Bash allows to use: cat <(echo "$FILECONTENT")
Bash also allow to use: while read i; do echo $i; done </etc/passwd
to combine previous two this can be used: echo $FILECONTENT | while read i; do echo $i; done
The problem with last one is that it creates sub-shell and after the while loop ends variable i cannot be accessed any more.
My question is:
How to achieve something like this: while read i; do echo $i; done <(echo "$FILECONTENT") or in other words: How can I be sure that i survives while loop?
Please note that I am aware of enclosing while statement into {} but this does not solves the problem (imagine that you want use the while loop in function and return i variable)
The correct notation for Process Substitution is:
while read i; do echo $i; done < <(echo "$FILECONTENT")
The last value of i assigned in the loop is then available when the loop terminates.
An alternative is:
echo $FILECONTENT |
{
while read i; do echo $i; done
...do other things using $i here...
}
The braces are an I/O grouping operation and do not themselves create a subshell. In this context, they are part of a pipeline and are therefore run as a subshell, but it is because of the |, not the { ... }. You mention this in the question. AFAIK, you can do a return from within these inside a function.
Bash also provides the shopt builtin and one of its many options is:
lastpipe
If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment.
Thus, using something like this in a script makes the modfied sum available after the loop:
FILECONTENT="12 Name
13 Number
14 Information"
shopt -s lastpipe # Comment this out to see the alternative behaviour
sum=0
echo "$FILECONTENT" |
while read number name; do ((sum+=$number)); done
echo $sum
Doing this at the command line usually runs foul of 'job control is not active' (that is, at the command line, job control is active). Testing this without using a script failed.
Also, as noted by Gareth Rees in his answer, you can sometimes use a here string:
while read i; do echo $i; done <<< "$FILECONTENT"
This doesn't require shopt; you may be able to save a process using it.
Jonathan Leffler explains how to do what you want using process substitution, but another possibility is to use a here string:
while read i; do echo "$i"; done <<<"$FILECONTENT"
This saves a process.
This function makes duplicates $NUM times of jpg files (bash)
function makeDups() {
NUM=$1
echo "Making $1 duplicates for $(ls -1 *.jpg|wc -l) files"
ls -1 *.jpg|sort|while read f
do
COUNT=0
while [ "$COUNT" -le "$NUM" ]
do
cp $f ${f//sm/${COUNT}sm}
((COUNT++))
done
done
}
I am wondering why this array expression in Bash doesn't give me an array. It just gives me the first element in the string:
IFS='\n' read -r -a POSSIBLE_ENCODINGS <<< $(iconv -l)
I want to try out all available encodings to see how reading different file encodings for a script in R works, and I am using this Bash-script to create text-files with all possible encodings:
#!/bin/bash
IFS='\n' read -r -a POSSIBLE_ENCODINGS <<< $(iconv -l)
echo "${POSSIBLE_ENCODINGS[#]}"
for CURRENT_ENCODING in "${POSSIBLE_ENCODINGS[#]}"
do
TRIMMED=$(echo $CURRENT_ENCODING | sed 's:/*$::')
iconv --verbose --from-code=UTF-8 --to-code="$TRIMMED" --output=encoded-${TRIMMED}.txt first_file.txt
echo "Current encoding: ${TRIMMED}"
echo "Output file:encoded-${TRIMMED}.txt"
done
EDIT: Code edited according to answers below:
#!/bin/bash
readarray -t possibleEncodings <<< "$(iconv -l)"
echo "${possibleEncodings[#]}"
for currentEncoding in "${possibleEncodings[#]}"
do
trimmedEncoding=$(echo $currentEncoding | sed 's:/*$::')
echo "Trimmed encoding: ${trimmedEncoding}"
iconv --verbose --from-code=UTF-8 --to-code="$trimmedEncoding" --output=encoded-${trimmedEncoding}.txt first_file.txt
echo "Current encoding: ${trimmedEncoding}"
echo "Output file:encoded-${trimmedEncoding}.txt"
done
You could just readarray/mapfile instead which are tailor made for reading multi-line output into an array.
mapfile -t possibleEncodings < <(iconv -l)
The here-strings are useless, when you can just run the command in a process-substitution model. The <() puts the command output as if it appears on a file for mapfile to read from.
As for why your original attempt didn't work, you are just doing the read call once, but there is still strings to read in the subsequent lines. You either need to read till EOF in a loop or use the mapfile as above which does the job for you.
As a side-note always use lowercase letters for user defined variable/array and function names. This lets you distinguish your variables from the shell's own environment variables which are upper-cased.
because read reads only one line, following while can be used
arr=()
while read -r line; do
arr+=( "$line" )
done <<< "$(iconv -l)"
otherwise, there is also readarray builtin
readarray -t arr <<< "$(iconv -l)"
Just some background, I have a file with 1000 servers in it new line delimted. I have to read them to an array the run about 5 commands over SSH. I have been using heredoc notation but that seems to fail. Currently I get an error saying the host isn't recognized.
IFS='\n' read -d '' -r -a my_arr < file
my_arr=()
for i in "${my_arr[#]}"; do
ssh "$1" bash -s << "EOF"
echo "making back up of some file"
cp /path/to/file /path/to/file.bak
exit
EOF
done
I get output that lists the first server but then all the ones in the array as well. I know that I am missing a redirect for STDIN that causes this.
Thanks for the help.
Do you need an array? What is wrong with:
while read -r host
do
ssh "$host" bash -s << "EOF"
echo "making back up of some file"
cp /path/to/file /path/to/file.bak
EOF
done < file
To be clear -- the problem here, and the only problem present in the code actually included in your question, is that you're using $1 inside your loop, whereas you specified $i as the variable that contains the entry being iterated over on each invocation of the loop.
That is to say: ssh "$1" needs to instead by ssh "$i".
I am trying to save the result from find as arrays.
Here is my code:
#!/bin/bash
echo "input : "
read input
echo "searching file with this pattern '${input}' under present directory"
array=`find . -name ${input}`
len=${#array[*]}
echo "found : ${len}"
i=0
while [ $i -lt $len ]
do
echo ${array[$i]}
let i++
done
I get 2 .txt files under current directory.
So I expect '2' as result of ${len}. However, it prints 1.
The reason is that it takes all result of find as one elements.
How can I fix this?
P.S
I found several solutions on StackOverFlow about a similar problem. However, they are a little bit different so I can't apply in my case. I need to store the results in a variable before the loop. Thanks again.
Update 2020 for Linux Users:
If you have an up-to-date version of bash (4.4-alpha or better), as you probably do if you are on Linux, then you should be using Benjamin W.'s answer.
If you are on Mac OS, which —last I checked— still used bash 3.2, or are otherwise using an older bash, then continue on to the next section.
Answer for bash 4.3 or earlier
Here is one solution for getting the output of find into a bash array:
array=()
while IFS= read -r -d $'\0'; do
array+=("$REPLY")
done < <(find . -name "${input}" -print0)
This is tricky because, in general, file names can have spaces, new lines, and other script-hostile characters. The only way to use find and have the file names safely separated from each other is to use -print0 which prints the file names separated with a null character. This would not be much of an inconvenience if bash's readarray/mapfile functions supported null-separated strings but they don't. Bash's read does and that leads us to the loop above.
[This answer was originally written in 2014. If you have a recent version of bash, please see the update below.]
How it works
The first line creates an empty array: array=()
Every time that the read statement is executed, a null-separated file name is read from standard input. The -r option tells read to leave backslash characters alone. The -d $'\0' tells read that the input will be null-separated. Since we omit the name to read, the shell puts the input into the default name: REPLY.
The array+=("$REPLY") statement appends the new file name to the array array.
The final line combines redirection and command substitution to provide the output of find to the standard input of the while loop.
Why use process substitution?
If we didn't use process substitution, the loop could be written as:
array=()
find . -name "${input}" -print0 >tmpfile
while IFS= read -r -d $'\0'; do
array+=("$REPLY")
done <tmpfile
rm -f tmpfile
In the above the output of find is stored in a temporary file and that file is used as standard input to the while loop. The idea of process substitution is to make such temporary files unnecessary. So, instead of having the while loop get its stdin from tmpfile, we can have it get its stdin from <(find . -name ${input} -print0).
Process substitution is widely useful. In many places where a command wants to read from a file, you can specify process substitution, <(...), instead of a file name. There is an analogous form, >(...), that can be used in place of a file name where the command wants to write to the file.
Like arrays, process substitution is a feature of bash and other advanced shells. It is not part of the POSIX standard.
Alternative: lastpipe
If desired, lastpipe can be used instead of process substitution (hat tip: Caesar):
set +m
shopt -s lastpipe
array=()
find . -name "${input}" -print0 | while IFS= read -r -d $'\0'; do array+=("$REPLY"); done; declare -p array
shopt -s lastpipe tells bash to run the last command in the pipeline in the current shell (not the background). This way, the array remains in existence after the pipeline completes. Because lastpipe only takes effect if job control is turned off, we run set +m. (In a script, as opposed to the command line, job control is off by default.)
Additional notes
The following command creates a shell variable, not a shell array:
array=`find . -name "${input}"`
If you wanted to create an array, you would need to put parens around the output of find. So, naively, one could:
array=(`find . -name "${input}"`) # don't do this
The problem is that the shell performs word splitting on the results of find so that the elements of the array are not guaranteed to be what you want.
Update 2019
Starting with version 4.4-alpha, bash now supports a -d option so that the above loop is no longer necessary. Instead, one can use:
mapfile -d $'\0' array < <(find . -name "${input}" -print0)
For more information on this, please see (and upvote) Benjamin W.'s answer.
Bash 4.4 introduced a -d option to readarray/mapfile, so this can now be solved with
readarray -d '' array < <(find . -name "$input" -print0)
for a method that works with arbitrary filenames including blanks, newlines, and globbing characters. This requires that your find supports -print0, as for example GNU find does.
From the manual (omitting other options):
mapfile [-d delim] [array]
-d
The first character of delim is used to terminate each input line, rather than newline. If delim is the empty string, mapfile will terminate a line when it reads a NUL character.
And readarray is just a synonym of mapfile.
The following appears to work for both Bash and Z Shell on macOS.
#! /bin/sh
IFS=$'\n'
paths=($(find . -name "foo"))
unset IFS
printf "%s\n" "${paths[#]}"
If you are using bash 4 or later, you can replace your use of find with
shopt -s globstar nullglob
array=( **/*"$input"* )
The ** pattern enabled by globstar matches 0 or more directories, allowing the pattern to match to an arbitrary depth in the current directory. Without the nullglob option, the pattern (after parameter expansion) is treated literally, so with no matches you would have an array with a single string rather than an empty array.
Add the dotglob option to the first line as well if you want to traverse hidden directories (like .ssh) and match hidden files (like .bashrc) as well.
you can try something like
array=(`find . -type f | sort -r | head -2`) , and in order to print the array values , you can try something like echo "${array[*]}"
None of these solutions suited me because I didn't feel like learning readarray and mapfile. Here is what I came up with.
#!/bin/bash
echo "input : "
read input
echo "searching file with this pattern '${input}' under present directory"
# The only change is here. Append to array for each non-empty line.
array=()
while read line; do
[[ ! -z "$line" ]] && array+=("$line")
done; <<< $(find . -name ${input} -print)
len=${#array[#]}
echo "found : ${len}"
i=0
while [ $i -lt $len ]
do
echo ${array[$i]}
let i++
done
You could do like this:
#!/bin/bash
echo "input : "
read input
echo "searching file with this pattern '${input}' under present directory"
array=(`find . -name '*'${input}'*'`)
for i in "${array[#]}"
do :
echo $i
done
In bash, $(<any_shell_cmd>) helps to run a command and capture the output. Passing this to IFS with \n as delimiter helps to convert that to an array.
IFS='\n' read -r -a txt_files <<< $(find /path/to/dir -name "*.txt")
This is my code:
grep $to_check $forbidden >${dir}variants_of_interest;
cat ${dir}variants_of_interest | (while read line; do
#process ${line} and echo result
done;
)
Thank to grep I get lines of data that I then process separately in loop. I would like to use variable instead of using file variants_of_interest.
Reason for this is that I am afraid that writing to file thousands of time (and consequently reading from it) rapidly slows down computation, so I am hoping that avoiding writing to file could help. What do you think?
I have to do thousands of grep commands and variants_of_interest contains up to 10 lines only.
Thanks for your suggestions.
You can just make grep pass its output directly to the loop:
grep "$to_check" "$forbidden" | while read line; do
#process "${line}" and echo result
done
I removed the explicit subshell in your example, since it is already in a separate one due to the piping. Also don't forget to quote the $line variable to prevent whitespace expansion on use.
You dont have to write a file. Simply iterate over the result of grep:
grep $to_check $forbidden | (while read line; do
#process ${line} and echo result
done;
)
This might work for you:
OIFS="$IFS"; IFS=$'\n'; lines=($(grep $to_check $forbidden)); IFS="$OIFS"
for line in "${lines[#]}"; do echo $(process ${line}); done
The first line places the results of the grep into the variable array lines.
The second line processes the array lines placing each line into the variable line