I am horribly perplexed.
I've written a bash script to sort lines into categories based on substrings within that line.
Here's my example "lines.txt"
i am line1
i am line2
If a line contains "line1", then it should be sorted into group "l1". If it contains "line2", then it should be sorted into group "l2"
The problem is that the variable which holds the category isn't retaining its value, and I have no clue why. Here's the script.
#!/bin/bash
categories="l1 l2"
l1="
line1
"
l2="
line2
"
# match line1
cat lines.txt | while read fline
do
cate="no match"
for c in $categories
do
echo "${!c}" | while read location
do
if [ ! -z "$location" ] && [[ "$fline" =~ "$location" ]]
then
echo "we are selecting category $c"
cate="$c"
break
fi
done
if [ "$cate" != "no match" ]
then
echo "we found a match"
break
fi
done
echo "$cate:$fline"
done
exit 0
And when I run it, I see the output
we are selecting category l1
no match:i am line1
we are selecting category l2
no match:i am line2
This means that we are selecting the correct group, but we don't remember it when we exit the nested "while" loop.
Why is my variable not retaining its value, and how could I fix that?
The while loop is executed in a subshell because of the pipe. That means that the name 'cate' really refers to two different variables. One outside the while loop and the other inside the loop inside the subshell. When the subshell exits that value is lost.
A way to get around this is to use a redirect like this
while read line; do
...
done < $myfile
If the expression is more complicated and you need something executed in a subshell, then you can use process substitution (Thanks to David Rankin for reminding me about this one).
while read -r line; do
...
done < <(find . -iname "*sh")
Related
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 line\nsecond line\nthird 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")"
You can get rid of the rather ugly echo in the here-string above by expanding the backslash sequences immediately when assigning lines. The $'...' form of quoting can be used there:
lines=$'first line\nsecond line\nthird line'
while read line; do
...
done <<< "$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 $lines can be converted to an array without starting a new sub-shell. The characters \ and n has to be converted to some character (e.g. a real new line character) and use the IFS (Internal Field Separator) variable to split the string into array elements. This can be done like:
lines="first line\nsecond line\nthird line"
echo "$lines"
OIFS="$IFS"
IFS=$'\n' arr=(${lines//\\n/$'\n'}) # Conversion
IFS="$OIFS"
echo "${arr[#]}", Length: ${#arr[*]}
set|grep ^arr
Result is
first line\nsecond line\nthird line
first line second line third line, Length: 3
arr=([0]="first line" [1]="second line" [2]="third line")
You are asking 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 $(/usr/local/bin/ipaddr))
Hmmm... I would almost swear that this worked for the original Bourne shell, but don't have access to a running copy just now to check.
There is, however, a very trivial workaround to the problem.
Change the first line of the script from:
#!/bin/bash
to
#!/bin/ksh
Et voila! A read at the end of a pipeline works just fine, assuming you have the Korn shell installed.
This is an interesting question and touches on a very basic concept in Bourne shell and subshell. Here I provide a solution that is different from the previous solutions by doing some kind of filtering. I will give an example that may be useful in real life. This is a fragment for checking that downloaded files conform to a known checksum. The checksum file look like the following (Showing just 3 lines):
49174 36326 dna_align_feature.txt.gz
54757 1 dna.txt.gz
55409 9971 exon_transcript.txt.gz
The shell script:
#!/bin/sh
.....
failcnt=0 # this variable is only valid in the parent shell
#variable xx captures all the outputs from the while loop
xx=$(cat ${checkfile} | while read -r line; do
num1=$(echo $line | awk '{print $1}')
num2=$(echo $line | awk '{print $2}')
fname=$(echo $line | awk '{print $3}')
if [ -f "$fname" ]; then
res=$(sum $fname)
filegood=$(sum $fname | awk -v na=$num1 -v nb=$num2 -v fn=$fname '{ if (na == $1 && nb == $2) { print "TRUE"; } else { print "FALSE"; }}')
if [ "$filegood" = "FALSE" ]; then
failcnt=$(expr $failcnt + 1) # only in subshell
echo "$fname BAD $failcnt"
fi
fi
done | tail -1) # I am only interested in the final result
# you can capture a whole bunch of texts and do further filtering
failcnt=${xx#* BAD } # I am only interested in the number
# this variable is in the parent shell
echo failcnt $failcnt
if [ $failcnt -gt 0 ]; then
echo $failcnt files failed
else
echo download successful
fi
The parent and subshell communicate through the echo command. You can pick some easy to parse text for the parent shell. This method does not break your normal way of thinking, just that you have to do some post processing. You can use grep, sed, awk, and more for doing so.
I use stderr to store within a loop, and read from it outside.
Here var i is initially set and read inside the loop as 1.
# reading lines of content from 2 files concatenated
# inside loop: write value of var i to stderr (before iteration)
# outside: read var i from stderr, has last iterative value
f=/tmp/file1
g=/tmp/file2
i=1
cat $f $g | \
while read -r s;
do
echo $s > /dev/null; # some work
echo $i > 2
let i++
done;
read -r i < 2
echo $i
Or use the heredoc method to reduce the amount of code in a subshell.
Note the iterative i value can be read outside the while loop.
i=1
while read -r s;
do
echo $s > /dev/null
let i++
done <<EOT
$(cat $f $g)
EOT
let i--
echo $i
How about a very simple method
+call your while loop in a function
- set your value inside (nonsense, but shows the example)
- return your value inside
+capture your value outside
+set outside
+display outside
#!/bin/bash
# set -e
# set -u
# No idea why you need this, not using here
foo=0
bar="hello"
if [[ "$bar" == "hello" ]]
then
foo=1
echo "Setting \$foo to $foo"
fi
echo "Variable \$foo after if statement: $foo"
lines="first line\nsecond line\nthird line"
function my_while_loop
{
echo -e $lines | while read line
do
if [[ "$line" == "second line" ]]
then
foo=2; return 2;
echo "Variable \$foo updated to $foo inside if inside while loop"
fi
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"
return 2;
fi
# Code below won't be executed since we returned from function in 'if' statement
# We aready reported the $foo var beint set to 2 anyway
echo "Value of \$foo in while loop body: $foo"
done
}
my_while_loop; foo="$?"
echo "Variable \$foo after while loop: $foo"
Output:
Setting $foo 1
Variable $foo after if statement: 1
Value of $foo in while loop body: 1
Variable $foo after while loop: 2
bash --version
GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13)
Copyright (C) 2007 Free Software Foundation, Inc.
Though this is an old question and asked several times, here's what I'm doing after hours fidgeting with here strings, and the only option that worked for me is to store the value in a file during while loop sub-shells and then retrieve it. Simple.
Use echo statement to store and cat statement to retrieve. And the bash user must chown the directory or have read-write chmod access.
#write to file
echo "1" > foo.txt
while condition; do
if (condition); then
#write again to file
echo "2" > foo.txt
fi
done
#read from file
echo "Value of \$foo in while loop body: $(cat foo.txt)"
I got such a piece of bash code:
var="empty"
find $path1 -maxdepth 3 | while read line; do
find $path2 -maxdepth 1 | while read line2; do
if [[ $line2 != $var ]]; then
echo "new value"
fi
var=$line2
done <<< "$line2"
done
The question is... how to make var stay changed? Because I would like to echo on every new value found by loops but it doesn't work ;( var="empty" every time that the second loop starts iteration.
How to make var=$line2 for every iteration?
You are reading the value into line2 with a read from stdin, and feeding the value of line2 into the loop at the done with a here-string on stdin. bash gives the here-string precedence, so line2 is only ever being assigned from line2, which means it's never set.
echo -e "one\nthree\nfive" | while read num
do echo $num
done <<< "two"
Output is two. The input stream is totally ignored.
You are also defining a nested loop for no reason, since you are never using the outer loop. Clean your code before posting please.
find ~ | while read f; do var=$f; echo $f; done
This works fine.
i have script. In this script i made search and replace of words. Word by word until word 'end'. It is ok and it works. You can see body of my script:
#!/bin/bash
end=end
until [ "$first" = "$end" ];do
echo "please write first word";
read first
if grep -q "$first" *txt; then
echo "word is exists"
grep "$first" *txt
echo "please write second word";
read second
sed -i 's/'"$first"'/'"$second"'/g' *txt
else
echo "second word does not exists"
exit 1
fi
done
It works for me. I have in the result console, where I can endlessly loop words, but if i want to do something like this: How can i write multiple words in line.
For example: "dog" "cat" "fish"
And search and replace all of these words. How can do it? For example, if i need to replace on these words ("elephat" "mouse" "bird"). How can you do it?
I mean search and replace words, like arguments.
You just need a loop to process the arguments.
Assuming you run the script passing pairs of original replacement words (myscript.sh original_word1 replacement1 original_word2 replacement2 ...) it would be something like the following:
while [[ $# -gt 1 ]]
do
original="$1"
replacement="$2"
# your code for actually replacing $original with $replacement
shift # discard already processed original arg
shift # discard already processed replacement arg
done
Note that if the user passes a last original word without replacement the script will just ignore it
Your English is rough, but I think you want to be able to prompt for multiple words, and replace them with a new set?
The below code will let you run a program like replace_words one two three and then be prompted for a list of words to replace, e.g. 1 2 3. After that, it exits.
declare -a replace_list=( "$#" ) # get the replace list as passed arguments
echo -n "Enter words to replace with: "; read -ra sub_list
for ((i=0; i < "${#replace_list[#]}"; ++i)); do
if grep -q "${replace_list[$i]}" *txt; then
echo "first word is exists"
sed -i "s/${replace_list[$i]}/${sub_list[$i]}/g" *txt
else
echo "${replace_list[$i]} does not exists"
exit 1
fi
done
I'm trying to parse a csv file I made with Google Spreadsheet. It's very simple for testing purposes, and is basically:
1,2
3,4
5,6
The problem is that the csv doesn't end in a newline character so when I cat the file in BASH, I get
MacBook-Pro:Desktop kkSlider$ cat test.csv
1,2
3,4
5,6MacBook-Pro:Desktop kkSlider$
I just want to read line by line in a BASH script using a while loop that every guide suggests, and my script looks like this:
while IFS=',' read -r last first
do
echo "$last $first"
done < test.csv
The output is:
MacBook-Pro:Desktop kkSlider$ ./test.sh
1 2
3 4
Any ideas on how I could have it read that last line and echo it?
Thanks in advance.
You can force the input to your loop to end with a newline thus:
#!/bin/bash
(cat test.csv ; echo) | while IFS=',' read -r last first
do
echo "$last $first"
done
Unfortunately, this may result in an empty line at the end of your output if the input already has a newline at the end. You can fix that with a little addition:
!/bin/bash
(cat test.csv ; echo) | while IFS=',' read -r last first
do
if [[ $last != "" ]] ; then
echo "$last $first"
fi
done
Another method relies on the fact that the values are being placed into the variables by the read but they're just not being output because of the while statement:
#!/bin/bash
while IFS=',' read -r last first
do
echo "$last $first"
done <test.csv
if [[ $last != "" ]] ; then
echo "$last $first"
fi
That one works without creating another subshell to modify the input to the while statement.
Of course, I'm assuming here that you want to do more inside the loop that just output the values with a space rather than a comma. If that's all you wanted to do, there are other tools better suited than a bash read loop, such as:
tr "," " " <test.csv
cat file |sed -e '${/^$/!s/$/\n/;}'| while IFS=',' read -r last first; do echo "$last $first"; done
If the last (unterminated) line needs to be processed differently from the rest, #paxdiablo's version with the extra if statement is the way to go; but if it's going to be handled like all the others, it's cleaner to process it in the main loop.
You can roll the "if there was an unterminated last line" into the main loop condition like this:
while IFS=',' read -r last first || [ -n "$last" ]
do
echo "$last $first"
done < test.csv
I am trying to write a script that:
a) reads the content of a .csv file
b) sets a variable to the value in the first position (ie to the left of the comma)
c) compare the variable value to each position in an array. If the value is in the array execute one command, if it isn't, insert that value into the first available slot in the array.
The .csv file is in the format:
co:7077,he17208am3200816internet.pdf,he17208am3200917internet.pdf
co:7077,he17208am3200817internet.pdf,he17208am3200918internet.pdf
co:7077,he17208am3200818internet.pdf,he17208am3200919internet.pdf
co:7077,he17208am3200819internet.pdf,he17208am3200915internet.pdf
co:7162,tra210051internet.pdf,tra21005101internet.pdf
co:7162,tra210051appinternet.pdf,tra21005102internet.pdf
co:7178,tra4157l11201021internet.pdf,tra4158l11201021internet.pdf
co:7178,tra4157l11201022internet.pdf,tra4158l11201022internet.pdf
My script so far looks like:
#!/bin/bash
declare -a array
anum=0
src=source.csv
pid=0
while read line;
do
pid=$( echo $line | awk '{print$1}' FS=",")
for n in "${array[#]}";
do
if [[ "$pid" = "$n" ]] ;
then
echo Duplicate value: "$pid";
else
array[$anum]="$pid"
anum=$(( $anum +1 ))
fi
done
done < $src
echo ${array[#]}
When the script is executed the pid is successfully set and reset with each iteration of the while loop, but apparently the nested for loop is never ran.
From my google'ing I suspect it has something to do with the pipe in pid line, but I'll be buggered if I can figure out how to make it work.
Any help is greatly appreciated.
You're not populating your array. The for loop is never executed because the array is empty.
Set a flag in the else clause instead of adding the array element there. After your for loop if the flag is set, add the array element. Don't forget to unset the flag.
You can do array[anum++] without the next line or (( anum++ )) instead of anum=$(($anum + 1)).
Also: while IFS=, read -r pid discard if you don't need the rest of the line (you could do it a little differently if you need it). Doing this, you won't need the echo and awk.
why did you use double square brackets? and also you used a single equals rather than double in the if?
try these one-liners...
$ if [ "a" == "b" ] ; then echo hello ; fi
$ if [ "a" == "a" ] ; then echo hello ; fi