Обсуждение: psql: print values and return the COUNT(*) value to bash?

On Mon, 29 Dec 2025 02:21:41 -0500
Ron Johnson <ronljohnsonjr@gmail.com> wrote:
…
> More stuff happens after this.  I want to print out these four lines, but
> also test the COUNT(*) value and bypass other bash commands if the count ==
> 0.

You should probably output your result from psql in a parsable way (CSV ? TSV ?
custom ?) for your bash script, then print values from bash. The commands
"column" or "printf" could help you format a pretty table if really needed.

> Is there any relatively simple way to do this (psql print and return
> count(*))?

Not in a clean way. To make psql return something else than 0, you need to
trigger an error with a failing query to make it return 3 when ON_ERROR_STOP is
enabled.

On Mon, Dec 29, 2025 at 02:21:41AM -0500, Ron Johnson wrote:
> Currently I do this in a bash script:
> Tbl=table_1
> Fld=field_1
> Sql="SELECT CURRENT_TIMESTAMP - MIN($Fld) AS days_ago
>           , MIN($Fld) as oldest_date
>           , COUNT(*) AS cnt
>      FROM $Tbl;"
> psql -h <host> <db> -Xc "${Sql}"
>  days_ago | oldest_date | cnt
> ----------+-------------+-----
>           |             |   0
> (1 row)
> 
> More stuff happens after this.  I want to print out these four lines, but
> also test the COUNT(*) value and bypass other bash commands if the count ==
> 0.
> 
> Is there any relatively simple way to do this (psql print and return
> count(*))?

1. Don't interpolate $variables to your sql. Instead use -v "t=$Tbl" -v
   "f=$Fld" options to psql, and then make your query use :"t" and :"f"
   - it's safer.

2. Catch output of the psql to variable, and check last line, there is
   rowcount there.

Example:

=$ cat z.sh
#!/usr/bin/env bash

table=pg_stat_all_tables
field=last_autovacuum

returned="$( psql -d depesz_explain -v "t=$table" -v "f=$field" -X << _SQL_
select current_timestamp - min(:"f") as days_ago
          , min(:"f") as oldest_date
          , count(*) as cnt
     from :"t"
_SQL_
)"
row_count="$( tail -n1 <<< "${returned}" | tr -cd 0-9 )"
echo "Rowcount = $row_count"
echo "Full output:"
echo "${returned}"

# vim: set filetype=bash shiftwidth=4 expandtab smarttab softtabstop=4 tabstop=4 textwidth=132 :

=$ bash z.sh
Rowcount = 1
Full output:
        days_ago        |          oldest_date          | cnt
------------------------+-------------------------------+-----
 7 days 03:48:07.348247 | 2025-12-22 07:22:32.593814+01 | 212
(1 row)

Best regards,

depesz

On Mon, 29 Dec 2025 11:11:58 +0100
hubert depesz lubaczewski <depesz@depesz.com> wrote:

Hi Ron, Hubert,

>
> Example:
>
> =$ cat z.sh
> #!/usr/bin/env bash
>
> table=pg_stat_all_tables
> field=last_autovacuum
>
> returned="$( psql -d depesz_explain -v "t=$table" -v "f=$field" -X << _SQL_
> select current_timestamp - min(:"f") as days_ago
>           , min(:"f") as oldest_date
>           , count(*) as cnt
>      from :"t"
> _SQL_
> )"
> row_count="$( tail -n1 <<< "${returned}" | tr -cd 0-9 )"
> echo "Rowcount = $row_count"
> echo "Full output:"
> echo "${returned}"
>
> # vim: set filetype=bash shiftwidth=4 expandtab smarttab softtabstop=4 tabstop=4 textwidth=132 :
>
> =$ bash z.sh
> Rowcount = 1
> Full output:
>         days_ago        |          oldest_date          | cnt
> ------------------------+-------------------------------+-----
>  7 days 03:48:07.348247 | 2025-12-22 07:22:32.593814+01 | 212
> (1 row)
>

IIUC, Ron wants the 212 number?

Maybe use the -t switch, and 'cut' :

#!/usr/bin/env bash

table=pg_stat_all_tables
field=last_autovacuum

returned="$( psql -t -d vv -v "t=$table" -v "f=$field" -X << _SQL_
select current_timestamp - min(:"f") as days_ago
          , min(:"f") as oldest_date
          , count(*) as cnt
     from :"t"
_SQL_
)"
row_count="$( tail -n1 <<< "${returned}" | cut -d "|" -f 3)"
echo "Rowcount = $row_count"
echo "Full output:"
echo "${returned}"



--
                Bien à vous, Vincent Veyron

https://compta.libremen.com
Logiciel libre de comptabilité générale et analytique en partie double

On Mon, Dec 29, 2025 at 05:40:23PM +0100, Vincent Veyron wrote:
> > =$ bash z.sh
> > Rowcount = 1
> > Full output:
> >         days_ago        |          oldest_date          | cnt
> > ------------------------+-------------------------------+-----
> >  7 days 03:48:07.348247 | 2025-12-22 07:22:32.593814+01 | 212
> > (1 row)
> > 
> 
> IIUC, Ron wants the 212 number?
> 
> Maybe use the -t switch, and 'cut' :

Ah, that part I misunderstood.

Well, if there is only 1 row returned, always, then the simplest thing
would be to change

> row_count="$( tail -n1 <<< "${returned}" | cut -d "|" -f 3)"

to this:

row_count="$( awk 'NR==3 {print $NF}' <<< "${returned}" )"

format can stay the same, we don't need to psql -t, or -a, and it will
just work :)

Best regards,

depesz