This question already has an answer here:
What is the difference between "for i" and "for i in 1 2 3 4" in shell scripting?
(1 answer)
Closed 2 years ago.
Could someone please clarify why the following code is valid in bash and what, if any, is the effect?
for value ; do
echo $value
done
As far as I can tell, bash simply ignores this code, but I do not see why it would not raise a syntax error or something?
As #Jetchisel pointed out, we can run help for in our shell to see the docstring for the for command.
$ help for
for: for NAME [in WORDS ... ] ; do COMMANDS; done
Execute commands for each member in a list.
The `for' loop executes a sequence of commands for each member in a
list of items. If `in WORDS ...;' is not present, then `in "$#"' is
assumed. For each element in WORDS, NAME is set to that element, and
the COMMANDS are executed.
Exit Status:
Returns the status of the last command executed.
The part that I did not know is highlighted below for emphasis:
[...] If `in WORDS ...;' is not present, then `in "$#"' is assumed. [...]
Related
This question already has answers here:
What is the meaning of the ${0##...} syntax with variable, braces and hash character in bash?
(4 answers)
Closed 11 months ago.
I found the following code in a shell script, but I am unsure of what the test condition is evaluating for
if test "${SOME_VAR#*$from asdf/qwer}" != "$SOME_VAR"; then
echo "##zxcv[message text='some text.' status='NORMAL']";
fi
The combination #*$ does not mean anything special. There are three special symbols and each of them has its own meaning in this context.
${SOME_VAR#abc} is a parameter expansion. Its value is the value of $SOME_VAR with the shortest prefix that matches abc removed.
In your example, abc is *${from} asdf/qwer. That means anything * followed by the value of variable $from (which is dynamic and replaced when the expression is evaluated), followed by a space and followed by asdf/qwer.
All in all, if the value of $SOME_VAR starts with a string that ends in ${from} asdf/qwer then everything before and including asdf/qwer is removed and the resulting value is passed as the first argument to test.
Type man bash in your terminal to read the documentation of bash or read it online.
This question already has answers here:
Linux/Unix 'man' page syntax conventions
(2 answers)
What does "[]" or "<>" mean in some command document? [duplicate]
(1 answer)
Closed 1 year ago.
How do I interpret square brackets? For example:
The syntax of the 'case' command is:
case WORD in
[ [(] PATTERN [| PATTERN]...) COMMAND-LIST ;;]...
esac
I understand how to do the loop but when asking for help from bash documentation I did not know how to interpret the brackets. I mean what the hell does it mean, [(]?
The syntax of bash compound commands is defined using the man page synopsis conventions. These are defined in man(1):
The following conventions apply to the SYNOPSIS section and can be used
as a guide in other sections.
bold text type exactly as shown.
italic text replace with appropriate argument.
[-abc] any or all arguments within [ ] are optional.
-a|-b options delimited by | cannot be used
together.
argument ... argument is repeatable.
[expression] ... entire expression within [ ] is repeatable.
This question already has answers here:
How do I pass in the asterisk character '*' in bash as arguments to my C program?
(5 answers)
The issue of * in Command line argument
(6 answers)
Closed 5 years ago.
How do I write a bash script/function that will take a command line argument with spaces and interpret it as if it had quotes around it.
ie: echo this is some text as if echo "this is some text"
What I want to do is create a simple CLI calculator script/function.
Here is what I have:
calc() {
echo $(($#))
}
On the CLI, all of these work:
"# +-/* ", #+-/*, # +-/ #
ie:
calc 10+2
12
but this one produces an error:
calc 10 * 2
-bash: 10 calc.sh 2: syntax error: invalid arithmetic operator (error token is ".sh 2")
Any ideas? It's not a big deal to include the quotes in the calculations, but if possible it would be quicker/more convenient to not include them. Is there any way to take a string after calc from first character to last and then pass it through quotes in the script?
In unix systems there is a variable called IFS. This variable decides at what signs the console separate the string into words. In theory you could chang the IFS variable, but this is really not advisable (as it will breake other bash commands).
Better: you could write your utillity in such way, that writing 'calc' will prompt the user to type his equation to standard in. Then you can read the text the user types and parse it in any way you want.
Even better: a shell script can specify that it wishes for an arbitrary nimber of words as input. Take all the words the user writes and then parse that. See: How to define a shell script with variable number of arguments?
This question already has answers here:
Command not found error in Bash variable assignment
(5 answers)
Closed 5 years ago.
I am trying to find te longest word in a given file. Before I check the lengtgh of each word I need to remove all of the following tokens {,.:} that may be attached (once or more) to the word. so for example, for this text:
:,cat dog, encyclopedia; remove:.,
i need the result:
cat dog encyclopedia remove
I am trying this, but I get a "command not found":
longest=0
for word in $(<$1)
do
#new_word = $(echo "${word//[.,:]/}")
new_word = "${word//[.,:]/}"
len=${#new_word}
if (( len > longest ))
then
longest=$len
longword=$new_word
fi
done
echo The longest word is $longword and its length is $longest.
thank you.
Your use of parameter expansion replacement pattern is correct.
The problem is that there must not be any whitespace around = while declaring variables in bash (any shell in general).
So, the following should work:
new_word="${word//[.,:]/}"
As an aside, use a while read ... construct to loop over the lines in a file, using for is pretty fragile.
Here's the code snippet from a shell script. (It's from MPFR library's configure script and it starts with #!/bin/sh. The original script is over 17000 lines long.. It's used when building gcc.)
Because I have so many questions in a short piece of code, I have embedded my questions in the code. Please can somebody explain to me why the code is like this? Also, though I have a vague idea, I would appreciate if someone could explain what this code is doing (I understand it will be difficult because it's only a part of a big script).
if { { ac_try="$ac_link"
# <---- question 1 : why is the first curly bracket used for if condition? (probably just for grouping and using the last return code)
# <---- question 2 : Is this second bracket for locally used code(probably)?
case "(($ac_try" in # <---- question 3 : what is this "((" symbol?
*\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
*) ac_try_echo=$ac_try;;
esac
eval ac_try_echo="\"\$as_me:${as_lineno-$LINENO}: $ac_try_echo\""
$as_echo "$ac_try_echo"; } >&5 # <---- question 4 : what is this >&5 redirection? I know >&{1,2,3} but not 5.
(eval "$ac_link") 2>&5
# <----- question 5 : why use sub-shell here? not to use eval result?
ac_status=$?
$as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
test $ac_status = 0; }; then : # <---- question 6 : is this ':'(nop) here ?
....
some commands
....
else
....
some commands
....
fi
From Bash man page:
{ list; }
list is simply executed in the current shell environment. list
must be terminated with a newline or semicolon. This is known
as a group command. The return status is the exit status of
list. Note that unlike the metacharacters ( and ), { and } are
reserved words and must occur where a reserved word is permitted
to be recognized. Since they do not cause a word break, they
must be separated from list by whitespace or another shell
metacharacter.
{} is just to list a few commands to run, very much like cmd1; cmd2; cmd3. For example, if you write cmd1 ; cmd2 | cmd3, do you mean {cmd1; cmd2;} | cmd3 or cmd1; {cmd2 | cmd3;}.
{{ }} is just nested command list, easy: e.g. {cmd1; cmd2; {cmd3; cmd4;}; }
For question 3, (( is just in a source string to be matched with the following patterns. If you are asking why it is used, we need possible values of $ac_try to analyze why. Honestly, I don't see many shell scripts purposely adding (( in front of a source string to be matched for patterns.
For question 4,
>&5: if file descriptor 5 is not yet created (i.e. mentioned in any part of the script... => be careful, you need to care the scope, some codes runs in sub-shell, which is counted as a sub-shell context/scope), create an unnamed file (well, temp file, if you like), with descriptor 5. This file can be used in other part of the script as an input.
For example, see the part mentioning "exchanges STDIN and STDOUT" in my answer to another question here.
For question 5, the eval, I am not quite sure, just a quick guess (and it depends on what command it evals) by providing you an example why sub-shell makes some differences:
cmd="Foo=1; ls"
(eval $cmd) # this command runs in sub-shell and thus $Foo in current shell will not be changed.
eval $cmd # this command runs in current shell and thus $Foo is changed, and it will affect all subsequent commands.
For question 6, look carefully at the man page I mentioned at top of the answer, the {} list syntax, require a final ;. i.e. {cmd1; cmd2 ; } The last ; is required.
--- UPDATE ---
Question 6: Sorry for not seeing the colon... :-)
It's no op: see this link.