I have this piece of code in Bash,
The goal is to compare two files and if files matches and also no force argument is passed, then exit.
Other way it should continue...
But I got this error [: !=: unexpected operator when I run with no argument and script is not stopped as expected,
When I pass force argument it works OK.
Any Idea please ?
if cmp -s file1 file2 && [ $1 != "-f" ] ; then
"do something"
exit 1
else "do something"
fi
[ ... ] is just a normal command, essentially equivalent to test. As such, if you pass an unquoted empty variable (like $1 when no arguments are provided to your program), it will try to run [ != "-f" ], which gives that error, since you need two sides to compare with !=.
To solve this, you can either use [[ ... ]] (which requires bash, not being POSIX compatible), which does not make unquoted variables "disappear", or you can also quote your variable to keep it POSIX compatible:
cmp -s file1 file2 && [[ $1 != "-f" ]]
cmp -s file1 file2 && [ "$1" != "-f" ]
Related
When I try to execute my script qwer.bash like this:
bash qwer.bash *whatever #whatever can really be anything
it will give back to me:
qwer.bash: line 5: [[: *whatever: syntax error: operand expected (error token is "*whatever")
here is my script:
#!/bin/bash
declare -a files
while [[ "$1" -ne "-p" ]]
do
echo "pwet"
shift
done
How can I avoid producing this error?
This is because the test $1 -ne ... generates an error when$1 is not an integer.
Try this:
while [[ "${#}" > 0 && "${1}" != "-p" ]]; do
echo "pwet"
shift
done
When shift is executed, ${#} decrements.
When "${#}" > 0 is false, the while loop ends. Note that && instructs the shell to not evaluate the second operand ("${1}" != "-p") when the first one is already false.
Though this is irrelevant in your case
Regarding
bash qwer.bash *whatever #whatever can really be anything
Case 1: If *whatever is meant to glob files :
bash qwer.bash ./*whatever
or
bash qwer.bash -- *whatever # here -- marks the end of options
or
./qwer.bash -- *whatever # you already have a shebang in your script.
This is to handle a case where you actually have a file named -file-with-dash in the current folder which may wrongly be taken as an option
Case 2: If *whatever is just a string:
bash qwer.bash \*whatever
or
bash qwer.bash "*whatever"
I have a bash statement to test a command line argument. If the argument passed to the script is "clean", then the script removes all .o files. Otherwise, it builds a program. However, not matter what is passed (if anything), the script still thinks that the argument "clean" is being passed.
#!/bin/bash
if test "`whoami`" != "root" ; then
echo "You must be logged in as root to build (for loopback mounting)"
echo "Enter 'su' or 'sudo bash' to switch to root"
exit
fi
ARG=$1
if [ $ARG == "clean" ] ; then
echo ">>> cleaning up object files..."
rm -r src/*.o
echo ">>> done. "
echo ">>> Press enter to continue..."
read
else
#Builds program
fi
Answer for first version of question
In bash, spaces are important. Replace:
[ $ARG=="clean" ]
With:
[ "$ARG" = "clean" ]
bash interprets $ARG=="clean" as a single-string. If a single-string is placed in a test statement, test returns false if the string is empty and true if it is non-empty. $ARG=="clean" will never be empty. Thus [ $ARG=="clean" ] will always return true.
Second, $ARG should be quoted. Otherwise, if it is empty, then the statement reduces to `[ == "clean" ] which is an error ("unary operator expected").
Third, it is best practices to use lower or mixed case for your local variables. The system uses upper-case shell variables and you don't want to accidentally overwrite one of them.
Lastly, with [...], the POSIX operator for equal, in the string sense, is =. Bash will accept either = or == but = is more portable.
first:
Every string must double quoted or will error absent argument.
second:
for string used only = or != not a == and also -n and -z commands.
third:
you may combine conditions by -a and -o commands but newer used enclose in () yous conditions so not to get error. Logical operators acts through operators presidence, fist calculate -o operator and then -a! For example
[ -n "$1" -a $1 = '-h' -o $1 = '--help' ] && { usage; exit 0; }
will work when passed to script at least 1 argument and is -h or --help. All spaces must be!!! Bush do short cycle logical evaluations. So don't trouble for case when $1 don't exist in second condition because of result of this expression is determined in first one. next don't calculate in this case. But if your argument may contains space symbols you need it double quote. You must do it also in command line too! Else you get error in script or split your arguments in two or more parts.
Operator == isn't used in test. For numbers(not siring) used -eq or -ne commands. See man 1 test for full descriptions. test EXPRESSION... equivalent of [ EXPRESSIONS... ]. More shirt form of test.
I'd like to add an argument to a command in bash only if a variable evaluates to a certain value. For example this works:
test=1
if [ "${test}" == 1 ]; then
ls -la -R
else
ls -R
fi
The problem with this approach is that I have to duplicate ls -R both when test is 1 or if it's something else. I'd prefer if I could write this in one line instead such as this (pseudo code that doesn't work):
ls (if ${test} == 1 then -la) -R
I've tried the following but it doesn't work:
test=1
ls `if [ $test -eq 1 ]; then -la; fi` -R
This gives me the following error:
./test.sh: line 3: -la: command not found
A more idiomatic version of svlasov's answer:
ls $( (( test == 1 )) && printf %s '-la' ) -R
Since echo understands a few options itself, it's safer to use printf %s to make sure that the text to print is not mistaken for an option.
Note that the command substitution must not be quoted here - which is fine in the case at hand, but calls for a more robust approach in general - see below.
However, in general, the more robust approach is to build up arguments in an array and pass it as a whole:
# Build up array of arguments...
args=()
(( test == 1 )) && args+=( '-la' )
args+=( '-R' )
# ... and pass it to `ls`.
ls "${args[#]}"
Update: The OP asks how to conditionally add an additional, variable-based argument to yield ls -R -la "$PWD".
In that case, the array approach is a must: each argument must become its own array element, which is crucial for supporting arguments that may have embedded whitespace:
(( test == 1 )) && args+= ( '-la' "$PWD" ) # Add each argument as its own array element.
As for why your command,
ls `if [ $test -eq 1 ]; then -la; fi` -R
didn't work:
A command between backticks (or its modern, nestable equivalent, $(...)) - a so-called command substitution - is executed just like any other shell command (albeit in a sub-shell) and the whole construct is replaced with the command's stdout output.
Thus, your command tries to execute the string -la, which fails. To send it to stdout, as is needed here, you must use a command such as echo or printf.
Print the argument with echo:
test=1
ls `if [ $test -eq 1 ]; then echo "-la"; fi` -R
I can't say how acceptable this is, but:
test=1
ls ${test:+'-la'} -R
See https://stackoverflow.com/revisions/16753536/1 for a conditional truth table.
Another answer without using eval and using BASH arrays:
myls() { local arr=(ls); [[ $1 -eq 1 ]] && arr+=(-la); arr+=(-R); "${arr[#]}"; }
Use it as:
myls
myls "$test"
This script builds whole command in an array arr and preserves the original order of command options.
Can anyone see what I did wrong here? I keep getting the following error message: [[: not found
read INPUT
if [[ "$INPUT" -ge 1 ]] && [[ "$INPUT" -le 10 ]]; then
Do something
else
printf "Please enter a value between 1 and 10"
fi
[[ is not available in scripts which start with #!/bin/sh, or which are started with sh yourscript. Start your script with #!/bin/bash if you want to use it.
See also http://mywiki.wooledge.org/BashGuide/Practices#Choose_Your_Shell
If you are going to use bash, by the way, there's a better syntax for numeric comparisons:
if (( input >= 1 && input <= 10 )); then ...
Note that lower-case variable names are preferred for local use -- all-upper-case names are reserved for environment variables and shell builtins.
If you're not going to use bash, use the POSIX test operator:
if [ "$input" -ge 1 ] && [ "$input" -le 10 ]; then ...
Note that when using [ ] correct quoting is essential, whereas with [[ ]] it is often superfluous; also, [ ] is missing some extensions such as pattern-matching and regular-expression operators.
It's complicated:
First, there are three separate ways of constructing your if statement. Each way has its own unique syntax on how to join two booleans. (Actually, there are four ways since one way allows you to use list operators).
A little background...
The if command is a compound command built into the shell. The if command executes the commands following the if. If that command returns a zero value, the if statement is considered true and the then clause executes. Otherwise, if it exists, the else clause will execute. Remember, the if is just a command. You can do things like this:
if ! mv "$foo" "$bar"
then
echo "I can't move $foo to $bar"
exit 2
fi
What we need is a command to do some testing for us. If the test succeeds, that test command returns an exit code of zero. If not, it returns a non-zero exit code. Then, it could be used with the if command!
The test command (Yes, there's really one!).
The [ is an alias for the test command which was created to allow you to test files, strings, and numbers for the if statement. (This is now a built in command in Bash, but its roots are actually part of /bin/test and /bin/[). These are the same:
if test "$foo" -eq "$bar"
then
...
fi
and
if [ "$foo" -eq "$bar" ]
then
...
fi
The test command (if you read the manpage has a -a And test and a -o Or test. You could have done:
if [ "$INPUT" -ge 1 -a "$INPUT" -le 10 ]
then
....
fi
This is a single test statement with three test parameters (-ge, -a, and -le).
Using List Operators
This isn't the only way to do a compound boolean test. The Bash shell has two list operators: && and ||. The list operators go in between two commands. If you use && and the left hand command returns a non-zero exit code, the right hand command is not executed, and the entire list returns the exit value of the left-hand command. If you use ||, and the left hand command succeeds, the right hand command is not executed, and the entire list returns a zero exit value. If the first command returns a non-zero exit value, the right-hand command is executed, and the entire list returns the exit value of the right-hand command.
That's why you can do things like this:
[ $bar -eq 0 ] || echo "Bar doesn't have a zero value"!
Since [ ... ] is just a command that returns a zero or non-zero value, we can use these list operators as part of our test:
if [ "$INPUT" -ge 1 ] && [ "$INPUT" -le 10 ]
then
...
fi
Note that this is two separate tests and are separated by a && list operator.
Bash's Special [[ compound command
In Kornshell, Zsh, and Bash, there are special compound commands for testing. These are the double square brackets. They appear to be just like the single square brackets command, but because they're compound commands, parsing is affected.
For example:
foo="This has white space"
bar="" #No value
if [ ! $foo = $bar ] # Doesn't work!
then
The shell expands $foo and $bar and the test will become:
if [ This has white space = ]
which just doesn't work. However,
if [[ $foo != $bar ]]
works fine because of special parsing rules. The double brackets allow you to use parentheses for grouping and && and || as boolean operators. Thus:
if [[ $INPUT -ge 1 && $INPUT -le 10 ]]
then
...
fi
Note that the && appears inside a single set of double square brackets. (Note there's no need for quotation marks)
Mathematical Boolean Expression
Bash has built in mathematical processing including mathematical boolean expressions. If you put something between double parentheses, Bash will evaluate it mathematically:
if (( $INPUT >= 1 && $INPUT <= 10 ))
then
...
fi
In this case, (( $INPUT >= 1 && $INPUT <= 10 )) is evaluated. If $INPUT is between 1 and 10 inclusively, the mathematical expression will evaluate as true (zero exit code), and thus the then clause will be executed.
So, you can:
Use the original test (single square brackets) command and use the -a to string together two boolean statements in a single test.
Use list operators to string together two separate test commands (single square brackets).
Use the newer compound test command (double square brackets) that now include && and || as boolean operators, so you have a single compound test.
Forget about test command and just use mathematical evaluation (double parentheses) to evaluate boolean expressions.
Test Constructs Can Vary by Shell
As has been mentioned in other posts, [[ is a Bash shell keyword that isn't present in the Bourne shell. You can see this from a Bash prompt with:
type '[['
[[ is a shell keyword
In a Bourne shell, you will instead get "command not found."
Be More Portable: Use the -a Test Operator
A more portable construct is to use the -a test operator to join conditions (see man test for details). For example:
if [ "$INPUT" -ge 1 -a "$INPUT" -le 10 ]; then
: # do something when both conditions are true
else
: # do something when either condition is false
fi
This will work in every Bourne-compatible shell I've ever used, and on any system that has a /bin/\[ executable.
if a script has
if [ $1 == "-?" ]; then #line 4
echo "usage: ...."
fi
when the script get runs without any parameter, it will complain that
./script.sh: line 4: [: ==: unary operator expected
but if instead
if [ "$1" == "-?" ]; then #line 4
echo "usage: ...."
fi
then everything's fine
why is that?
thanks
If the first argument is missing or empty, your first script evaluates to:
if [ == "-?" ] ; then
... which is a syntax error. As you noticed, to prevent that you need to make use of "", then it evaluates to:
if [ "" == "-?" ] ; then
AFAIK this is due to the way the original Bourne shell was working. You should make it a habit of enclosing variables in "" to also work correctly with arguments that have spaces in it. For example, if you would call your script like this:
./myScript "first argument has spaces"
Then your first script would evaluate to:
if [ first argument has spaces == "-?" ] ; then
which is also a syntax error. Also things like rm $1 will not do what you want if you pass filenames with spaces. Do rm "$1" instead.
Because [ replaces the values before executing. [[ doesn't, so will work as expected.