {{{read}}} or some containing command to run in a subshell, so its effect is never witnessed in the parent process. Again, in both cases the pipeline causes `read` or some containing command to run in a subshell, so its effect is never witnessed in the parent process.
Line 50: Line 50:
 It should be stressed that this issue isn't specific to loops. It's a general property of all pipes, though the "{{{while/read}}}" loop might be considered the canonical example that crops up over and over when people read the help or manpage description of the {{{read}}} builtin and notice that it accepts data on stdin. They might recall that data redirected into a compound command is available throughout that command, but not understand why all the fancy process substitutions and redirects they run across in places like [[BashFAQ/001|FAQ #1]] are necessary. Naturally they proceed to put their funstuff directly into a pipeline, and confusion ensues. It should be stressed that this issue isn't specific to loops. It's a general property of all pipes, though the `while/read` loop might be considered the canonical example that crops up over and over when people read the help or manpage description of the `read` builtin and notice that it accepts data on stdin. They might recall that data redirected into a compound command is available throughout that command, but not understand why all the fancy process substitutions and redirects they run across in places like [[BashFAQ/001|FAQ #1]] are necessary. Naturally they proceed to put their funstuff directly into a pipeline, and confusion ensues.
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    # POSIX
    while read -r line; do linecount=$((linecount + 1)); done < file
    echo $linecount
 }}}
# POSIX
while IFS= read -r line; do linecount=$((linecount + 1)); done < file
echo "$linecount"
}}}
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    # POSIX
    linecount=0
# POSIX
linecount=0
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    cat /etc/passwd |
    {
 
while read -r line
 do
  linecount=$((linecount + 1))
 done
cat /etc/passwd |
{
while IFS= read -r line
    do
     linecount=$((linecount + 1))
    done
Line 77: Line 77:
 echo "total number of lines: $linecount" ;
    }
 }}}
    echo "total number of lines: $linecount"
}
}}}
Line 83: Line 83:
 * Use ProcessSubstitution (Bash only):  * Use ProcessSubstitution (Bash/Zsh/Ksh93 only):
Line 86: Line 86:
    # Bash
    while read -r line
    do
     ((linecount++))
    done < <(grep PATH /etc/profile)
# Bash/Ksh93/Zsh
while IFS= read -r line
do
    ((linecount++))
done < <(grep PATH /etc/profile)
Line 92: Line 92:
    echo "total number of lines: $linecount"
 }}}
echo "total number of lines: $linecount"
}}}
Line 100: Line 100:
    # POSIX
    mkfifo mypipe
    grep PATH /etc/profile > mypipe &
# POSIX
mkfifo mypipe
grep PATH /etc/profile > mypipe &
Line 104: Line 104:
    while read -r line
    do
     linecount=$((linecount + 1))
    done < mypipe
while IFS= read -r line
do
    linecount=$((linecount + 1))
done < mypipe
Line 109: Line 109:
    echo "total number of lines: $linecount"
 }}}
echo "total number of lines: $linecount"
}}}
Line 115: Line 115:
    # ksh
    grep PATH /etc/profile |&
# ksh
grep PATH /etc/profile |&
Line 118: Line 118:
    while read -r -p line
    do
     linecount=$((linecount + 1))
    done
while IFS= read -r -p line
do
    linecount=$((linecount + 1))
done
Line 123: Line 123:
    echo "total number of lines: $linecount"
 }}}
 Bash 4 also has coproc, but its syntax is very different from ksh's syntax, and not really applicable for this task.

 * Use a HereString (Bash only):
echo "total number of lines: $linecount"
}}}
Line 130: Line 127:
     # Options:
     # -r Backslash does not act as an escape character; \n is not taken as LF.
     # -a The words are assigned to sequential indices of the array "words"
# bash>4
coproc grep PATH /etc/profile
Line 134: Line 130:
     read -ra words <<< 'hi ho hum'
     printf 'total number of words: %d' "${#words[@]}"
 }}}
while IFS= read -r line
do
    linecount=$((linecount + 1))
done <&"${COPROC[0]}"
Line 138: Line 135:
 The {{{<<<}}} operator is specific to bash (2.05b and later), however it is a very clean and handy way to specify a small string of literal input to a command. echo "total number of lines: $linecount"
}}}

 * Use a HereString (Bash/Zsh/Ksh93 only, though the example uses the Bash-specific {{{read -a}}} (Ksh93 and Zsh using {{{read -A}}} instead)):

 {{{
# Options:
# -r Backslash does not act as an escape character for the word separators or line delimiter.
# -a The words are assigned to sequential indices of the array "words"

read -ra words <<< 'hi ho hum'
printf 'total number of words: %d\n' "${#words[@]}"
}}}

 The `<<<` operator is available in Bash (2.05b and later), Zsh (where it was first introduced inspired from a similar operator in the Unix port of the `rc` shell), Ksh93 and Yash.
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    # Bash
    declare -i linecount
# POSIX
linecount=0
while IFS= read -r line; do
    linecount=$((linecount+1))
done <<EOF
hi
ho
hum
EOF
Line 146: Line 164:
    while read -r; do
        ((linecount++))
    done <<EOF
    hi
    ho
    hum
    EOF

    
printf 'total number of lines: %d' "$linecount"
 }}}
printf 'total number of lines: %d\n' "$linecount"
}}}
Line 160: Line 170:
     # Bash 4.2
     # +m: Monitor mode (enables job control). Background processes display their
     # exit status on their completion.
     set +m
     shopt -s lastpipe
# Bash 4.2
# +m: Disable monitor mode (job control). Background processes display their
# exit status upon completion when in monitor mode (we don't want that).
set +m
shopt -s lastpipe
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     printf '%s\n' hi{,,,,,} | while read -r "lines[x++]"; do :; done
     printf 'total number of lines: %d' "${#lines[@]}"
 }}}
x=0
printf '%s\n' hi{,,,,,} | while IFS= read -r "lines[x++]"; do :; done
printf 'total number of lines: %d\n' "${#lines[@]}"
}}}

I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?

In most shells, each command of a pipeline is executed in a separate SubShell. Non-working example:

# Works only in ksh88/ksh93, or zsh or bash 4.2 with lastpipe enabled
# In other shells, this will print 0
linecount=0

printf '%s\n' foo bar |
while IFS= read -r line
do
    linecount=$((linecount + 1))
done

echo "total number of lines: $linecount"

The reason for this potentially surprising behaviour, as described above, is that each SubShell introduces a new variable context and environment. The while loop above is executed in a new subshell with its own copy of the variable linecount created with the initial value of '0' taken from the parent shell. This copy then is used for counting. When the while loop is finished, the subshell copy is discarded, and the original variable linecount of the parent (whose value hasn't changed) is used in the echo command.

Different shells exhibit different behaviors in this situation:

  • BourneShell creates a subshell when the input or output of anything (loops, case etc..) but a simple command is redirected, either by using a pipeline or by a redirection operator ('<', '>').

  • BASH, Yash and PDKsh-derived shells create a new process only if the loop is part of a pipeline.

  • KornShell and Zsh creates it only if the loop is part of a pipeline, but not if the loop is the last part of it. The read example above actually works in ksh88, ksh93, zsh! (but not MKsh or other PDKsh-derived shells)

  • POSIX specifies the bash behaviour, but as an extension allows any or all of the parts of the pipeline to run without a subshell (thus permitting the KornShell behaviour, as well).

More broken stuff:

# Bash 4
# The problem also occurs without a loop
printf '%s\n' foo bar | mapfile -t line
printf 'total number of lines: %s\n' "${#line[@]}" # prints 0

f() {
    if [[ -t 0 ]]; then
        echo "$1"
    else
        read -r var
    fi
}

f 'hello' | f
echo "$var" # prints nothing

Again, in both cases the pipeline causes read or some containing command to run in a subshell, so its effect is never witnessed in the parent process.

It should be stressed that this issue isn't specific to loops. It's a general property of all pipes, though the while/read loop might be considered the canonical example that crops up over and over when people read the help or manpage description of the read builtin and notice that it accepts data on stdin. They might recall that data redirected into a compound command is available throughout that command, but not understand why all the fancy process substitutions and redirects they run across in places like FAQ #1 are necessary. Naturally they proceed to put their funstuff directly into a pipeline, and confusion ensues.

Workarounds

  • If the input is a file, a simple redirect will suffice:
    # POSIX
    while IFS= read -r line; do linecount=$((linecount + 1)); done < file
    echo "$linecount"

    Unfortunately, this doesn't work with a Bourne shell; see sh(1) from the Heirloom Bourne Shell for a workaround.

  • Use command grouping and do everything in the subshell:

    # POSIX
    linecount=0
    
    cat /etc/passwd |
    {
        while IFS= read -r line
        do
            linecount=$((linecount + 1))
        done
    
        echo "total number of lines: $linecount"
    }
    This doesn't really change the subshell situation, but if nothing from the subshell is needed in the rest of your code then destroying the local environment after you're through with it could be just what you want anyway.
  • Use ProcessSubstitution (Bash/Zsh/Ksh93 only):

    # Bash/Ksh93/Zsh
    while IFS= read -r line
    do
        ((linecount++))
    done < <(grep PATH /etc/profile)
    
    echo "total number of lines: $linecount"
    This is essentially identical to the first workaround above. We still redirect a file, only this time the file happens to be a named pipe temporarily created by our process substitution to transport the output of grep.
  • Use a named pipe:

    # POSIX
    mkfifo mypipe
    grep PATH /etc/profile > mypipe &
    
    while IFS= read -r line
    do
        linecount=$((linecount + 1))
    done < mypipe
    
    echo "total number of lines: $linecount"
  • Use a coprocess (ksh, even pdksh, oksh, mksh..):

    # ksh
    grep PATH /etc/profile |&
    
    while IFS= read -r -p line
    do
        linecount=$((linecount + 1))
    done
    
    echo "total number of lines: $linecount"
    # bash>4
    coproc grep PATH /etc/profile
    
    while IFS= read -r line
    do
        linecount=$((linecount + 1))
    done <&"${COPROC[0]}"
    
    echo "total number of lines: $linecount"
  • Use a HereString (Bash/Zsh/Ksh93 only, though the example uses the Bash-specific read -a (Ksh93 and Zsh using read -A instead)):

    # Options:
    # -r Backslash does not act as an escape character for the word separators or line delimiter.
    # -a The words are assigned to sequential indices of the array "words"
    
    read -ra words <<< 'hi ho hum'
    printf 'total number of words: %d\n' "${#words[@]}"

    The <<< operator is available in Bash (2.05b and later), Zsh (where it was first introduced inspired from a similar operator in the Unix port of the rc shell), Ksh93 and Yash.

  • With a POSIX shell, or for longer multi-line data, you can use a here document instead:
    # POSIX
    linecount=0
    while IFS= read -r line; do
        linecount=$((linecount+1))
    done <<EOF
    hi
    ho
    hum
    EOF
    
    printf 'total number of lines: %d\n' "$linecount"
  • Use lastpipe (Bash 4.2)
    # Bash 4.2
    # +m: Disable monitor mode (job control). Background processes display their
    #     exit status upon completion when in monitor mode (we don't want that).
    set +m
    shopt -s lastpipe
    
    x=0
    printf '%s\n' hi{,,,,,} | while IFS= read -r "lines[x++]"; do :; done
    printf 'total number of lines: %d\n' "${#lines[@]}"
    Bash 4.2 introduces the aforementioned ksh-like behavior to Bash. The one caveat is that job control must not be enabled, thereby limiting its usefulness in an interactive shell.

For more related examples of how to read input and break it into words, see FAQ #1.


CategoryShell

BashFAQ/024 (last edited 2023-12-12 13:15:33 by 195)