A variable modified inside a while loop is not remembered
In the following program, if I set the variable $foo to the value 1 inside the first if statement, it works in the sense that its value is remembered after the if statement. However, when I set the same variable to the value 2 inside an if which is inside a while statement, it's forgotten after the while loop. It's behaving like I'm using some sort of copy of the variable $foo inside the while loop and I am modifying only that particular copy. Here's a complete test program:
#!/bin/bash
set -e
set -u
foo=0
bar="hello"
if [[ "$bar" == "hello" ]]
then
foo=1
echo "Setting $foo to 1: $foo"
fi
echo "Variable $foo after if statement: $foo"
lines="first linensecond linenthird line"
echo -e $lines | while read line
do
if [[ "$line" == "second line" ]]
then
foo=2
echo "Variable $foo updated to $foo inside if inside while loop"
fi
echo "Value of $foo in while loop body: $foo"
done
echo "Variable $foo after while loop: $foo"
# Output:
# $ ./testbash.sh
# Setting $foo to 1: 1
# Variable $foo after if statement: 1
# Value of $foo in while loop body: 1
# Variable $foo updated to 2 inside if inside while loop
# Value of $foo in while loop body: 2
# Value of $foo in while loop body: 2
# Variable $foo after while loop: 1
# bash --version
# GNU bash, version 4.1.10(4)-release (i686-pc-cygwin)
echo -e $lines | while read line
...
done
The while loop is executed in a subshell. So any changes you do to the variable will not be available once the subshell exits.
Instead you can use a here string to re-write the while loop to be in the main shell process; only echo -e $lines will run in a subshell:
while read line
do
if [[ "$line" == "second line" ]]
then
foo=2
echo "Variable $foo updated to $foo inside if inside while loop"
fi
echo "Value of $foo in while loop body: $foo"
done <<< "$(echo -e "$lines")"
UPDATED#2
Explanation is in Blue Moons's answer.
Alternative solutions:
Eliminate echo
while read line; do
...
done <<EOT
first line
second line
third line
EOT
Add the echo inside the here-is-the-document
while read line; do
...
done <<EOT
$(echo -e $lines)
EOT
Run echo in background:
coproc echo -e $lines
while read -u ${COPROC[0]} line; do
...
done
Redirect to a file handle explicitly (Mind the space in < < !):
exec 3< <(echo -e $lines)
while read -u 3 line; do
...
done
Or just redirect to the stdin :
while read line; do
...
done < <(echo -e $lines)
And one for chepner (eliminating echo ):
arr=("first line" "second line" "third line");
for((i=0;i<${#arr[*]};++i)) { line=${arr[i]};
...
}
Variable $lines can be converted to an array without starting a new sub-shell. The characters and n has to be converted to some character (eg a real new line character) and use the IFS (Internal Field Separator) variable to split the string into array elements. This can be done like:
lines="first linensecond linenthird line"
echo "$lines"
OIFS="$IFS"
IFS=$'n' arr=(${lines//n/$'n'}) # Conversion
IFS="$OIFS"
echo "${arr[@]}", Length: ${#arr[*]}
set|grep ^arr
Result is
first linensecond linenthird line
first line second line third line, Length: 3
arr=([0]="first line" [1]="second line" [2]="third line")
You are the 742342nd user to ask this bash FAQ. The answer also describes the general case of variables set in subshells created by pipes:
E4) If I pipe the output of a command into read variable , why doesn't the output show up in $variable when the read command finishes?
This has to do with the parent-child relationship between Unix processes. It affects all commands run in pipelines, not just simple calls to read . For example, piping a command's output into a while loop that repeatedly calls read will result in the same behavior.
Each element of a pipeline, even a builtin or shell function, runs in a separate process, a child of the shell running the pipeline. A subprocess cannot affect its parent's environment. When the read command sets the variable to the input, that variable is set only in the subshell, not the parent shell. When the subshell exits, the value of the variable is lost.
Many pipelines that end with read variable can be converted into command substitutions, which will capture the output of a specified command. The output can then be assigned to a variable:
grep ^gnu /usr/lib/news/active | wc -l | read ngroup
can be converted into
ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
This does not, unfortunately, work to split the text among multiple variables, as read does when given multiple variable arguments. If you need to do this, you can either use the command substitution above to read the output into a variable and chop up the variable using the bash pattern removal expansion operators or use some variant of the following approach.
Say /usr/local/bin/ipaddr is the following shell script:
#! /bin/sh
host `hostname` | awk '/address/ {print $NF}'
Instead of using
/usr/local/bin/ipaddr | read A B C D
to break the local machine's IP address into separate octets, use
OIFS="$IFS"
IFS=.
set -- $(/usr/local/bin/ipaddr)
IFS="$OIFS"
A="$1" B="$2" C="$3" D="$4"
Beware, however, that this will change the shell's positional parameters. If you need them, you should save them before doing this.
This is the general approach -- in most cases you will not need to set $IFS to a different value.
Some other user-supplied alternatives include:
read A B C D << HERE
$(IFS=.; echo $(/usr/local/bin/ipaddr))
HERE
and, where process substitution is available,
read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
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