Face it, backticks are a pain. BASH $() construct provides a simpler, more effective approach. This talk uses examples from automating git branches and command line processing with getopt(1) to show how $() works in shell scripts.

1. Getting into BSDM with BASH:
Command Interpolation
Steven Lembark
Workhorse Computing
lembark@wrkhors.com

2. Commanding the shell
Think interpolating variables is fun?

3. Commanding the shell
Think interpolating variables is fun?
Commands feel even better :-)
Extract command output into variables.
Control shell script execution.

4. BASH really is interpreted
Unlike C, C++, Perl, Python, Ruby, Raku, Haskell...
BASH:
$foo;
Valid shell command.

5. BASH really is interpreted
Unlike C, C++, Perl, Python, Ruby, Raku, Haskell...
$foo $args;
Runs something... maybe.

6. Control
Real utility: Extract from one ouput.
Feed it into another.

7. Control
Real utility: Extract from one ouput.
Feed it into another.
Not just pipes.

8. Control
Real utility: Extract from one ouput.
Feed it into another.
Saves cut+paste.
Use one shell command to control another.

9. BSDM
BSD used ksh.
Backticks were the way to run commands:
foo=`ls -l bar`;
Q: How many escapes does this require?
foo=`ls -l /var/tmp/`whoami``?

10. BSDM
BSD used ksh.
Backticks were the way to run commands:
foo=`ls -l bar`;
Avoid BSD Masochism: use BASH.

11. What branch am I on?
Extract a branch with pipes.
Now you know what to type.
$ git status | head -1 | cut -d ' ' -f3;
master

12. What branch am I on?
Extract a branch with pipes.
Now you know what to cut+paste.
$ git status | head -1 | cut -d ' ' -f3;
master

13. What branch am I on?
Save a branch with $().
Now you don’t have to type.
br=$(git status | head -1 | cut -d ' ' -f3);
echo $br;
master

14. What branch am I on?
Save a branch with $().
Now you don’t have to type.
br=$(git status | head -1 | cut -d ' ' -f3);
echo $br;
master

15. Resync your fork
br=$(git status | head -1 | cut -d ' ' -f3);
git merge upstream/master;
git checkout master;
git rebase upstream;
git checkout $br; # back to start

16. Build & install the current release.
Postgresql tags releases as REL_X_Y.
Upgrades or smoke tests use most current branch.
Be nice to automate it.

17. Build & install the current release.
Find the release tag:
$ git tag | egrep '^REL_[0-9_]*$';
REL_10_0
REL_10_1
REL_10_10
REL_10_11
$ git tag | egrep ... | sort -rn | head -n1;
REL_12_1

18. Build & install the current release.
rx=’^REL_[0-9_]*$’;
rel=$(git tag|grep “$rx”|sort -rn|head -1);
ver=$(echo ${rel#*_} | tr ‘_’ ‘.’);
prefix=”/opt/postgresql/$ver;
[ -d $prefix ] && exit 1;
git clean -fdx;
git checkout $rel;
./configure --prefix=$prefix ... ;
make -wk all check install 2>&1 | tee ... ;

19. Build & install the current release.
rx=’^REL_[0-9_]*$’;
rel=$(git tag|grep “$rx”|sort -rn|head -1);
ver=$(echo ${rel#*_} | tr ‘_’ ‘.’);
prefix=”/opt/postgresql/$ver;
[ -d $prefix ] && exit 1;
git clean -fdx;
git checkout $rel;
./configure --prefix=$prefix ... ;
make -wk all check install 2>&1 | tee ... ;

20. Using force
Re-install an existing version?
Select a REL_* tag?
OK:
foobar --force REL_10_0;

21. Using force
Re-install an existing version?
Select a REL_* tag?
OK:
foobar --force REL_10_0;
if [ “$1” = ‘--force’ ]; then ... fi;
shift;

22. Using force
Re-install an existing version?
Select a REL_* tag?
OK:
foobar --force REL_10_0;
if [ “$1” = ‘--force’ ]; then ... fi;
shift;

23. Using force
Re-install an existing version?
Select a REL_* tag?
OOPS:
foobar --force REL_10_0;
foobar REL_10_0 --force;

24. Having options
getopt(1) shuffles command line args.
Leaves them suitable for iterating in code.

25. Having options
getopt(1) shuffles command line args.
Leaves them suitable for iterating in code.
ARGS=$( getopt ... "$@" );
eval set -- "$ARGS”;
while :; do
case $1 in
’--’) shift; break;;
...
done;

26. Having options
getopt(1) shuffles command line args.
Leaves them suitable for iterating in code.
ARGS=$( getopt ... "$@" );
eval set -- "$ARGS”;
while :; do
case $1 in
’--’) shift; break;;
...
done;

27. Having options
eval + set
re-assign arguments.
ARGS=$( getopt ... "$@" );
eval set -- "$ARGS”;
while :; do
case $1 in
’--’) shift; break;;
...
done;

28. Having options
-- marks end of switches.
ARGS=$( getopt ... "$@" );
eval set -- "$ARGS”;
while :; do
case $1 in
’--’) shift; break;;
...
done;

29. Having options
Leave program args on the stack.
ARGS=$( getopt ... "$@" );
eval set -- "$ARGS”;
while :; do
case $1 in
’--’) shift; break;;
...
done;

30. Example: Glob vs. File Path
‘--file’ or ‘-f’ for a path.
‘--glob’ or ‘-g’ for a glob.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help’
-n "$(basename $0)" -- "$@”
);

31. Example: Glob vs. File Path
-f & -g have args (f:g:).
-h & -? do not (h?).
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

32. Example: Glob vs. File Path
-f & -g have args (f:g:).
-h & -? do not (h?).
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

33. Example: Glob vs. File Path
-f & -g have options, -h & -? do not.
Long options are --file , --glob & --help.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

34. Example: Glob vs. File Path
Errors use the executable basename.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

35. Example: Glob vs. File Path
Errors use the executable basename.
With nested commands.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

36. Example: Glob vs. File Path
Errors use the executable basename.
Without escapes, escaped escapes, doubled backticks...
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

37. Example: Glob vs. File Path
One more reason to use $(...).
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);

38. Example: Glob vs. File Path
two-pass interpolation.
Handles whitespace.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);
eval set -- "$ARGS”;

39. Example: Glob vs. File Path
Consume with a case.
ARGS=$(
getopt -o 'f:g:h?'
--long 'file:,glob:,help'
-n "$(basename $0)" -- "$@”
);
eval set -- "$ARGS”;

40. while :; # loop until ‘--’ end-of-args.
do
case $1
in
'--') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

41. while :;
do
case $1 # switch always $1
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

42. while :;
do
case $1 # switch always $1
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift; # shift off the switch
done;

43. while :;
do
case $1 # arg always $2, may be ‘’
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

44. while :;
do
case $1 # help is a one-shot.
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

45. while :;
do
case $1 # anything else is fatal
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

46. while :;
do
case $1 # consumes only switches
in
'–-') shift; break ;;
'-g'|'--glob') glob="$2" ; shift ;;
'-f'|'--file') file="$2" ; shift ;;
'-h'|'--help'|'-?') usage; exit 1 ;;
*) usage; exit -1 ;;
esac
shift;
done;

47. getopt + eval + case
Handle argument order.
Optional args.
Short & long options.
Unknown arguments.

48. Avoid infestations!
$(...) saves you from backticks.
See also:
bash(1) set, eval, case
getopt(1)