My whole script is currently this:
#!/bin/sh
clear;
blanko="";
# Dummy-Variablen
variable=Testvariable;
if [[$variable == $blanko]];
then
echo "Nichts da!"
else
echo $variable
fi
and if I enter
TestSelect.sh
I get
/usr/bin/TestSelect.sh: line 6: [[Testvariable: command not found
Testvariable
How can I fix this?
This is problem:
if [[$variable == $blanko]];
Spaces are required inside square brackets, use it like this:
[[ "$variable" == "$blanko" ]] && echo "Nichts da!" || echo "$variable"
On a related note, spaces are required around [ ] as well:
if [ "$variable" = "$blanko" ]; then
# more code here
fi
Note that variables do need to be enclosed in double quotes inside [ ] to prevent word splitting and globbing. Double quotes also help when either of the variables being compared is not set - shell will throw a syntax error otherwise.
Look at the following post to understand why we need spaces around [ ]:
Why should there be a space after '[' and before ']' in Bash?
Another related post that talks about other syntax elements that need spaces as well:
Why is whitespace sometimes needed around metacharacters?
Finally, this post talks about the difference between [[ ]] and [ ]:
Difference between single and double square brackets in Bash
Related:
“0: command not found” in Bash
Just use #!/bin/bash on tope of script if you are using bash scripting like: if [[ $partition == "/dev/sda2" ]]; then to compare string and run script with ./scriptname.sh or bash scriptname.sh
If your script runs on your local with /bin/bash but not on your container with sh, then consider adding bash to your container by apk add --no-cache bash.
Related
My whole script is currently this:
#!/bin/sh
clear;
blanko="";
# Dummy-Variablen
variable=Testvariable;
if [[$variable == $blanko]];
then
echo "Nichts da!"
else
echo $variable
fi
and if I enter
TestSelect.sh
I get
/usr/bin/TestSelect.sh: line 6: [[Testvariable: command not found
Testvariable
How can I fix this?
This is problem:
if [[$variable == $blanko]];
Spaces are required inside square brackets, use it like this:
[[ "$variable" == "$blanko" ]] && echo "Nichts da!" || echo "$variable"
On a related note, spaces are required around [ ] as well:
if [ "$variable" = "$blanko" ]; then
# more code here
fi
Note that variables do need to be enclosed in double quotes inside [ ] to prevent word splitting and globbing. Double quotes also help when either of the variables being compared is not set - shell will throw a syntax error otherwise.
Look at the following post to understand why we need spaces around [ ]:
Why should there be a space after '[' and before ']' in Bash?
Another related post that talks about other syntax elements that need spaces as well:
Why is whitespace sometimes needed around metacharacters?
Finally, this post talks about the difference between [[ ]] and [ ]:
Difference between single and double square brackets in Bash
Related:
“0: command not found” in Bash
Just use #!/bin/bash on tope of script if you are using bash scripting like: if [[ $partition == "/dev/sda2" ]]; then to compare string and run script with ./scriptname.sh or bash scriptname.sh
If your script runs on your local with /bin/bash but not on your container with sh, then consider adding bash to your container by apk add --no-cache bash.
when I read the book linux shell scripting cookbook
they say when you wanna print !,you shouldn't put it in double quote，or you can add \ before ! to escape it.
e.g.
$echo "Hello,world!"
bash: !:event not found error
$echo "Hello,world\\!"
Hello,world!
but in my situation(ubuntu14.04), I get the answer like that:
$echo "Hello,world!"
Hello,world!
$echo "Hello,world\\!"
Hello,world\!
So, why in my machine can't get the same answer?
Why the escape symbol \ was printed as a normal symbol?
When you're typing interactively to the shell, ! has special meaning, it's the history expansion character. To prevent this special meaning, you need to put it in single quotes or escape it.
echo 'Hello, world!'
echo "Hello, world\!'
The reason it's not happening on Ubuntu may be because it's running a newer version of bash, which is apparently more selective about when history expansion occurs. It seems to require ! to be followed by alphanumerics, not punctuation.
You don't need to do this in scripts, because history is not normally enabled there. It's just for interactive shells.
Create a shell script called file.sh:
#!/bin/bash
# file.sh: a sample shell script to demonstrate the concept of Bash shell functions
# define usage function
usage(){
echo "Usage: $0 filename"
exit 1
}
# define is_file_exits function
# $f -> store argument passed to the script
is_file_exits(){
local f="$1"
[[ -f "$f" ]] && return 0 || return 1
}
# invoke usage
# call usage() function if filename not supplied
[[ $# -eq 0 ]] && usage
# Invoke is_file_exits
if ( is_file_exits "$1" )
then
echo "File found"
else
echo "File not found"
fi
Run it as follows:
chmod +x file.sh
./file.sh
./file.sh /etc/resolv.conf
In an attempt to stay consistent i have tried to use double brackets [[ ]] in all my if statements. I did however get into a problem when i was going to check the return value for a command i wanted to run. After testing several ways of creating an if statement i found that only without brackets could i execute a command.
The following does not work:
if [[ $command ]] ; then
echo "something"
fi
if [[ $(command) ]] ; then
echo "something"
fi
if [[ ${command} ]] ; then
echo "something"
fi
and the code above makes the if loop true even when the command was not run.
since the code above doesnt work with braces it doesnt work to use this either:
[[ $command ]] || echo "failed"
and it doesnt work in a subshell either.
The following works:
if $command ; then
echo "something"
fi
if $(command) ; then
echo "something"
fi
Why doesnt it work to place a command in an if loop with brackets, and why does the if loops above report true when it didnt even run the command ? I'm using bash version 4.1.9. Ive tried this many times and the if loops are just as simple as the ones i typed above, it just checks if a command was run successfully and exits if it wasnt.
The short answer is:
[ and [[ expect an expression.
if expects a command.
Saying:
[[ $(command) ]]
would essentially execute:
[[ -n <command_output> ]]
which may or may not be what you want. On the other hand, saying:
$command && echo something || echo other
would echo something or other based on the return code of the command (0 and non-zero respectively).
Double braces are a shortcut for test. In your examples, what's happening is that you're testing the shell variable $command for existence.
if [[ $PWD ]]; then
echo PWD is set to a value
fi
if [[ $NOT_A_REAL_VAR ]]; then
echo Nope, its not set
fi
In your second example, you're using command substitution to check that command output something on standard output.
if [[ $(echo hi) ]]; then
echo "echo said hi'
fi
if [[ $(true) ]]; then #true is a program that just quits with successful exit status
echo "This shouldn't execute"
fi
Your third example is the same as your first, pretty much. You use the curly braces if you want to group your variables. for example if you want to put an 's' after something.
WORD=Bike
echo "$WORDS" #won't work because "WORDS" isn't a variable
echo "${WORD}S" # will output "BikeS"
Then in your fifth example, you are running the program that is sitting inside command.
So, if you want to test some strings, use [[ ]] or [ ]. If you just want to test the exit status of a program, then don't use those, just use a bare if.
Check man test for details on the braces.
If you're just checking the return value of the command, drop the double brackets.
if $command
then
echo "Command succeeded"
else
echo "Command failed: $!"
fi
The double brackets are a test command. (Well, not really, but their a takeoff of the single square brackets that were an alias to the test command.) In early Bourne shell, you would see things like:
if test -z "$string"
then
echo "This is an empty string"
fi
The square brackets were syntactic sugar:
if [ -z "$string" ]
then
echo "This is an empty string"
fi
So, if you're not doing an actual test, you can eliminate the double or single square brackets.
If you're using square brackets, you should use the double ones and not the single ones because the double ones are a bit more forgiving and can do a bit more:
if [ -z $string ] # No quotes: This will actually fail if string is zero bytes!
if [[ -z $string ]] # This will work despite the lack of quotes
I have the following bash script file callee.sh which is being called from another script file caller.sh.
The callee.sh is as follows:
if [ $1 -eq 1 ];
then
echo inside $1
source ~/MYPROGRAMSRC/standAloneWordCount.sh $2
#echo "inside standalone branch"
#echo $1
elif [ $1 -eq 2 ];
then
#echo "inside distributed branch"
#echo $1
else
echo invalid option for first argument-\n Options:\n "distributed"\n or\n "standalone"\n
fi
As most people might be able to tell, this is a script I use to decide whether to run hadoop in distributed or standAlone mode depending on the arguments.
This script is called from caller.sh as follows
source callee.sh $2 $counterGlobal
where $2 is a number either 1 or 2 and $counterGlobal is some integer.
My problem is that the if condition in callee.sh never evaluates to True and hence my script standAloneWordCount.sh which I call from within callee.sh is never called. I am running with bash shell and have tried many variants of the if statement like:
if [ $(($1 == 1 )) ] -- (1)
In an echo statement just above the line -- (1) , the expression $(($1 == 1)) evaluates to 1 so I am baffled as to why I am unable to satisfy the if condition.
Also I keep getting the error where it says:
syntax error near unexpected token `else'
if anyone could help me out with these two errors, it would be much appreciated. As I've run out of ideas.
Thanks in advance!
have tried many variants of the if statement like:
if [ $(($1 == 1 )) ]
You should instead be saying:
if (($1 == 1)); then
...
fi
Regarding the Syntax error near unexpected tokenelse'`, it's not because of any code that you've shown above. It seems to originate from some other portion of your script.
If you're using bash, try using double square brackets:
if [[ $1 -eq 1 ]]; then
echo "inside 1"
fi
As for the syntax error, you need quotes around your text (which also means escaping the existing quotes or use single quotes):
echo -e "invalid option for first argument-\n Options:\n \"distributed\"\n or\n \"standalone\"\n"
The -e flag is there to let bash know you want the \n to evaluate to a newline.
My whole script is currently this:
#!/bin/sh
clear;
blanko="";
# Dummy-Variablen
variable=Testvariable;
if [[$variable == $blanko]];
then
echo "Nichts da!"
else
echo $variable
fi
and if I enter
TestSelect.sh
I get
/usr/bin/TestSelect.sh: line 6: [[Testvariable: command not found
Testvariable
How can I fix this?
This is problem:
if [[$variable == $blanko]];
Spaces are required inside square brackets, use it like this:
[[ "$variable" == "$blanko" ]] && echo "Nichts da!" || echo "$variable"
On a related note, spaces are required around [ ] as well:
if [ "$variable" = "$blanko" ]; then
# more code here
fi
Note that variables do need to be enclosed in double quotes inside [ ] to prevent word splitting and globbing. Double quotes also help when either of the variables being compared is not set - shell will throw a syntax error otherwise.
Look at the following post to understand why we need spaces around [ ]:
Why should there be a space after '[' and before ']' in Bash?
Another related post that talks about other syntax elements that need spaces as well:
Why is whitespace sometimes needed around metacharacters?
Finally, this post talks about the difference between [[ ]] and [ ]:
Difference between single and double square brackets in Bash
Related:
“0: command not found” in Bash
Just use #!/bin/bash on tope of script if you are using bash scripting like: if [[ $partition == "/dev/sda2" ]]; then to compare string and run script with ./scriptname.sh or bash scriptname.sh
If your script runs on your local with /bin/bash but not on your container with sh, then consider adding bash to your container by apk add --no-cache bash.