I understand the format below...
while read line
do
etc...
However, I saw this yesterday and haven't been able to figure out what var would be in the following:
while read pkg var
do
etc...
Thanks
while loop will read the var one by one , but assign the last parts to one var.
For example, I have a file like:
a b c d
when run the command
$ while read x y
do
echo $x
echo $y
done < file
Resule:
a
b c d
You will get "b c d" to $y.
Of course, if you only assign one var (line), then $line will get the whole line.
The read builtin will read multiple whitespace-separated (or, really, separated by whatever is in $IFS) values.
echo a b c | (read x y z; echo "$y")
#=> b
If there are more fields than variables passed to read, the last variable gets the rest of the line.
Related
Given a file so that in any line can be more than one word, and exists a single space between any word to other, for example:
a a a a
b b b b
c c
d d
a a a a
How can I create array so that in the cell number i will be the line number i , but WITHOUT DUPLICATES BETWEEN THE ELEMENTS IN THE ARRAY !
In according to the file above, we will need create this array:
Array[0]="a a a a" , Array[1]="b b b b" , Array[2]="c c" , Array[3]=d d.
(The name of the file pass to the script as argument).
I know how to create array that will contain all the lines. Something like that:
Array=()
while read line; do
Array=("${Array[#]}" "${line}")
done < $1
But how can I pass to the while read.. the sorting (and uniq) output of the file?
You should be able to use done < <(sort "$1" | uniq) in place of done < $1.
The <() syntax creates a file-like object from a subshell to execute a separate set of commands.
hi i'm new to shell and got stuck on this:
i have file A like
[area1]
a
b
[area2]
c
d
[area3]
e
f
i want to read the lines in a certain area and append the lines to file B. like for [area2], i'm expecting to read
c
d
also the area names are random and stored in a variable say $AREA, so i'll also need to match the variable instead of directly using "[area2]", so what i need to cut is from the line equals $AREA to the next line start with "["
how can i achieve this? any help would be appreciated!
a="area2"
sed '/\['"$a"'\]/,+4!d' A|sed '1,2d' >>B
Sed has a write command(w) that you can use to redirect output to a file:
AREA="area2";
sed '/\['"$AREA"'\]/,/^\[/!d;//d;w B' A
/\['"$AREA"'\]/,/^\[/!d: prints lines from your area value up to next line starting with [
//d: removes lines matching the addresses(here [area2] and [area3])
w B: writes to file B
To append output to file B:
sed '/\['"$AREA"'\]/,/^\[/!d;//d;' A >> B
Check out the following transcript. With all possible rigor and formality, what is going on at each step?
$> ls -1 #This command prints 3 items. no explanation required.
a
b
c
$> X=$(ls -1) #Capture the output (as what? a string?)
$> Y=($(ls -1)) #Capture it again (as an array now?)
$> echo ${#X[#]} #Why is the length 1?
1
$> echo ${#Y[#]} #This works because Y is an array of the 3 items?
3
$> echo $X #Why are the linefeeds now spaces?
a b c
$> echo $Y #Why does the array echo as its first element
a
$> for x in $X;do echo $x; done #iterate over $X
a
b
c
$> for y in $Y;do echo $y; done #iterating over y doesn't work
a
$> echo ${X[2]} #I can loop over $X but not index into it?
$> echo ${Y[2]} #Why does this work if I can't loop over $Y?
c
I assume bash has well established semantics about how arrays and text variables (if that's even what they're called) work, but the user manual is not organized in an optimal fashion for someone who wants to reason about scripts based on whatever small set of underlying principles the language designer intended.
Let me preface the following with the very strong suggestion that you never use ls to populate an array. The correct code would be
Z=( * )
to create an array with each (non-hidden) file in the current directory as a distinct array element.
$> ls -1 #This command prints 3 items. no explanation required.
a
b
c
Correct. Each file name is printed on a separate line (although, beware of file names containing newlines; the parts before and after each newline would appear as separate file names.)
$> X=$(ls -1) #Capture the output (as what? a string?)
Yes. The output of ls is concatenated by the command substitution into a single string using a single space to separate each line. (The command substitution would be subject to word-splitting if it weren't the right-hand side of an assignment; word-splitting will come up below.)
$> Y=($(ls -1)) #Capture it again (as an array now?)
Same as with X, but now each of the words in the result of the command substitution is treated as a separate array element. As long as none of the output lines contain any characters in the value of IFS, each file name is one word and will be treated as a separate array element.
$> echo ${#X[#]} #Why is the length 1?
1
X, not being a real array, is treated as an array with a single element, namely the value of $X.
$> echo ${#Y[#]} #This works because Y is an array of the 3 items?
3
Correct.
$> echo $X #Why are the linefeeds now spaces?
a b c
When $X is unquoted, the resulting expansion is subject to word-splitting. In this case, the newlines are simply treated the same as any other whitespace, separating the result into a sequence of words that are passed to echo as distinct arguments, which are then displayed separated by a single space each.
$> echo $Y #Why does the array echo as its first element
a
For a true array, $Y is equivalent to ${Y[0]}.
$> for x in $X;do echo $x; done #iterate over $X
a
b
c
This works, but has caveats.
$> for y in $Y;do echo $y; done #iterating over y doesn't work
a
See above; $Y only expands to the first element. You want for y in "${Y[#]}"; do to iterate over all the elements.
$> echo ${X[2]} #I can loop over $X but not index into it?
Correct. X is not an array, but $X expanded to a space-separated list which the for loop could iterate over.
$> echo ${Y[2]} #Why does this work if I can't loop over $Y?
c
Indexing and iteration are two completely different things in shell. You don't actually iterate over an array; you iterate over the resulting sequence of words of a properly expanded array.
Regardless of the number of arguments passed to my script, I would like for the second to the last argument to always represent a specific variable in my code.
Executing the program I'd type something like this:
sh myprogram.sh -a arg_a -b arg_b special specific
test=("${3}")
echo $test
The results will show 'special'. So using that same idea if I try this (since I won't know that number of arguments):
secondToLastArg=$(($#-1))
echo $secondToLastArg
The results will show '3'. How do I dynamically assign the second to last argument?
You need a bit of math to get the number you want ($(($#-1))), then use indirection (${!n}) to get the actual argument.
$ set -- a b c
$ echo $#
a b c
$ n=$(($#-1))
$ echo $n
2
$ echo ${!n}
b
$
Indirection (${!n}) tells bash to use the value of n as the name of the variable to use ($2, in this case).
You can use $# as array & array chopping methods:
echo ${#:$(($#-1)):1}
It means, use 1 element starting from $(($#-1))...
If some old versions of shells do not support ${array:start:length} syntax but support only ${array:start} syntax, use below hack:
echo ${#:$(($#-1))} | { read x y ; echo $x; } # OR
read x unused <<< `echo ${#:$(($#-1))}`
I am trying to write a program that will print out every line from a file with another line of that file added at the end, basically creating pairs from a portion of each line. If the line is the same, it will do nothing. Also, it must avoid repeating the same pairs. A B is the same as B A
In short
FileInput:
otherstuff A
otherstuff B
otherstuff C
otherstuff D
Output:
A B
A C
A D
B C
B D
C D
I was trying to do this with a BASH script, but was having trouble because I could not get my nested while loops to work. It would read the first line, compare it to each other line, and then stop (Basically only outputting the first 3 lines in the example output above, the outer while loop only ran once).
I also suspect I might be able to do this using MATLAB, so suggestions using that are also welcome.
Here is the bash script that I have thus far. As I said, it is no printing out correctly for me, as the outer loop only runs once.
#READS IN file from terminal
FILE1=$1
#START count at 0
count0=
exec 3<&0
exec 0< $FILE1
while read LINEa; do
while read LINEb; do
eventIDa=$(echo $LINEa | cut -c20-23)
eventIDb=$(echo $LINEb | cut -c20-23)
echo $eventIDa $eventIDb
done
done
Using bash:
#!/bin/bash
[ -f "$1" ] || { echo >&2 "File not found"; exit 1; }
mapfile -t lines < <(cut -c20-23 <"$1" | sort | uniq)
for i in ${!lines[#]}; do
elem1=${lines[$i]}
unset lines[$i]
for elem2 in "${lines[#]}"; do
echo "$elem1" "$elem2"
done
done
This will read a file given as a parameter on the command line, sort and filter out duplicates, and output all combinations. You can modify the parameter to cut to adjust to your particular input file.
Due to the particular way you seem to indent to use cut, your input example above won't work. Instead, use something with the correct line length, such as:
123456789012345678 A
123456789012345678 B
123456789012345678 C
123456789012345678 D
Assuming the otherstuff is not relevant (otherwise you can of course add it later) this should do the trick in Matlab:
combnk({'A' 'B' 'C' 'D'},2)