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Bash if run a command

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# 1  
Old 10-09-2014
Bash if run a command
I am having trouble with bash. I am trying to put a command in an if statement and then compare it to a string.

This works perfectly.

Code:
echo $(ipcs | grep Shared | awk '{print $2}')

When I put it in an if statement I get some problems.

Code:
$ if [ "$(ipcs | grep Shared | awk '{print $2}')" -eq "Shared"]; then                echo expression evaluated as true;             else                echo expression evaluated as false;             fi
bash: [: missing `]'
expression evaluated as false

$ if [ "$(ipcs | grep Shared | awk '{print $2}')" = "Shared"]; then                echo expression evaluated as true;             else                echo expression evaluated as false;             fi
bash: [: missing `]'
expression evaluated as false

$ if [ "$(ipcs | grep Shared | awk '{print $2}')" == "Shared"]; then                echo expression evaluated as true;             else                echo expression evaluated as false;             fi
bash: [: missing `]'
expression evaluated as false

I tried ==, =, and -eq because I wasn't sure which one to use.
# 2  
Old 10-09-2014
Hi,

Need to enclose in single quotes like so;

Code:
if [ `$(ipcs | grep Shared | awk '{print $2}')` == "Shared"]

Regards

Dave
Last edited by gull04; 10-09-2014 at 07:31 AM.. Reason: Error - Cut and paste
# 3  
Old 10-09-2014
You need a space after the [ and before the closing ]
Code:
if [ "$(ipcs | grep Shared | awk '{print $2}')" = "Shared" ]; then

Shorter is
Code:
if [ "$(ipcs | awk '/Shared/ {print $2}')" = "Shared" ]; then

Both are not portable.
# 4  
Old 10-09-2014
Quote:
Originally Posted by gull04
Hi,

Need to enclose in single quotes like so;

Code:
if [ `$(ipcs | grep Shared | awk '{print $2}')` == "Shared"]

Regards

Dave
Those look more backticks to me `. On my system single quotes aren't slanted '. Are backticks better than double quotes?

Quote:
Originally Posted by MadeInGermany
You need a space after the [ and before the closing ]
Code:
if [ "$(ipcs | grep Shared | awk '{print $2}')" = "Shared" ]; then

Shorter is
Code:
if [ "$(ipcs | awk '/Shared/ {print $2}')" = "Shared" ]; then

Both are not portable.
That worked Smilie. Can you please explain why you need spaces at the beginning and end [ ] brackets for if statements? I always forget when I don't do shellscripting for awhile.

I sometimes forget how awesome awk is Smilie.

What do you mean by not portable? Is it not posix compatible? Is there a way to make it portable?
# 5  
Old 10-09-2014
I don't understand what you're trying to do. On a system with no active message queues, shared memory segments, or semaphores, the output from ipcs will be something like:
Code:
iIPC status from <running system> as of Thu Oct  9 11:22:09 PDT 2014
T     ID     KEY        MODE       OWNER    GROUP
Message Queues:

T     ID     KEY        MODE       OWNER    GROUP
Shared Memory:

T     ID     KEY        MODE       OWNER    GROUP
Semaphores:

So, the output from:
Code:
ipcs|grep Shared|awk '{print $2}'

will be:
Code:
Memory:

and the condition in your if statement will always evaluate to false.

If, in addition to showing us code that doesn't work, you would also tell us what you're trying to do, we might be better able to help.

And, no. You do not want to enclose a command substitution in backquotes unless you are trying to perform a command substitution on the results of a command substitution.
# 6  
Old 10-10-2014
Quote:
Originally Posted by Don Cragun
I don't understand what you're trying to do. On a system with no active message queues, shared memory segments, or semaphores, the output from ipcs will be something like:
Code:
iIPC status from <running system> as of Thu Oct  9 11:22:09 PDT 2014
T     ID     KEY        MODE       OWNER    GROUP
Message Queues:

T     ID     KEY        MODE       OWNER    GROUP
Shared Memory:

T     ID     KEY        MODE       OWNER    GROUP
Semaphores:

So, the output from:
Code:
ipcs|grep Shared|awk '{print $2}'

will be:
Code:
Memory:

and the condition in your if statement will always evaluate to false.

If, in addition to showing us code that doesn't work, you would also tell us what you're trying to do, we might be better able to help.

And, no. You do not want to enclose a command substitution in backquotes unless you are trying to perform a command substitution on the results of a command substitution.
It looks like we are using two different distros of Linux. Our output is a bit different. I am using Fedora.

Code:
$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 262145     bob        600        393216     2          dest         
0x00000000 2523138    bob        600        393216     2          dest         
0x00000000 2555907    bob        600        393216     2          dest         
0x00000000 3375108    bob        600        998400     2          dest         
0x00000000 3440645    bob        666        40         1                       

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x000005b1 262146     bob        600        6         

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages

I am working on a C project that spawns threads, creates shared memory, and creates semaphore arrays. It is annoying to have to keep typing or searching my bash history for:

Code:
ipcrm -s $(ipcs | grep bob | awk '{printf "%s ",$2}')
ipcrm  shm $(ipcs | grep bob | awk '{printf "%s ",$2}')

So I was thinking I could do something like this.

Code:
if [ `$(ipcs | grep Shared | awk '{print $2}')` == "Shared"]
ipcrm  shm $(ipcs | grep bob | awk '{printf "%s ",$2}')

I wanna do that first behavior until $2 equals Semaphore.

Code:
if [ `$(ipcs | grep Semaphore | awk '{print $2}')` == "Semaphore"]
ipcrm -s $(ipcs | grep bob | awk '{printf "%s ",$2}'

)


I am not sure how to this. I have been reading the bash documentation for several days with no luck.
# 7  
Old 10-10-2014
You had several options given to you in your duplicate thread, and it's not clear whether you tried any of them. Particularly, my suggestion with the while loop and printing the options to help figure out why it's not matching when it apparently should.

Please try that, and get back to me, otherwise I'm just wild guessing.
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