How to use an environment variable inside a quoted string in Bash

Question

I ve tried various forms of the following in a bash script      bin bash svn diff    --diff-cmd  usr bin diff -x  -y -w -p -W  COLUMNS    But I can t get the syntax to correctly expand the COLUMNS environment variable   I ve tried various forms of the following   svn diff    --diff-cmd  usr bin diff -x  -y -w -p -W  COLUMNS    and  svn diff    --diff-cmd  usr bin diff -x  -y -w -p -W   COLUMNS     and  eval svn diff    --diff-cmd  usr bin diff -x  -y -w -p -W  COLUMNS    Suggestions

User · Accepted Answer

If unsure  you might use the  cols  request on the terminal  and forget COLUMNS   COLS   tput cols

User · Answer

Note that COLUMNS is    NOT an environment variable   It is an ordinary bash parameter that is set by bash itself  Set automatically upon receipt of a SIGWINCH signal    That second point usually means that your COLUMNS variable will only be set in your interactive shell  not in a bash script   If your script s stdin is connected to your terminal you can manually look up the width of your terminal by asking your terminal   tput cols   And to use this in your SVN command   svn diff      --diff-cmd  usr bin diff -x  -y -w -p -W   tput cols      Note  you should quote      and stay away from eval  -

User · Answer

The following script works for me for multiple values of  COLUMNS   I wonder if you are not setting COLUMNS prior to this call      bin bash COLUMNS 30 svn diff    --diff-cmd  usr bin diff -x  -y -w -p -W  COLUMNS    Can you echo  COLUMNS inside your script to see if it set correctly

User · Answer

You are doing it right  so I guess something else is at fault  not export-ing COLUMNS      A trick to debug these cases is to make a specialized command  a closure for programming language guys   Create a shell script named diff-columns doing   exec  usr bin diff -x -y -w -p -W   COLUMNS         and just use   svn diff      --diff-cmd  diff-columns   This way your code is cleaner to read and more modular  top-down approach   and you can test the diff-columns code thouroughly separately  bottom-up approach

User · Answer

Just a quick note summary for any who came here via Google looking for the answer to the general question asked in the title  as I was    Any of the following should work for getting access to shell variables inside quotes   echo   VARIABLE  echo    VARIABLE     Use of single quotes is the main issue   According to the Bash Reference Manual      Enclosing characters in single quotes     preserves the literal value of each character within the quotes  A single quote may not occur between single quotes  even when preceded by a backslash            Enclosing characters in double quotes     preserves the literal value of all characters within the quotes  with the exception of          and  when history expansion is enabled     The characters   and   retain their special meaning within double quotes  see Shell Expansions   The backslash retains its special meaning only when followed by one of the following characters              or newline  Within double quotes  backslashes that are followed by one of these characters are removed  Backslashes preceding characters without a special meaning are left unmodified  A double quote may be quoted within double quotes by preceding it with a backslash  If enabled  history expansion will be performed unless an   appearing in double quotes is escaped using a backslash  The backslash preceding the   is not removed    The special parameters   and   have special meaning when in double quotes  see Shell Parameter Expansion     In the specific case asked in the question   COLUMNS is a special variable which has nonstandard properties  see lhunath s answer above

[bash] How to use an environment variable inside a quoted string in Bash

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