This question already has answers here:
How can I view only the first n lines of the file?
(2 answers)
Closed 4 years ago.
I am working on ARM-based processor and I am preparing inside it a shell script that writes into a text file a set of commands output.
I want it to write exactly 10 lines of a command's output (for example top command) but I don't know how, would you help me please ?
Thank you.
Which operating system are you working in ? If you have awk installed, you
can do:
command | awk 'NR<=10' > f.txt
command | head -n 10 > file.txt
If you want a pure Bash solution:
n=0
command | while (( n++ != 10 )) && IFS= read -r line; do
printf '%s\n' "$line"
done
command | sed 1,10p > f.txt
sed filters lines based on a pattern and performs an action on them. In this case, pattern is to filter lines whose number is between 1 and 10, and action is just to 'p'rint them.
Related
This question already has answers here:
Conditional step in a pipeline
(2 answers)
Can I make a shell function in as a pipeline conditionally "disappear", without using cat?
(1 answer)
Closed 4 months ago.
EMPTY_VAR=''
MMDDYYYY='6.18.1997'
PIPE_VAR=' | xargs echo "1+" | bc'
echo "$MMDDYYYY" | cut -d "." -f 2${EMPTY_VAR}
>> 18
Command above would give me correct output, which is 18, but if I try to use PIPE_VAR instead it would give me bunch of errors:
echo "$MMDDYYYY" | cut -d "." -f 2${PIPE_VAR}
cut: '|': No such file or directory
cut: xargs: No such file or directory
cut: echo: No such file or directory
cut: '"1+"': No such file or directory
cut: '|': No such file or directory
cut: bc: No such file or directory
OR:
echo "$MMDDYYYY" | cut -d "." -f 2"$PIPE_VAR"
cut: invalid field value ‘| xargs echo "1+" | bc’
Try 'cut --help' for more information.
What I'm really trying to find out is that even possible to combine commands like this?
You can't put control operators like | in a variable, at least not without resorting to something like eval. Syntax parsing comes before parameter expansion when evaluating the command line, so Bash is only ever going to see that | as a literal character and not pipeline syntax. See BashParsing for more details.
Conditionally adding a pipeline is hard to do well, but having a part of the pipeline conditionally execute one command or another is more straightforward. It might look something like this:
#!/bin/bash
MMDDYYYY='6.18.1997'
echo "$MMDDYYYY" | cut -d "." -f 2 |
if some_conditional_command ; then
xargs echo "1+" | bc
else
cat
fi
It looks like you're trying to calculate the next day. That's hard to do with plain arithmetic, particularly with month/year ends.
Let date do the work. This is GNU date. It can't parse 6.18.1997 but it can parse 6/18/1997
for MMDDYYYY in '2.28.1996' '2.28.1997'; do
date_with_slashes=${MMDDYYYY//./\/}
next_day=$(date -d "$date_with_slashes + 1 day" '+%-m.%-d.%Y')
echo "$next_day"
done
2.29.1996
3.1.1997
This question already has answers here:
How to save both matching and non-matching from grep
(3 answers)
Closed 1 year ago.
Currently My shell script iterate the lines in one huge file two times:
(What I want to do is just like the shell script below.)
grep 'some_text' huge_file.txt > lines_contains_a.txt
grep -v 'some_text' huge_file.txt > lines_not_contains_a.txt
but it is slow.
How to do the same thing only iterate the lines once?
Thanks!
With GNU awk:
awk '/some_text/ { print >> "lines_contains_a.txt" }
!/some_text/ { print >> "lines_not_contains_a.txt" }' huge_file.txt
With sed:
sed -n '/some_text/ w lines_contains_a.txt
/some_text/! w lines_not_contains_a.txt' huge_file.txt
This question already has answers here:
How to replace one character with two characters using tr
(5 answers)
Closed 3 years ago.
Hi I have a shell script that has
variable="apple banana monkey"
I want it to be
apple\nbanana\nmonkey
But when I try and execute
echo $variable | tr ' ' '\n'
It results to
apple
banana
monkey
I want to get the actual literal of new line and not the evaluated value.
I have tried echo -e or echo -n or even put numerous escapes \\ but to no avail.
Please help. Thanks
tr command translates chars into chars by performing a 1 to 1 mapping. You are asking the tool to translate a space into two chars, which is something that cannot be done with tr.
If you accept a command switch, you can try with sed:
echo "$variable" | sed 's/ /\\n/g'
This question already has answers here:
Displaying only single most recent line of a command's output
(2 answers)
Closed 5 years ago.
I been trying to print a variable in the same time for a scrip that pretends automatize a process the content is the output of this
sed "s/Read/\n/g" /tmp/Air/test.txt | tail -1 test.txt | grep ARP
so i put this in a while loop
do
out= sed "s/Read/\n/g" /tmp/Air/test.txt | tail -1 test.txt | grep ARP
echo -n "$out"
sleep 1
done
i read other questions here and i try with different option like echo -ne, echo -ne "$out" \r, printf "\r" or printf "%s" and no luck with no one, all the other example don't have a variable to print just counter o system variables
Update
it seems to appear that the echo -n repeat $out in the same line, if out="this is a test" the output of echo -n is "this is a test this is a test this is a test this is a test ...." maybe im missing some option ?
Update 2
sorry for the miss understood perhaps i was not very clear but what i want is overwrite the same line with the value of $out, the source of $out is the output of the aireplay-ng command that executes along with the script and i get the output with
the ouput is something like this
102415 packets (got 5 ARP requests and 15438 ACKs), sent 37085 packets...(499 pps)
but the number of ARP request is changing constantly
this code for example use echo -ne and overwrite in the same line
#!/bin/bash
for pc in $(seq 1 100); do
echo -ne "$pc%\033[0K\r"
sleep 1
done
the output of this is like a percent indicator that shows "10%" and going instead of "1% 2% 3% 4% 5% .." in the same line and i already try like this but with no luck
if you are trying to execute the sed Please use
`sed "s/Read/\n/g" /tmp/Air/test.txt | tail -1 test.txt | grep ARP`
First of all you are assigning a value of bash command wrongly to variable.
out=$(sed "s/Read/\n/g" /tmp/Air/test.txt | tail -1 test.txt | grep ARP)
Then you can print all your output in one line as you wrote:-
echo -n $out
The recent addendum to your question reads like you're miscommunicating your intent: this is a test this is a test this is a test is what a plain reading of your question indicates you to be asking for (printing this is a test over and over in a loop without newlines, after all, can be expected to do nothing else); why you'd describe this in a context that makes it sound like a bug is thus surprising.
If you want to send the cursor back to the beginning of your current line and overwrite that line, that might be something like the following:
#!/bin/bash
# ^^^^ not /bin/sh; this enables bash extensions
# ask the shell to keep $COLUMNS up-to-date
shopt -s checkwinsize
# defaults to 80-character terminal width, but uses $COLUMNS if available
printf "%-${COLUMNS:-80}s\r" "$out"`
...which prints your string, pads out to 80 characters with spaces, and then returns the cursor to the beginning of the line, such that the next thing you write will overwrite that string.
Of course, if you print that line and then return to a shell prompt, the prompt will start at the beginning of the same line and overwrite the text, so be sure to follow up with an echo.
This question already has answers here:
how can I combine these lines
(4 answers)
Closed 8 years ago.
I want convert this text on a given file:
87665
S
3243423
S
334243
N
...
to something like this:
87665,S
3243423,S
334243,N
...
I've been reading some similar questions, but it didn't work... is there a way to do this with a single line command in linux?
Thanks!
Using sed:
sed '$!N;s/\n/,/' filename
Using paste:
paste -d, - - < filename
paste would leave a trailing , in case the input has an odd number of lines.
Something like this might work for you:
$ awk 'NR%2{a=$0;next}{print a","$0}' file
87665,S
3243423,S
334243,N
To handle files with odd lines, you can do:
awk '{printf "%s%s", $0, NR%2?",":ORS}' file
Just for fun, a pure bash solution:
while IFS= read -r l1; do
read -r l2
printf '%s\n' "$l1${l2:+,$l2}"
done < file
If there's an odd number of lines, the last line will not have a trailing comma.