How to iterate through a set of variables, then expand the variables in Bash












0














I'm trying to set up a "check if machines are online" script with Bash, but running into an issue of when and where to define the variables so they're expanded properly. Something like:



#!/bin/bash
rm01="c01 c02 c03"
rm02="d01 d02 d03"
rm10="e11 e22 e33"
for room in rm01 rm02 rm03; do
echo $room
for computer in $room; do
#run various nslookup/ping tests and report
done
done
exit 0


I'm running into issues because I can't find a way to expand $room for its corresponding set of computers (in $rm01, $rm02, $rm10) listed at the beginning.



What am I doing wrong?










share|improve this question





























    0














    I'm trying to set up a "check if machines are online" script with Bash, but running into an issue of when and where to define the variables so they're expanded properly. Something like:



    #!/bin/bash
    rm01="c01 c02 c03"
    rm02="d01 d02 d03"
    rm10="e11 e22 e33"
    for room in rm01 rm02 rm03; do
    echo $room
    for computer in $room; do
    #run various nslookup/ping tests and report
    done
    done
    exit 0


    I'm running into issues because I can't find a way to expand $room for its corresponding set of computers (in $rm01, $rm02, $rm10) listed at the beginning.



    What am I doing wrong?










    share|improve this question



























      0












      0








      0







      I'm trying to set up a "check if machines are online" script with Bash, but running into an issue of when and where to define the variables so they're expanded properly. Something like:



      #!/bin/bash
      rm01="c01 c02 c03"
      rm02="d01 d02 d03"
      rm10="e11 e22 e33"
      for room in rm01 rm02 rm03; do
      echo $room
      for computer in $room; do
      #run various nslookup/ping tests and report
      done
      done
      exit 0


      I'm running into issues because I can't find a way to expand $room for its corresponding set of computers (in $rm01, $rm02, $rm10) listed at the beginning.



      What am I doing wrong?










      share|improve this question















      I'm trying to set up a "check if machines are online" script with Bash, but running into an issue of when and where to define the variables so they're expanded properly. Something like:



      #!/bin/bash
      rm01="c01 c02 c03"
      rm02="d01 d02 d03"
      rm10="e11 e22 e33"
      for room in rm01 rm02 rm03; do
      echo $room
      for computer in $room; do
      #run various nslookup/ping tests and report
      done
      done
      exit 0


      I'm running into issues because I can't find a way to expand $room for its corresponding set of computers (in $rm01, $rm02, $rm10) listed at the beginning.



      What am I doing wrong?







      bash variables iteration






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 21 '18 at 15:50









      Benjamin W.

      20.3k134656




      20.3k134656










      asked Nov 21 '18 at 15:37









      phonedog365

      133




      133
























          1 Answer
          1






          active

          oldest

          votes


















          1














          The quick fix is to use variable indirection:



          for computer in ${!room}; do


          Relying on word splitting is rarely the best idea, though. You could use arrays and namerefs instead (requires Bash 4.3 or newer):



          #!/usr/bin/env bash

          # Declare arrays
          rm01=(c01 c02 c03)
          rm02=(d01 d02 d03)
          rm03=(e11 e22 e33)

          # Declare room as nameref
          declare -n room

          # Using nameref as control variable sets room as reference to each variable in turn
          for room in rm{01..03}; do
          # Properly quoted array expansion
          for computer in "${room[@]}"; do
          echo "$computer" # or whatever needs to be done
          done
          done
          exit 0





          share|improve this answer























          • Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
            – phonedog365
            Nov 22 '18 at 19:19










          • @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
            – Benjamin W.
            Nov 22 '18 at 19:31













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          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1














          The quick fix is to use variable indirection:



          for computer in ${!room}; do


          Relying on word splitting is rarely the best idea, though. You could use arrays and namerefs instead (requires Bash 4.3 or newer):



          #!/usr/bin/env bash

          # Declare arrays
          rm01=(c01 c02 c03)
          rm02=(d01 d02 d03)
          rm03=(e11 e22 e33)

          # Declare room as nameref
          declare -n room

          # Using nameref as control variable sets room as reference to each variable in turn
          for room in rm{01..03}; do
          # Properly quoted array expansion
          for computer in "${room[@]}"; do
          echo "$computer" # or whatever needs to be done
          done
          done
          exit 0





          share|improve this answer























          • Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
            – phonedog365
            Nov 22 '18 at 19:19










          • @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
            – Benjamin W.
            Nov 22 '18 at 19:31


















          1














          The quick fix is to use variable indirection:



          for computer in ${!room}; do


          Relying on word splitting is rarely the best idea, though. You could use arrays and namerefs instead (requires Bash 4.3 or newer):



          #!/usr/bin/env bash

          # Declare arrays
          rm01=(c01 c02 c03)
          rm02=(d01 d02 d03)
          rm03=(e11 e22 e33)

          # Declare room as nameref
          declare -n room

          # Using nameref as control variable sets room as reference to each variable in turn
          for room in rm{01..03}; do
          # Properly quoted array expansion
          for computer in "${room[@]}"; do
          echo "$computer" # or whatever needs to be done
          done
          done
          exit 0





          share|improve this answer























          • Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
            – phonedog365
            Nov 22 '18 at 19:19










          • @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
            – Benjamin W.
            Nov 22 '18 at 19:31
















          1












          1








          1






          The quick fix is to use variable indirection:



          for computer in ${!room}; do


          Relying on word splitting is rarely the best idea, though. You could use arrays and namerefs instead (requires Bash 4.3 or newer):



          #!/usr/bin/env bash

          # Declare arrays
          rm01=(c01 c02 c03)
          rm02=(d01 d02 d03)
          rm03=(e11 e22 e33)

          # Declare room as nameref
          declare -n room

          # Using nameref as control variable sets room as reference to each variable in turn
          for room in rm{01..03}; do
          # Properly quoted array expansion
          for computer in "${room[@]}"; do
          echo "$computer" # or whatever needs to be done
          done
          done
          exit 0





          share|improve this answer














          The quick fix is to use variable indirection:



          for computer in ${!room}; do


          Relying on word splitting is rarely the best idea, though. You could use arrays and namerefs instead (requires Bash 4.3 or newer):



          #!/usr/bin/env bash

          # Declare arrays
          rm01=(c01 c02 c03)
          rm02=(d01 d02 d03)
          rm03=(e11 e22 e33)

          # Declare room as nameref
          declare -n room

          # Using nameref as control variable sets room as reference to each variable in turn
          for room in rm{01..03}; do
          # Properly quoted array expansion
          for computer in "${room[@]}"; do
          echo "$computer" # or whatever needs to be done
          done
          done
          exit 0






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Nov 21 '18 at 15:59

























          answered Nov 21 '18 at 15:49









          Benjamin W.

          20.3k134656




          20.3k134656












          • Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
            – phonedog365
            Nov 22 '18 at 19:19










          • @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
            – Benjamin W.
            Nov 22 '18 at 19:31




















          • Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
            – phonedog365
            Nov 22 '18 at 19:19










          • @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
            – Benjamin W.
            Nov 22 '18 at 19:31


















          Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
          – phonedog365
          Nov 22 '18 at 19:19




          Is there a way to do the arrays/nameref method with older bash? I'm stuck on 3.4 because Apple updates its BSD binaries infrequently. The first example is working just fine, thanks.
          – phonedog365
          Nov 22 '18 at 19:19












          @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
          – Benjamin W.
          Nov 22 '18 at 19:31






          @phonedog365 You could install a more recent Bash with Homebrew; I think you're actually on Bash 3.2 with macOS. If you can't use namerefs, you can use indirection as in this Q&A. Another good read is BashFAQ/006.
          – Benjamin W.
          Nov 22 '18 at 19:31




















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