Shell – Multiple Command List After Shell Conditionals, &&, ||. Shell Script, Dash

control flowshell

I'm trying to do multiple commands after a condition, so for example…

[ $VAR ] || echo "Error msg" ; echo "exit"

and the Inverse

[ -z $VAR ] &&  echo "Error msg" ; echo "exit"

I know that won't work as intended, I actually knew how previously and forgot how to do this. I'm fully aware of the many alternatives, such as using if's or bracketing via () and {} . Using () will create a sub process which wouldn't exit a running script. Using {} will work, but I know a more readable alternative exists.

I have done this with a :, and it was perfect!!! I just can't remember now for the life of me, and I lost that previously written script.

If anyone knows how to write this with the :'s, I would really appreciate any help!

Best Answer

I tend to use something like this, which I consider nicely readable:

[ -z $VAR ] && {
  echo "Error msg"
  exit ${LINENO}
} >&2

For :, the only thing I can imagine is that you somehow defined a function but I have no idea how that would translate into a block that allows multiple commands to execute.

From man bash:

: [arguments]
        No effect; the command does nothing beyond expanding arguments and
        performing any specified redirections. A zero exit code is returned.

So the only possibility I see is if you had redefined : to be something else. I'll be interested in seeing any possibilities as to what that could be.

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Shell backup script, missing `]`

The problem is that && is not a valid operator for [. It needs -a instead, or [ condition ] && [ other_condition ].

&& is a separator (as is ;, ||, & and \n) in a posix shell. It sees the statement [ -f www/archives/pdf/pdf_201207021048.tar; the statement ends becaues there is a &&, then there is another statement (which also would cause an error if it got to it) that looks like 11294720 -gt 0 ], and that statement is terminated with ;.

Shell – do…while or do…until in POSIX shell script

A very versatile version of a do ... while has this structure:

while 
      Commands ...
do :; done

An example is:

#i=16
while
      echo "this command is executed at least once $i"
      : ${start=$i}              # capture the starting value of i
      # some other commands      # needed for the loop
      (( ++i < 20 ))             # Place the loop ending test here.
do :; done
echo "Final value of $i///$start"
echo "The loop was executed $(( i - start )) times "

As it is (no value set for i) the loop executes 20 times.
UN-Commenting the line that sets i to 16 i=16, the loop is executed 4 times.
For i=16, i=17, i=18 and i=19.

If i is set to (let's say 26) at the same point (the start), the commands still get executed the first time (until the loop break command is tested).

The test for a while should be truthy (exit status of 0).
The test should be reversed for an until loop, i.e.: be falsy (exit status not 0).

A POSIX version needs several elements changed to work:

i=16
while
       echo "this command is executed at least once $i"
       : ${start=$i}              # capture the starting value of i
       # some other commands      # needed for the loop
       i="$((i+1))"               # increment the variable of the loop.
       [ "$i" -lt 20 ]            # test the limit of the loop.
do :;  done
echo "Final value of $i///$start"
echo "The loop was executed $(( i - start )) times "

./script.sh
this command is executed at least once 16
this command is executed at least once 17
this command is executed at least once 18
this command is executed at least once 19
Final value of 20///16
The loop was executed 4 times

Best Answer

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Shell backup script, missing `]`

Shell – do…while or do…until in POSIX shell script

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