I am learning to play around with functions in bash. I have the first function read_file() that reads /etc/file and replaces ':'with a space between words (e.g root:x:0:0:root ... becomes root x 0 0 root ... ). I then want to be able to manipulate output from individual words in each of the lines.
My second function- display__user_shell() prints our the shell for each corresponding users as is in the /etc/file.
My problem is figuring out how to call the first function read_file() and using its variables in the display__user_shell function.
I have been able to do the above when using input from a single line rather than reading from a file.
i just called new_data -i.e $new_data from the display__user_shell() function
read_file() {
read -p "Enter file" file
while read line
do
newlin=$(echo $line | tr ":" " ")
echo newlin
done
}
oldIFS=$IFS
IFS=" "
ct=0
display__user_shell() {
readfile
for item in $newlin;
do
[ $ct -eq 0 ] && name="$item";
[ $ct -eq 6 ] && name="$item";
done
echo "$user's shell is $shell"
}
IFS=$oldIFS
display__user_shell
the first line of the output should be..
root's shell is /bin/bash
Irrespective of the implementation there is an interesting question here: how to reference variables from one function in another function. The short answer is that you can:
$ a() { aye=bee; }
$ b() { echo "$aye"; }
$ a
$ b
bee
But this is a very bad idea - Bash has "unfortunate" scoping rules different from safer languages like Java, Python, or Ruby, and code like this is very hard to follow. Instead there are several patterns you can use to produce more readable code:
Print the value in the inner function and assign that to a value in the outer function:
a() {
echo 'bee'
}
b() {
aye="$(a)"
echo "$aye"
}
b # Prints "bee"
Call and assign to a variable the first function in the outer scope and use it in the second function:
a() {
echo 'bee'
}
aye="$(a)"
b() {
echo "$aye"
}
b # Prints "bee"
Treat the first and second functions as a pipeline, passing standard output of the first one to the standard input of the second one (read is a slow way to process a large file, but it'll serve as an example):
a() {
echo 'bee'
}
b() {
while read -r line
do
echo "$line"
done
}
a | b # Prints "bee"
Which one you choose depends on things like what else you intend to do with what a returns and whether a produces huge amounts of output.
Related
i have bash script which do some function with website domains like following
#!/usr/bin/env bash
domain="$1";
function A(){
ping $domain -c 4
}
function B(){
host $domain
}
A;
B;
and when i run this script by doing ./script.sh whateverdomain.com it works well.
BUT i heard that with array i can run the script against some of domains seperated by comma for example.
like ./script.sh domain1.com,domain2.com and it will excute the whole script function against the first one then the second one and i tried the following code.
my_arr=($(echo $DOMAIN | tr "," "\n"))
d=$(for domain in "${my_arr[#]}" ;
do echo $domain
done)
pingme(){
ping -c 4 $d
}
but it hanging and not passing each domain to variable $d
so i need to define array as function and when i run the script it pass domain and execute the script functions then repeat the whole script against the second domain and so on like.
#!/usr/bin/env bash
domain="$1";
function myarray(){
# the array function which will pass the domains one by one
}
function A(){
ping $domain -c 4
}
function B(){
host $domain
}
myarray;
A;
B;
Converting comments into an answer.
The natural way to write shell scripts is to pass the arguments separately:
./script.sh domain1.com domain2.com
and iterate over the elements of "$#" in the script, passing the domain names explicitly to the function (as shown) — global variables are just as problematic in shell scripts as in other programming languages.
#!/usr/bin/env bash
A() { ping "$1" -c 4; }
B() { host "$1"; }
for domain in "$#"
do
A $domain
B $domain
done
If you must use the comma-separated argument, then use:
domains=($(echo "$1" | tr "," "\n"))
pingme() { ping "$1" -c 4; }
for domain in "${domains[#]}"
do
pingme $domain
done
and (as before) the function should process $1, not a global variable.
Inside a function, $1 is the first argument passed to the function, not what's passed to the script as a whole.
Note that in the original code, none of the semicolons is needed. In the code I wrote, the semicolons before the close braces are needed because I lazily wrote the functions on a single line (because they are so simple — I would probably not use functions for such simple commands, especially as they're each only invoked once in the script). If they were spread over three or four lines, the semicolons would not be needed.
A()
{
ping "$1" -c 4
}
Also, you don't need to use the keyword function — and it is generally regarded as a bad idea to do so. As Charles Duffy noted in a comment, the Bash Hackers wiki indicates you should not use function.
There's a getStrings() function that calls getPage() function that returns some html page. That html is piped through egrep and sed combination to get only 3 strings. Then I try to put every string into separate variable link, profile, gallery respectively using while read.. construction. But it works only inside the while...done loop because it runs in subprocess. What should I do to use those variables outside the getStrings() function?
getStrings() {
local i=2
local C=0
getPage $(getPageLink 1 $i) |
egrep *some expression that results in 3 strings* |
while read line; do
if (( (C % 3) == 0 )); then
link=$line
elif (( (C % 3) == 1 )); then
profile=$line
else
gallery=$line
fi
C=$((C+1)) #Counter
done
}
Simple: don't run the loop in a subprocess :)
To actually accomplish that, you can use process substitution.
while read line; do
...
done < <(getPage $(getPageLink 1 $i) | egrep ...)
For the curious, a POSIX-compatible way is to use a named pipe (and its possible that bash uses named pipes to implement process substitution):
mkfifo pipe
getPage $(getPageLink 1 $i) | egrep ... > pipe &
while read line; do
...
done < pipe
Starting in bash 4.2, you can just set the lastpipe option, which causes the last command in a pipeline to run in the current shell, rather than a subshell.
shopt -s lastpipe
getPage $(getPageLink 1 $i) | egrep ... | while read line; do
...
done
However, using a while loop is not the best way to set the three variables. It's easier to just call read three times within a command group, so that they all read from the same stream. In any of the three scenarios above, replace the while loop with
{ read link; read profile; read gallery; }
If you want to be a little more flexible, put the names of the variables you might want to read in an array:
fields=( link profile gallery )
then replace the while loop with this for loop instead:
for var in "${fields[#]}"; do read $var; done
This lets you easily adjust your code, should the pipeline ever return more or fewer lines, by just editing the fields array to have the appropriate field names.
One more solving using array:
getStrings() {
array_3=( `getPage | #some function
egrep | ...` ) #pipe conveyor
}
function get_arguments()
{
read -p 'data : ' data
read -p 'lambda: ' lambda
echo $data $lambda
}
data,lambda=$(get_arguments)
But i am getting an error
data : /home/wolfman/Downloads/data
lambda value: 2
./shell_script.sh: line 25: data,lambda,= /home/wolfman/Downloads/data: No such file or directory
But
1) Why is it even evaluating that whether that file exists or not.. its just a string??
2) what am i doing wrong :(
THanks
sh syntax does not allow that. But, the variables in the function are global, so you can just invoke the function and data and lambda will be set in the caller.
functions return an integer value, but they can print arbitrary data which can be read by the caller. For example, you could do:
get_arguments | { read data lambda; echo $data $lambda; }
The drawback is that the values are only available in that block. (The pipe creates a subshell, and the values read by read are only valid in that subshell.)
Just for fun here are a couple of other possible methods.
read -r data lambda <<< $(get_arguments)
or
set -- $(get_arguments)
data=$1
lambda=$2
shells don't allow direct assignment to lists of variables, you have manage that with shell string parsing (or possibly other methods). Try
data_lambda=$(get_arguments)
data=${data_lambda% *}
#-----------------^^space char
lambda=${data_lambda#* }
#------------------^^space char
$d=123 l=345
$data_lambda=$(echo $d $l)
$echo $data_lambda
123 345
$data=${data_lambda% *}
$lambda=${data_lambda#* }
$echo $data
123
$echo $lambda
345
Substituting $(echo $d $l) for data_lambda=$(get_arguments)`.
See my write-up on shell parameter modifiers
IHTH
I have some set of bash functions which output some information:
find-modelname-in-epson-ppds
find-modelname-in-samsung-ppds
find-modelname-in-hp-ppds
etc ...
I've been writing functions which read output and filter it:
function filter-epson {
find-modelname-in-epson-ppds | sed <bla-blah-blah>
}
function filter-hp {
find-modelname-in-hp-ppds | sed <the same bla-blah-blah>
}
etc ...
But the I thought that it would be better do something like this:
function filter-general {
(somehow get input) | sed <bla-blah-blah>
}
and then call in another high-level functions:
function high-level-func {
# outputs filtered information
find-modelname-in-hp/epson/...-ppds | filter-general
}
How can I achieve that with the best bash practices?
If the question is How do I pass stdin to a bash function?, then the answer is:
Shellscript functions take stdin the ordinary way, as if they were commands or programs. :)
input.txt:
HELLO WORLD
HELLO BOB
NO MATCH
test.sh:
#!/bin/sh
myfunction() {
grep HELLO
}
cat input.txt | myfunction
Output:
hobbes#metalbaby:~/scratch$ ./test.sh
HELLO WORLD
HELLO BOB
Note that command line arguments are ALSO handled in the ordinary way, like this:
test2.sh:
#!/bin/sh
myfunction() {
grep "$1"
}
cat input.txt | myfunction BOB
Output:
hobbes#metalbaby:~/scratch/$ ./test2.sh
HELLO BOB
To be painfully explicit that I'm piping from stdin, I sometimes write
cat - | ...
A very simple means to get stdin into a variable is to use read. By default, it reads file descriptor "0", i.e. stdin i.e., /dev/stdin.
Example Function:
input(){ local in; read in; echo you said $in; }
Example implementation:
echo "Hello World" | input
Result:
you said Hello World
Additional info
You don't need to declare a variable as being local, of course. I just included that for the sake of good form. Plain old read in does what you need.
So you understand how read works, by default it reads data off the given file descriptor (or implicit stdin) and blocks until it encounters a newline. Much of the time, you'll find that will implicitly be attached to your input, even if you weren't aware of it. If you have a function that seems to hang with this mechanism just keep this detail in mind (there are other ways of using read to deal with that...).
More robust solutions
Adding on to the basic example, here's a variation that lets you pass the input via a stdin OR an argument:
input()
{
local in=$1; if [ -z "$in" ]; then read in; fi
echo you said $in
}
With that tweak, you could ALSO call the function like:
input "Hello World"
How about handling an stdin option plus other arguments? Many standard nix utilities, especially those which typically work with stdin/stdout adhere to the common practice of treating a dash - to mean "default", which contextually means either stdin or stdout, so you can follow the convention, and treat an argument specified as - to mean "stdin":
input()
{
local a=$1; if [ "$a" == "-" ]; then read a; fi
local b=$2
echo you said $a $b
}
Call this like:
input "Hello" "World"
or
echo "Hello" | input - "World"
Going even further, there is actually no reason to only limit stdin to being an option for only the first argument! You might create a super flexible function that could use it for any of them...
input()
{
local a=$1; if [ "$a" == "-" ]; then read a; fi
local b=$2; if [ "$b" == "-" ]; then read b; fi
echo you said $a $b
}
Why would you want that? Because you could formulate, and pipe in, whatever argument you might need...
myFunc | input "Hello" -
In this case, I pipe in the 2nd argument using the results of myFunc rather than the only having the option for the first.
Call sed directly. That's it.
function filter-general {
sed <bla-blah-blah>
}
I want to pass an array parameter to a function in bash, and writing some testing code as：
#!/bin/sh
function foo {
a=$1;
for i in ${a[#]} ; do
echo $i
done
}
names=(jim jerry jeff)
foo ${names[#]}
the above code just show jim, rather than the three j*. so my question is:
why my code doesn't work
what's the right way to do it
#!/bin/bash
function foo {
a=($*)
for i in ${a[#]}
do
echo $i
done
}
names=(jim jerry jeff)
foo ${names[#]}
Your code did not show jim to me, but "names", literally. You have to pass the whole array. And you have to recapture it with a=$($).
The manpage part in bash about Arrays is rather long. I only cite one sentence:
Referencing an array variable without a subscript is equivalent to referencing the array with a subscript of 0.
You're fairly close; the biggest problem was the command a=$1, which assigns only the first parameter ($1) to a, while you want to assign the entire list of parameters ($#), and assign it as an array rather than as a string. Other things I corrected: you should use double-quotes around variables whenever you use them to avoid confusion with special characters (e.g. spaces); and start the script with #!/bin/bash, since arrays are a bash extension, not always available in a brand-X shell.
#!/bin/bash
function foo {
a=("$#")
for i in "${a[#]}" ; do
echo "$i"
done
}
names=(jim jerry jeff "jim bob")
foo "${names[#]}"
For example like this:
my_array[0]="jim"
my_array[1]="jerry"
function foo
{
#get the size of the array
n=${#my_array[*]}
for (( Idx = 0; Idx < $n; ++Idx )); do
echo "${my_array[$Idx]}"
done
}