Hi I want to achieve #cat sample.txt > abc.txt
But "> abc.txt" am getting as argument.
How to cascade these two strings and execute the combined one.
I tried the below steps
/home/root# export str=" > abc.txt;"
/home/root# echo $str
> abc.txt;
/home/root# echo "cat sample.txt $str"
cat sample.txt > abc.txt;
/home/root# `echo "cat sample.txt $str"`
Hello world
cat: can't open '>': No such file or directory
cat: can't open 'abc.txt;': No such file or directory
like this?
#!/bin/bash
args="> foo"
command="date"
eval "$command $args"
cat foo
use of eval is not really a recommended method but sometimes it comes in handy for doing things in a quick and dirty way. I will probably get downrated for suggesting this.
Be aware of its side effects.
If you need to do this from command line write :
cat sample.txt | tee $str
Related
I'm sorry for my poor English, first.
I want to read a file (tel.txt) that contains many tel numbers (a number per line) and use that line to grep command to search about the specific number in the source file (another file)!
I wrote this code :
dir="/home/mujan/Desktop/data/ADSL_CDR_Text_Parts_A"
file="$dir/tel.txt"
datafile="$dir/ADSL_CDR_Like_Tct4_From_960501_to_97501_Part0.txt"
while IFS= read -r line
do
current="$line"
echo `grep -F $current "$datafile" >> output.txt`
done < $file
the tel file sample :
44001547
44001478
55421487
but that code returns nothing!
when I declare 'current' variable with literals it works correctly!
what happened?!
Your grep command is redirected to write its output to a file, so you don't see it on the terminal.
Anyway, you should probably be using the much simpler and faster
grep -Ff "$file" "$datafile"
Add | tee -a output.txt if you want to save the output to a file and see it at the same time.
echo `command` is a buggy and inefficient way to write command. (echo "`command`" would merely be inefficient.) There is no reason to capture standard output into a string just so that you can echo that string to standard output.
Why don't you search for the line var directly? I've done some tests, this script works on my linux (CentOS 7.x) with bash shell:
#!/bin/bash
file="/home/mujan/Desktop/data/ADSL_CDR_Text_Parts_A/tel.txt"
while IFS= read -r line
do
echo `grep "$line" /home/mujan/Desktop/data/ADSL_CDR_Text_Parts_A/ADSL_CDR_Like_Tct4_From_960501_to_97501_Part0.tx >> output.txt`
done < $file
Give it a try... It shows nothing on the screen since you're redirecting the output to the file output.txt so the matching results are saved there.
You should use file descriptors when reading with while loop.instead use for loop to avoid false re-directions
dir="/home/mujan/Desktop/data/ADSL_CDR_Text_Parts_A"
file="$dir/tel.txt"
datafile="$dir/ADSL_CDR_Like_Tct4_From_960501_to_97501_Part0.txt"
for line in `cat $file`
do
current="$line"
echo `grep -F $current "$datafile" >> output.txt`
done
I want to be able to read and write to a file with a bash script. I am able to read the variables from the file but I am not sure how to write to it.
The file data.txt contains the variable names and their values like this:
var1=2
var2=3
The shell script looks like this:
#!bin/bash
echo "Read"
var1=$(grep -Po "(?<=^var1=).*" data.txt)
echo "Do Something"
var2=${var1}
echo "Write"
# var2 should be saved in data.txt like this: var2=<Here is the Value>, e.g. var2=2
I can read the specific values of the variables with the grep command, which I got from this answer. But I am not sure how to implement the functionality to be able to save the values of the variables at the correct position (see the last comment in the bash script).
The variable names are unique in data.txt.
Any ideas how to achieve this?
I found a solution with the sed command:
echo "Write"
sed -i -e "s/$(grep -Po '(?<=^var2=).*' data.txt)/${var2}/g" data.txt
You can use sed to do the job.
!/bin/bash
echo "Read"
var1=$(grep -Po "(?<=^var1=).*" data.txt)
echo "Do Something"
var2=${var1}
echo "Write"
sed -i "/var2/ s/.*/var2=${var2}/" data.txt # replace the line contains 'var2' with var2=2
I assume you use GNU sed, for BSD sed, you need to feed an extra suffix to -i.
Basically I want to take as input text from a file, remove a line from that file, and send the output back to the same file. Something along these lines if that makes it any clearer.
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name > file_name
however, when I do this I end up with a blank file.
Any thoughts?
Use sponge for this kind of tasks. Its part of moreutils.
Try this command:
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | sponge file_name
You cannot do that because bash processes the redirections first, then executes the command. So by the time grep looks at file_name, it is already empty. You can use a temporary file though.
#!/bin/sh
tmpfile=$(mktemp)
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name > ${tmpfile}
cat ${tmpfile} > file_name
rm -f ${tmpfile}
like that, consider using mktemp to create the tmpfile but note that it's not POSIX.
Use sed instead:
sed -i '/seg[0-9]\{1,\}\.[0-9]\{1\}/d' file_name
try this simple one
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | tee file_name
Your file will not be blank this time :) and your output is also printed to your terminal.
You can't use redirection operator (> or >>) to the same file, because it has a higher precedence and it will create/truncate the file before the command is even invoked. To avoid that, you should use appropriate tools such as tee, sponge, sed -i or any other tool which can write results to the file (e.g. sort file -o file).
Basically redirecting input to the same original file doesn't make sense and you should use appropriate in-place editors for that, for example Ex editor (part of Vim):
ex '+g/seg[0-9]\{1,\}\.[0-9]\{1\}/d' -scwq file_name
where:
'+cmd'/-c - run any Ex/Vim command
g/pattern/d - remove lines matching a pattern using global (help :g)
-s - silent mode (man ex)
-c wq - execute :write and :quit commands
You may use sed to achieve the same (as already shown in other answers), however in-place (-i) is non-standard FreeBSD extension (may work differently between Unix/Linux) and basically it's a stream editor, not a file editor. See: Does Ex mode have any practical use?
One liner alternative - set the content of the file as variable:
VAR=`cat file_name`; echo "$VAR"|grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' > file_name
Since this question is the top result in search engines, here's a one-liner based on https://serverfault.com/a/547331 that uses a subshell instead of sponge (which often isn't part of a vanilla install like OS X):
echo "$(grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name)" > file_name
The general case is:
echo "$(cat file_name)" > file_name
Edit, the above solution has some caveats:
printf '%s' <string> should be used instead of echo <string> so that files containing -n don't cause undesired behavior.
Command substitution strips trailing newlines (this is a bug/feature of shells like bash) so we should append a postfix character like x to the output and remove it on the outside via parameter expansion of a temporary variable like ${v%x}.
Using a temporary variable $v stomps the value of any existing variable $v in the current shell environment, so we should nest the entire expression in parentheses to preserve the previous value.
Another bug/feature of shells like bash is that command substitution strips unprintable characters like null from the output. I verified this by calling dd if=/dev/zero bs=1 count=1 >> file_name and viewing it in hex with cat file_name | xxd -p. But echo $(cat file_name) | xxd -p is stripped. So this answer should not be used on binary files or anything using unprintable characters, as Lynch pointed out.
The general solution (albiet slightly slower, more memory intensive and still stripping unprintable characters) is:
(v=$(cat file_name; printf x); printf '%s' ${v%x} > file_name)
Test from https://askubuntu.com/a/752451:
printf "hello\nworld\n" > file_uniquely_named.txt && for ((i=0; i<1000; i++)); do (v=$(cat file_uniquely_named.txt; printf x); printf '%s' ${v%x} > file_uniquely_named.txt); done; cat file_uniquely_named.txt; rm file_uniquely_named.txt
Should print:
hello
world
Whereas calling cat file_uniquely_named.txt > file_uniquely_named.txt in the current shell:
printf "hello\nworld\n" > file_uniquely_named.txt && for ((i=0; i<1000; i++)); do cat file_uniquely_named.txt > file_uniquely_named.txt; done; cat file_uniquely_named.txt; rm file_uniquely_named.txt
Prints an empty string.
I haven't tested this on large files (probably over 2 or 4 GB).
I have borrowed this answer from Hart Simha and kos.
This is very much possible, you just have to make sure that by the time you write the output, you're writing it to a different file. This can be done by removing the file after opening a file descriptor to it, but before writing to it:
exec 3<file ; rm file; COMMAND <&3 >file ; exec 3>&-
Or line by line, to understand it better :
exec 3<file # open a file descriptor reading 'file'
rm file # remove file (but fd3 will still point to the removed file)
COMMAND <&3 >file # run command, with the removed file as input
exec 3>&- # close the file descriptor
It's still a risky thing to do, because if COMMAND fails to run properly, you'll lose the file contents. That can be mitigated by restoring the file if COMMAND returns a non-zero exit code :
exec 3<file ; rm file; COMMAND <&3 >file || cat <&3 >file ; exec 3>&-
We can also define a shell function to make it easier to use :
# Usage: replace FILE COMMAND
replace() { exec 3<$1 ; rm $1; ${#:2} <&3 >$1 || cat <&3 >$1 ; exec 3>&- }
Example :
$ echo aaa > test
$ replace test tr a b
$ cat test
bbb
Also, note that this will keep a full copy of the original file (until the third file descriptor is closed). If you're using Linux, and the file you're processing on is too big to fit twice on the disk, you can check out this script that will pipe the file to the specified command block-by-block while unallocating the already processed blocks. As always, read the warnings in the usage page.
The following will accomplish the same thing that sponge does, without requiring moreutils:
shuf --output=file --random-source=/dev/zero
The --random-source=/dev/zero part tricks shuf into doing its thing without doing any shuffling at all, so it will buffer your input without altering it.
However, it is true that using a temporary file is best, for performance reasons. So, here is a function that I have written that will do that for you in a generalized way:
# Pipes a file into a command, and pipes the output of that command
# back into the same file, ensuring that the file is not truncated.
# Parameters:
# $1: the file.
# $2: the command. (With $3... being its arguments.)
# See https://stackoverflow.com/a/55655338/773113
siphon()
{
local tmp file rc=0
[ "$#" -ge 2 ] || { echo "Usage: siphon filename [command...]" >&2; return 1; }
file="$1"; shift
tmp=$(mktemp -- "$file.XXXXXX") || return
"$#" <"$file" >"$tmp" || rc=$?
mv -- "$tmp" "$file" || rc=$(( rc | $? ))
return "$rc"
}
There's also ed (as an alternative to sed -i):
# cf. http://wiki.bash-hackers.org/howto/edit-ed
printf '%s\n' H 'g/seg[0-9]\{1,\}\.[0-9]\{1\}/d' wq | ed -s file_name
You can use slurp with POSIX Awk:
!/seg[0-9]\{1,\}\.[0-9]\{1\}/ {
q = q ? q RS $0 : $0
}
END {
print q > ARGV[1]
}
Example
This does the trick pretty nicely in most of the cases I faced:
cat <<< "$(do_stuff_with f)" > f
Note that while $(…) strips trailing newlines, <<< ensures a final newline, so generally the result is magically satisfying.
(Look for “Here Strings” in man bash if you want to learn more.)
Full example:
#! /usr/bin/env bash
get_new_content() {
sed 's/Initial/Final/g' "${1:?}"
}
echo 'Initial content.' > f
cat f
cat <<< "$(get_new_content f)" > f
cat f
This does not truncate the file and yields:
Initial content.
Final content.
Note that I used a function here for the sake of clarity and extensibility, but that’s not a requirement.
A common usecase is JSON edition:
echo '{ "a": 12 }' > f
cat f
cat <<< "$(jq '.a = 24' f)" > f
cat f
This yields:
{ "a": 12 }
{
"a": 24
}
Try this
echo -e "AAA\nBBB\nCCC" > testfile
cat testfile
AAA
BBB
CCC
echo "$(grep -v 'AAA' testfile)" > testfile
cat testfile
BBB
CCC
I usually use the tee program to do this:
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | tee file_name
It creates and removes a tempfile by itself.
I want to save a command to a file (for example I want to save the string "cat /etc/passwd" to a file) but I can't use the echo command.
How can I create and save string to a file directly without using echo command?
You can redirect cat to a file, type the text, and press Control-D when you're done, like this:
cat > file.txt
some text
some more text
^D
By ^D I mean to press Control-D at the end. The line must be empty.
It will not be part of the file, it is just to terminate the input.
Are you avoiding ECHO for security purposes (e.g. you're using a shared terminal and you don't want to leave trace in the shell history of what you've written inside your files) or you're just curious for an alternative method?
Simple alternative to echo:
As someone said, redirecting cat is probably the simpler way to go.
I'd suggest you to manually type your end-of-file, like this:
cat <<EOF > outputfile
> type here
> your
> text
> and finish it with
> EOF
Here's the string you're asking for, as an example:
cat <<EOF > myscript.sh
cat /etc/passwd
EOF
You probably don't want everyone to know you've peeked into that file, but if that's your purpose please notice that wrapping it inside an executable file won't make it more private, as that lines will be logged anyway...
Security - Avoiding history logs etc..
In modern shell, just try adding a space at the beginning of every command and use freely whatever you want.
BTW, my best hint is to avoid using that terminal at all, if you can. If you got two shells (another machine or even just another secure user in the same machine), I'd recommend you using netcat. See here: http://www.thegeekstuff.com/2012/04/nc-command-examples/?utm_source=feedburner
{ { command ls $(dirname $(which cat)) |
grep ^ca't$'; ls /etc/passwd; } |
tr \\n ' '; printf '\n'; } > output-file
But it's probably a lot simpler to just do : printf 'cat /etc/passwd\n'
To be clear, this is a tongue-in-cheek solution. The initial command is an extraordinarily convoluted way to get what you want, and this is intended to be a humorous answer. Perhaps instructive to understand.
I am not sure I understood you correctly but
cat /etc/passwd > target.file
use the > operator to write it to file without echoing
If you need to use it, inside a program :
cat <<EOF >file.txt
some text
some more text
EOF
I would imagine that you are probably trying to print the content of a string to a file, hence you mentioned echo.
You are avoiding this:
echo "cat /etc/passwd" > target.file
You can use a here string combined with cat.
cat > target.file <<< "cat /etc/passwd"
Now the file target.file will contain a string cat /etc/passwd.
$ cat target.file
cat /etc/passwd
$
To create string:
var1=your command
to save a file or variable in a file without echo use:
cat $FILE/VAR1 > /new/file/path
My code is given below. Echo works fine. But, the moment I redirect output of echo to touch, I get an error "no such file or directory". Why ? How do i fix it ?
If I copy paste the output of only echo, then the file is created, but not with touch.
while read line
do
#touch < echo -e "$correctFilePathAndName"
echo -e "$correctFilePathAndName"
done < $file.txt
If you have file names in each line of your input file file.txt then you don't need to do any loop. You can just do:
touch $(<file.txt)
to create all the files in one single touch command.
You need to provide the file name as argument and not via standard input. You can use command substitution via $(…) or `…`:
while read line
do
touch "$(echo -e "$correctFilePathAndName")"
done < $file.txt
Ehm, lose the echo part... and use the correct variable name.
while read line; do
touch "$line"
done < $file.txt
try :
echo -e "$correctFilePathAndName" | touch
EDIT : Sorry correct piping is :
echo -e "$correctFilePathAndName" | xargs touch
The '<' redirects via stdin whereas touch needs the filename as an argument. xargs transforms stdin in an argument for touch.