I'm totally new in writing shell scripts so I could use some help here.
I would like to write a script that when run with no parameters it just echo backs, and when it is given a data (.dat) file it displays the content of it.
Excuse me for my bad English,
R.
This script, when run with no parameters it just echo backs and when a filename is passed as argument, it displays the content of it:
#!/bin/sh
# Explanation - We use'$#' to count number of arguments.
if ! [ $# -gt 0 ]; then
# Explanation - Zeroth argument '$0' is scriptname itself. Print it.
cat "$0"
else
# Explanation - Print (cat) 1st argument.
cat "$1"
fi
NOTE: Since you've used 'minix' tag, I tested it on minix3. The script works well on minix as well as linux.
Related
I am creating a script (myscript.sh) in BASH that reads from STDOUT, typically a stream of data that comes from cat, or from a file and outputs the stream of data (amazing!), like this:
$cat myfile.txt
hello world!
$cat myfile.txt | myscript.sh
hello world!
$myscript.sh myfile.txt
hello world!
But I also would like the following behaviour: if I call the script without arguments I'd like it to output a brief help:
$myscript.sh
I am the help: I just print what you say.
== THE PROBLEM ==
The problem is that I am capturing the stream of data like this:
if [[ $# -eq 0 ]]; then
stream=$(cat <&0)
elif [[ -n "$stream" ]]; then
echo "I am the help: I just print what you say."
else
echo "Unknown error."
fi
And when I call the script with no arguments like this:
$myscript.sh
It SHOULD print the "help" part, but it just keep waiting for a stream of data in line 2 of code above...
Is there any way to tell bash that if nothing comes from STDOUT just break and continue executing?
Thanks in advance.
There's always a standard input stream; if no arguments are given and input isn't redirected, standard input is the terminal.
If you want to treat that specially, use test -t to test if standard input is connected to a terminal.
if [[ $# -eq 0 && -t 0 ]]; then
echo "I am the help: I just print what you say."
else
stream=$(cat -- "$#")
fi
There's no need to test $#. Just pass your arguments to cat; if it gets filenames it will read from them, otherwise it will read from standard input.
I agree to #Barmar's solution.
However, it might be better to entirely avoid a situation where your program behavior depends on whether the input file descriptor is a terminal (there are situations where a terminal is mimicked even though there's none -- in such a situation, your script would just produce the help string).
You could instead introduce a special - argument to explicitly request reading from stdin. This will result in simpler option handling and uniform behavior of your script, no matter what's the environment.
First answer is to help yourself - try running the script with bash -x myscript.sh. It will include lot of information to help you.
If you specific case, the condition $# -eq 0 was flipped. As per requirement, you want to print the help message is NOT ARGUMENT ARE PROVIDED:
if [[ $# -eq 0 ]] ; then
echo "I am the help: I just print what you say."
exit 0
fi
# Rest of you script, read data from file, etc.
cat -- "$#"
Assuming this approach is taken, and if you want to process standard input or a file, simple pass '-' as parameter: cat foobar.txt | myscript.sh -
What I'm attempting to do is receive values from the command line (instead of using the read method and asking the user to enter the values and/or file names in multiple steps).
./hello.sh 5 15 <file_name.txt
I have heard that simply using an array can help do the same, but I am not able to-
Avoid printing
5 15
on the next line
Since 5 and 15 are being printed, I'd expect the string 'abcdefgh' (contents of file_name.txt) to be printed; however, the output stops at
5 15
I would really appreciate it if someone could point out why my code isn't sufficient, and if possible, point me in the direction of some learning resources to broaden my knowledge of this concept.
Here is the code:
#! /usr/bin/bash
echo "$#"
I am simply testing things out (wanted to print out the variables before doing anything with and to them).
<file_name.txt is a redirection. It is not passed as a parameter. The parameters of the script are 5 and 15. The < redirects the file file_name.txt to standard input stdin of the script. You can read from stdin with for example cat.
#!/usr/bin/bash
echo "$#" # outputs parameters of the script joined with spaces
cat # redirects standard input to standard output, i.e. reads from the fiel
why my code isn't sufficient
Your script is not reading from the file, so the content of the file is ignored.
point me in the direction of some learning resources
File descriptors and redirections and standard streams are basic tools in shell - you should learn about them in any shell and linux introduction. My 5 min google search resulted in this link https://www.digitalocean.com/community/tutorials/an-introduction-to-linux-i-o-redirection , which looks like some introduction to the topic.
Will this work?
./hello.sh 5 15 `catfile_name.txt`
And update hello.sh to:
#! /usr/bin/bash
shift 2
echo $#
Here is a more generic solution. It looks at each input parameter in turn. If it is a valid file, it outputs the contents of the file. Otherwise if just prints the parameter.
#! /usr/bin/bash
for $parameter in "${#}"; do # Quotation marks avoid splitting parameters with spaces.
if [ -f $parameter ]; then # '-f {value}' tests if {value} is a file.
cat $parameter
else
echo $parameter # You could also use 'echo -n ...' to skip newlines.
fi
done
I need to read command line arguments. First arg is script name. second one is redirection operator i.e. "<" and third one is input filename. When I tried to use "$#", I got 0. When I used "$*", it gave me nothing. I have to use "<" this operator. My input file consists of all user input data. If I don't use the operator, It asks user for the input. Can someone please help me? Thank you !
Command Line :
./script_name < input_file
Script:
echo "$*" # gave nothing
echo "$#" # gave me 0
I need to read input filename and store it to some variable. Then I have to change the extension of it. Any help/suggestions should be appreciated.
When a user runs:
./script_name <input_file
...that's exactly equivalent to if they did the following:
(exec <input_file; exec ./script_name)
...first redirecting stdin from input_file, then invoking the script named ./script_name without any arguments.
There are operating-system-specific interfaces you can use to get the filename associated with a handle (when it has one), but to use one of these would make your script only able to run on an operating system providing that interface; it's not worth it.
# very, very linux-specific, won't work for "cat foo | ./yourscript", generally evil
if filename=$(readlink /proc/self/fd/0) && [[ -e $filename ]]; then
set -- "$#" "$filename" # append filename to the end of the argument list
fi
If you want to avoid prompting for input when an argument is given, and to have the filename of that argument, then don't take it on stdin but as an argument, and do the redirection yourself within the script:
#!/bin/bash
if [[ $1 ]]; then
exec <"$1" # this redirects your stdin to come from the file
fi
# ...put other logic here...
...and have users invoke your script as:
./script_name input_file
Just as ./yourscript <filename runs yourscript with the contents of filename on its standard input, a script invoked with ./yourscript filename which invokes exec <"$1" will have the contents of filename on its stdin after executing that command.
< is used for input redirection. And whatever is at the right side of < is NOT a command line argument.
So, when you do ./script_name < input_file , there will be zero (0) command line arguments passed to the script, hence $# will be zero.
For your puprpose you need to call your script as:
./script_name input_file
And in your script you can change the extension with something like:
mv -- "$1" "${1}_new_extension"
Edit: This was not what OP wanted to do.
Altough, there is already another spot on answer, I will write this for the sake of completeness. If you have to use the '<' redirection you can do something like this in your script.
while read filename; do
mv -- "$filename" "${filename}_bak"
done
And call the script as, ./script < input_file. However, note that you will not be able to take inputs from stdin in this case.
Unfortunately, if you're hoping to take redirection operators as arguments to your script, you're not going to be able to do that without surrounding your command line arguments in quotes:
./script_name "<input_file"
The reason for this is that the shell (at least bash or zsh) processes the command before ever invoking your script. When the shell interprets your command, it reads:
[shell command (./script_name)][shell input redirection (<input_file)]
invoking your script with quotes effectively results in:
[shell command (./script_name)][script argument ("<input_file")]
Sorry this is a few years late; hopefully someone will find this useful.
Recently i got an assignment at school, where we are to write a small program in Bash Scripting Language.
This shell script is supposed to accept some Positional Parameters or Arguments on the command line.
Now, with the help of an if-else statement i check if the argument is present, if the argument is present the script does what it is supposed to do, but if the argument is not present - i display an error message, prompting the user to input an argument and pass the argument again to the same shell script...
Now, i want to know if this approach is good or bad in Bash Programming paradigm. I'm slightly suspicious that this might run too many tasks in the background that are kept open or that are never ended and keep on consuming memory... please help.
Here's a small snippet (assume the script name to be script1.bash):
#!/bin/bash
if [ $# -gt 0 ]; then
read -p "Please enter your name: " name
script1.bash $name
elif [ $# -eq 1 ]; then
echo "Your name is $1!"
fi
It's ... questionable :-)
The main problem is that you're spawning a subshell everytime someone refuses to input their name.
You would be better of with something like:
#!/bin/bash
name=$1
while [[ -z "$name" ]] ; do
read -p "Please enter your name: " name
done
echo "Your name is $name!"
which spawns no subshells.
I have a shell script where a user passes different sizes as command line arguments. For each of those sizes, I want to perform some task, then save the output of that task into a .txt file with the size in the name.
How can I take the command line passed and make that part of a stringname for the output file? I save the output file in a directory specified by another command line argument. Perhaps an example will clear it up.
In the foor lop, the i value represents the command line argument I need to use, but $$i doesnt work.
./runMe arg1 arg2 outputDir [size1 size2 size3...]
for ((i=4; i<$#; i++ ))
do
ping -s $$i google.com >> $outputDir/$$iresults.txt
done
I need to know how to build the $outputDir/$$iresults.txt string. Also, the ping -s $$i doesnt work. Its like I need two levels of replacement. I need to replace the $[$i] inner $i with the value in the loop, like 4 for ex, making it $4. Then replace $4 with the 4th command line argument!
Any help would be greatly appreciated.
Thanks!
Indirection uses the ! substitution prefix:
echo "${!i}"
But you should be using a bare for after shifting the earlier ones out:
shift
shift
for f
do
echo "$f"
done
for ARG in $#; do
$COMMAND > ${ARG}.txt
done