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I have this script I'm studying and I would like to know what is cat doing in this section.
if cat downloaded.txt | grep "$count" >/dev/null
then
echo "File already downloaded!"
else
echo $count >> downloaded.txt
cat $count | egrep -o "http://server.*(png|jpg|gif)" | nice -n -20 wget --no-dns-cache -4 --tries=2 --keep-session-cookies --load-cookies=cookies.txt --referer=http://server.com/wallpaper/$number -i -
rm $count
fi
Like most cats, this is a useless cat.
Instead of:
if cat downloaded.txt | grep "$count" >/dev/null
It could have been written:
if grep "$count" download.txt > /dev/null
In fact, because you've eliminated the pipe, you've eliminated issues with which exit value the if statement is dealing with.
Most Unix cats you'll see are of the useless variety. However, people like cats almost as much as they like using a grep/awk pipe, or using multiple grep or sed commands instead of combining everything into a single command.
The cat command stands for concatenate which is to allow you to concatenate files. It was created to be used with the split command which splits a file into multiple parts. This was useful if you had a really big file, but had to put it on floppy drives that couldn't hold the entire file:
split -b140K -a4 my_really_big_file.txt my_smaller_files.txt.
Now, I'll have my_smaller_files.txt.aaaa and my_smaller_files.txt.aaab and so forth. I can put them on the floppies, and then on the other computer. (Heck, I might go all high tech and use UUCP on you!).
Once I get my files on the other computer, I can do this:
cat my_smaller_files.txt.* > my_really_big_file.txt
And, that's one cat that isn't useless.
cat prints out the contents of the file with the given name (to the standard output or to wherever it's redirected). The result can be piped to some other command (in this case, (e)grep to find something in the file contents). Concretely, here it tries to download the images referenced in that file, then adds the name of the file to downloaded.txt in order to not process it again (this is what the check in if was about).
http://www.linfo.org/cat.html
"cat" is a unix command that reads the contents of one or more files sequentially and by default prints out the information the user console ("stdout" or standard output).
In this case cat is being used to read the contents of the file "downloaded.txt", the pipe "|" is redirecting/feeding its output to the grep program, which is searching for whatever is in the variable "$count" to be matched with.
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I have an hexadecimal string s and a file f, i need to search the first occurence of that string in the file and save that in a variable with his offset. I thought that the right way to do that is convert the file to hex and search that with a grep. The main problem is that i saw a lot of commands(hexdump,xxd,etc.) to convert but none of them actually work. Any suggestion?
My attempt was like this:
xxd -plain $f > $f
grep "$s" .
output should be like:
> offset:filename
A first approach without any error handling could look like
#!/bin/bash
BINFILE=$1
SEARCHSTRING=$2
HEXSTRING=$(xxd -p ${BINFILE} | tr -d "\n")
echo "${HEXSTRING}"
echo "Searching ${SEARCHSTRING}"
OFFSET=$(grep -aob ${SEARCHSTRING} <<< ${HEXSTRING} | cut -d ":" -f 1)
echo ${OFFSET}:${BINFILE}
I've used xxd here because of Does hexdump respect the endianness of its system?. Please take also note that according How to find a position of a character using grep? grep will return multiple matches, not only the first one. The offset will be counted beginning from 1, not 0. To substract 1 from the variable ${OFFSET} you may use $((${OFFSET}-1)).
I.e. search for the "string" ELF (HEX 454c46) in a system binary will look like
./searchHEX.sh /bin/yes 454c46
7f454c460201010000000000000000000...01000000000000000000000000000000
Searching 454c46
2:/bin/yes
I would use regex for this as well:
The text file:
$ cat tst.txt
1234567890x1fgg0x1cfffrr
A script you can easily change/extend yourself.
#! /bin/bash
part="$(perl -0pe 's/^((?:(?!0(x|X)[0-9a-fA-F]+).)*)(0(x|X)[0-9a-fA-F]+)(.|\n)*/\1:\3\n/g;' tst.txt)"
tmp=${part/:0x*/}
tmp=${#tmp}
echo ${part/*:0x/$tmp:0x} # Echoes 123456789:0x1f
Regex:
^((?:(?!0x[0-9a-fA-F]+).)*) = Search for the first entry that's a hexadecimal number and create a group of it (\1).
(0x[0-9a-fA-F]+) = Make a group of the hexadecimal number (\3).
(.|\n)* = Whatever follows.
Please note that tmp=${part/:0x*/} could cause problems if you have text like :0x before the hexadecimal number that is caught.
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I want to search for a String by navigating to a particular line, How to do this in shell scripting?
For example,
I have
this is the first line
this is the Second line
This is the Third line
Now here i would want to look for string "Third" by going to 3rd line.
Any help is appreciated, Thank you.
Try stringing together cat, sed, and grep.
sed '3!d' filename | grep Third
The unnamed or anonymous pipe (|) and redirection (<, >) are powerful features of many shells. They allow one to combine a set of commands to perform a more complex function.
In the case of this question there were two clear steps,
1) Operate on a specific line of a file (e.g. filter a file)
2) Search the output of the filter for a specific string
Recognizing that there were two steps is a strong indicator that two commands will need to be combined. Therefore, the problem can be solved by finding a solution to each step and then combining them in to one command with pipes and redirection.
If you know about the Stream Editor (sed), it may come to your mind when thinking about how to accomplish the first step of filtering the file. If not searching for, "linux get a specific line of a file" this OS question comes up high in the search results.
$ cat tmp.txt
this is the first line
this is the Second line
This is the Third. line
$ sed '3!d' tmp.txt
This is the Third. line
Knowing that grep can be search for lines with the string of interest the next challenge is to figure out how to get the output of sed as the input to grep. The pipe (|) solves this problem.
sed '3!d' filename | grep Third
Example output:
$ sed '3!d' tmp.txt | grep Third
This is the Third. line
$
Another powerful concept in shell scripting is the exit status. The grep command will set the exit status to 0 when a match is found and 1 when a match is not found. The shell stores the exit status in a special variable named $? (for bash). Therefore, one could use the exit status to conditionally determine the next step in the shell script. The example below does not implement conditions (like if, else). The example below shows the exit status value using the echo command.
$ sed '3!d' tmp.txt | grep Third
This is the Third. line
$ echo $?
0
$ sed '3!d' tmp.txt | grep third
$ echo $?
1
$
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I want to search for a string (which I don't know unless I look inside the files) on Linux command-line.
Example:
A file 1 with text inside
A file 2 with text inside
In both files the word "Apple" is existent.
I want to echo this word (which exists in both files) in a file or store it in a variable.
How is this possible?
You can get a list of all the unique words in a file using:
grep -o -E '\w+' filename | sort -u
where -E '\w+' is matching words and -o outputs the matching parts. We can then use the join command which identifies matching lines in two files, along with process substitution to pass in the results of our word finder:
join <(grep -o -E '\w+' filename1 | sort -u) <(grep -o -E '\w+' filename2 | sort -u)
If there are no duplicates whiten a single file you could use cat file1 file2 |sort | uniq -d
$ cat input_one.txt
FIREFOX a
FIREFOX b
Firefox a
firefox b
$ cat input_two.txt
CHROME a
FIREFOX a
EXPLORER a
$ while read line; do grep "$line" input_two.txt ; done < input_one.txt
FIREFOX a
Explanation:
while will loop every line with input_two.txt file as input and will store the temporary line in the line variable.
In every line will search for it in the input_one.txt file and -o option will make to print only the matched part.
EDIT: See comments
You can write the script to handle this.
You need loop a word on file 1, in the loop use grep command (grep -nwr "" -e "$word") to find a word in file 2.
If match, echo a word.
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Suppose I have a file with warnings. Each warning in a new line with an id that has 3 capital letters followed by 3 digits only, should be replaced by its id.
Example:
SIM_WARNING[ANA397]: Node q<159> for vector output signal does not exist
The output should be ANA397 and the rest of line is deleted.
How to do so using sed?
I don't think you need sed for that. A simple grep with --only-matching could do, as in:
grep -E 'SIM_WARNING\[(.)\]' --only-matching
should work for you.
Where:
-E does "enhanced regular expressions. I think we need those for capturing with ( )
then follows the pattern, which consists of the fixed SIM_WARNING, followed by a match inside the square brackets
--only-matching simply makes grep print only matching content
In other words: by using ( match ) you tell grep that you only care about the thing in that match pattern.
for id in $(grep -o "^SIM_WARNING\[[A-Z][A-Z][A-Z][0-9][0-9][0-9]\]" test1.bla | grep -o "[A-Z][A-Z][A-Z][0-9][0-9][0-9]" test1.bla ); do echo $id; done
This finds ANA397 from the below.
SIM_WARNING[ANA397]: Node q<159> for vector output signal does not exist
First of all, you have to choose how to use the IDs, for example if you need to cycle the file first or the IDs later...
E.G. (Cycle file first)
exec 3<file
while read -r line <&3; do
id="$(printf "%s" "${line}" | sed -e "s/.*\[\([[:alnum:]]\+\)\].*/\1/")"
### Do something with id
done
exec 3>&-
Otherwise you can decide to cycle the output of sed...
E.G.
for id in $(sed -e "s/.*\[\([[:alnum:]]\+\)\].*/\1/" file); do
### Do something with id
done
Both of the examples should work with posix shell (If I am not missing something...), but shell like posh may not support classes as [[:alnum:]], you can substitute them with the equivalent [a-zA-Z0-9], as every guide will teach you.
Note that the check is not on 3 letters and 3 digits, but for any letter and digit between brackets ([ and ]).
EDIT:
If your lines start with SIM_WARNING you can discriminates those lines with -e "/^SIM_WARNING/! d"
For a strict check on 3 letters and 3 digits you can use -e "s/.*\[\([a-zA-Z][a-zA-Z][a-zA-Z][0-9][0-9][0-9]\)\].*/\1/"
So taking the example above you can do somethin like:
for id in $(sed -e "/^SIM_WARNING/! d" -e "s/.*\[\([a-zA-Z][a-zA-Z][a-zA-Z][0-9][0-9][0-9]\)\].*/\1/" file)
### Do something with id
done
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I am writing a bash script to check if an array element is in a file.
For example:
I have an array of errors errors=("1234" "5678" "9999")
I have a file that contains patterns of strings
123400 452612 9999A0 1010EB
I am looking to loop over the file that contains the errors and check to see if any of the array elements matches any string pattern in the file. If it does then give me back the exact array pattern it matched in the file for further processing.
Any ideas on how I can do this?
Here's a way where you only need to invoke grep once:
$ grep -oFf <(printf "%s\n" "${errors[#]}") file
1234
9999
The -f option is to specify a file that contains the pattern. I use a process substitution to "contain" the patterns, one per line.
The -F option specifies plain-text matching: I assume your "errors" array won't contain regular expressions.
Sounds like you just want a loop:
for error in "${errors[#]}"; do
if grep -qE "(^| )$error( |\$)" file; then
# $error was found in the file
fi
done
This matches the error preceded by the start of the line or a space, and followed by a space or the end of the line.
I made an effort to not match appearances of the errors within substrings but if you don't care, then you could change the grep command to this:
grep -qF "$error" file
This will return success if the error string occurs anywhere on the line.
The script goes like this,
#/bin/bash
errors=("1234" "5678" "9999")
for error in "${errors[#]}"
do
grep -o "$error" file
done
For a sample file,
$ cat file
123400 452612 9999A0 1010EB
The script produces an output
$ ./script.sh
1234
9999
meaning the above two keys from the array have matched in the file. The -o flag in grep is to identify only the matching parts from the array. An excerpt from the man grep page.
-o, --only-matching
Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.