Suppose I want to compare gcc version to see whether the system has the minimum version installed or not. To check the gcc version, I executed the following gcc --version | head -n1 | cut -d" " -f4 The output was 4.8.5 So, I wrote a simple if statement to check this version against some other value if [ "$(gcc --version | head -n1 | cut -d" " -f4)" -lt 5.0.0 ]; then echo "Less than 5.0.0" else echo "Greater than 5.0.0" fi But it throws an error: [: integer expression expected: 4.8.5 I understood my mistake that I was using strings to compare and the -lt requires integer. So, is there any other way to compare the versions?

Continuing https://unix.stackexchange.com/questions/48805/semicolon-in-conditional-structures (which handles single brackets), what's the point of having a semicolon after the closing DOUBLE bracket ]]? In my tests, running

#!/bin/zsh --
if [[ "a" == "a" ]] then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "a" ]]; then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "a" ]]
then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "a" ]];
then
	echo "true"
else
	echo "false"
fi

yields

true
true
true
true

, and running

#!/bin/zsh --
if [[ "a" == "b" ]] then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "b" ]]; then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "b" ]]
then
	echo "true"
else
	echo "false"
fi

if [[ "a" == "b" ]];
then
	echo "true"
else
	echo "false"
fi

yields

false
false
false
false

No error is reported. In zsh, what's the difference between the conditionals with a semicolon ; after the closing double bracket and the conditionals without a semicolon after the closing double bracket? The same question goes for bash.

When I run the following code

declare -A X # get an associative array
index="a'b" # a somewhat weird index
X[$index]="this is set"
echo ">"
if test -v X[$index]; then # behaves in an unexpected way
  echo indeed, this is set
else
  echo no, it is not
fi

the result is no, it is not in my bash version 5.1.16(1). As soon as i remove the single quote from the index, it works as expected, telling that indeed, this is set. From the bash manual : > Indexed arrays are referenced using integers [...]; associative arrays use arbitrary strings. Given that I can properly use the index to set and retrieve the value, I wonder if this is a bug in test -v or if there is some explanation why this does not work or whether different quoting can get test -v to work. (I could use test -z in this particular case, but the question is about -v).

*Extremely "noob" question:* I have a running sudo memtester ... on an Ubuntu 22.04 machine, but I gave it too much memory to test and it's taking [too much time](https://unix.stackexchange.com/q/789492/515980) (I see it running and updating, I made sure the RAM given to test was less than the free one). I'd like to stop it. I was thinking of trying with a simple Ctrl-c, but I wonder if that would cause some sort of lock-up, forcing me to reboot. Unfortunately the [man pages](https://man.archlinux.org/man/memtester.8.en) don't say anything about how to stop it.

In bash, the conditional expression with the unary test -v myvariable tests wether the variable myvariable has been set. Note that myvariable should not be expanded by prefixing it with a dollar, so **not** $myvariable. Now I find that for array elements the conditional expression -v myarray[index] works well too, without the full expansion syntax ${myarray[$index]}. Try this: myarray=myvalue for i in 1 2 3 do [ -v myarray\[i] ] && echo element $i is set done (note the escape \[ to prevent globbing, as alternative to using quotes) gives the desired output: element 2 is set **Question** Is this behaviour safe to use *aka* is this documented behaviour? **Addendum** After reading the answer https://unix.stackexchange.com/a/677920/376817 of Stéphane Chazelas, I expanded my example: myarray=val myarray=val myarray=val myarray=val myarray=val myarray="" myarray="" unset myarray myarray myarray touch myarray4 myarrayi myarray4=val myarrayi=val then for i in {0..7}; do [ -v myarray\[i] ] && echo element $i is set; done gives element 1 is set element 2 is set element 6 is set Without quoting or escaping the index expression [i] : for i in {0..7}; do [ -v myarray[i] ] && echo element $i is set; done gives element 0 is set element 1 is set element 2 is set element 3 is set element 4 is set element 5 is set element 6 is set element 7 is set The same with the variable myarrayi unset : unset myarrayi for i in {0..7}; do [ -v myarray[i] ] && echo element $i is set; done gives %nothing% And finally with expansion of the index as $i (still without quoting or escaping the bracket) : for i in {0..7}; do [ -v myarray[$i] ] && echo element $i is set; done it gives element 1 is set element 2 is set element 4 is set because ls -l myarray* shows -rw-rw-r-- 1 me us 0 nov 17 15:37 myarray4 -rw-rw-r-- 1 me us 0 nov 17 15:37 myarrayi

I'm trying to check if the value inside a file is "0".

COUNT_EXECUTION=$(< /tmp/count)

if [ "$COUNT_EXECUTION" == "0" ]; then
  echo "Try restart service."
else
  echo "There is execution in progress."
fi

File content is just a 0, but the test always falls into the else

I am a little bit confused on what do these operators do differently when used in bash (brackets, double brackets, parenthesis and double parenthesis). [[ , [ , ( , (( I have seen people use them on if statements like this: if [[ condition ]] if [ condition ] if ((condition)) if (condition)

I have a shell script where we have following lines if [ -z "$xyz" ] and if [ -n "$abc" ], but I am not sure what their purpose is. Can anyone please explain?

I want to test if the output of a function matches the text I envision it should write to standard output. But the code below never prints

PASS

and I can't figure out why. I also tried putting various

\n

characters or deleting them in the variable

but that didn't help.

error_out()
{
    printf "%s\n" "${ERROR}"
    printf "Return to exit\n"
    read -r THROW_AWAY
    exit
}

ERROR="Test output."
OUTPUT_TO_TEST=$(error_out)
GROUND_TRUTH="Test output.\nReturn to exit\n"
{ [ "${OUTPUT_TO_TEST}" = "${GROUND_TRUTH}" ] \
         && printf "error_out test PASS\n"; } \
    || { \
        printf "error_out test FAIL\n"; \
    }

Here's what I'm trying to do (in a script):

#!/usr/bin/env bash

if [[ ! $("/usr/bin/scp seamus@fumier.local:/Users/seamus/Downloads/imgutils/image-utils* /home/pi/testscp") ]]; then
    printf "\nERROR & EXIT: 'scp' failed \n"
    exit 1
fi
printf "success!\n"

I've tried doing this in a few ways, but nothing seems to work. Here's what I get from the above:

./testscp.sh: line 3: /usr/bin/scp seamus@fumier.local:/Users/seamus/Downloads/imgutils/image-utils* /home/pi/testscp: No such file or directory

ERROR & EXIT: 'scp' failed

The scp command runs OK when it's on a line by itself. If I do that, I can test for $0 - which works OK... but I'd prefer to do it all in a single line. What am I missing?

I see there is an executable called "[" in /usr/bin. What is its purpose?

The script is called **isFile.sh** and looks like this:

#!/bin/sh

echo $1
echo $2

if [ ! -f $1 ]; then
  echo "$1 (arg1) is not a file"
fi

if [ ! -f $2 ]; then
  echo "$2 (arg2) is not a file"
fi

First I created a file by doing touch file.exist. And I ran bash isFile.sh file.exist file.notexist The output was: >file.exist > >file.notexist > >file.notexist (arg2) is not a file ---------------------------------------------------- Then I ran bash isFile.sh "" file.notexist The output was: >(# empty line) > >file.notexist > >file.notexist (arg2) is not a file Expected output is: >(# empty line) > >file.notexist > >(arg1) is not a file > >file.notexist (arg2) is not a file Can somebody explain why?

I'm currently building Linux From Scratch 12.1 with the book, I've followed the instructions line by line. I'm struggling with the GCC testsuite, I have lots of FAIL and timeout. In addition to that, I'm running a Lubuntu VM with an LFS.vdi where I'm building LFS, during the test it is being filled to the point where the tests can't continue. My VM runs lubuntu 64bit with 4proc and 10Gb RAM. I have a 30Go drive dedicated to LFS. I've tried this for the "not enough space" problem but it didn't work: https://superuser.com/questions/529149/how-to-compact-virtualboxs-vdi-file-size I don't really know where to look, any help would be greatly appreciated. If I can access some helpful logs, let me know. Thanks in advance

I'm writing a script which will have some arguments and so I am using getopts but i want to solve the problem with one argument. I use a switch, for example -d, and I want the argument for -d will be the path to a directory, like ./work. I want to test the user's input for a string or path, not a number. Is there any solution to solve this problem? I want to solve it with something like: If (test) then echo it is string else echo it is not string

Stéphane Chazelas wrote: > a few rules like > > - always quote variables > - never use the -a or -o operator (use several [ commands and the && and || shell operators) > > Make [ reliable with POSIX shells. Why "never use the -a or -o operator"? How can I do "use several [ commands and the && and || shell operators)"?

I have following in a bash script: echo "status: [$status] and message: [$message]" if [ "${status}" -eq 0 && "$message" = "ERROR" ]; then echo "Exit 0"; exit 0 fi # status: message: [ERROR] However, it gives below error, and the logic under then i.e. exit 0 never gets executed: line 2: [: missing `]' It seems to be complaining about the missing closing ], however, it is there. What am i missing here?

From man bash:

-n string
    True if the length of string is non‐zero.

Examples:

# expected
$ var=""; [ -n "$var" ]; echo $?
1

# unexpected?
$ var=""; [ -n $var ]; echo $?
0

Here we see that -n with unquoted variable containing empty string returns true. Why? Why is $var required to be quoted here?

I want to check if an input string matches whitelisted file paths, then only I want to perform some operation. I am doing something like this path1="mydir/**" path2="mydir/testing" [ [ "$path2" =~ "$path1" ] ] && echo "Matches" Where, if paths are like mydir/test/dir mydir/othertest/dir These should pass. Above check is giving me an error assets/bin/copymain: line 8: [: too many arguments

Bash code to print all folders: for f in ~/*; do if [ $f == '/home/sk/.' -o $f == '/home/sk/..' ]; then true else echo "$f" fi done It works on bash. When i ran the code on z shell, it threw error: = not found Then I converted [ into [[, ] into ]] to avoid this error in z shell and ran it on z shell. It threw next error: condition expected: $f With [[ and ]], bash also throws error as: syntax error in conditional expression syntax error near `-o' Is there a POSIX standard to do string comparison in shell, that works across shells?

#!/bin/sh re="\/$" if [ $1 =~ $re ]; then echo "${ATTENTION_PREFIX}$1 DIRECTORY MAY NOT CONTAIN A \"/\" OR LITERAL SLASH!${ATTENTION_POSTFIX}" exit 1 fi Executing ./file.sh hello/ results in [: 29: hello: unexpected operator It looks like that this regular expression method is incorrect for shell scripting.