Advanced Bash-Scripting Guide exec 3<> myfile.txt
while read line <&3 do {
echo "$line"
(( Lines++ )); # Incremented values of this variable
#+ accessible outside loop.
# No subshell, no problem.
} done exec 3>&echo "Number of lines read = $Lines" # 8 echo exit 0
# Lines below not seen by script.
$ cat myfile.txt
Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8.
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
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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 [ "$name" != Smith ] do
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read name # Reads from redirected stdin ($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 $1 }'`
# Number of lines in target file.
#
# Very contrived and kludgy, nevertheless shows that
#+ it's possible to redirect stdin within a "for" loop...
#+ if you're clever enough.
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#
# More concise is line_count=$(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=$1 fi
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TRUE=1 if [ "$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.
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