Question

I have the following script in bash:

#!/bin/bash
PSQL="psql -a -e -d users -U postgres --no-align -F$'\t' -c"
FILENAME="file.ext"
QUERY="select * from users limit 1;"
${PSQL} "${QUERY}" # > ${FILENAME}

But the -F$'\t' is evaluated differently. How can we make sure that the string will not be evaluated to return a different string, i.e. finally we would like to execute the command as: psql -a -e -d users -U postgres --no-align -F$'\t' -c "select * from users limit 1;" > file.ext

Answer 1

The best way to solve your problem is to use arrays:

PSQL=( psql -a -e -d users -U postgres --no-align $'-F\t' -c )
FILENAME="file.ext"
QUERY="select * from users limit 1;"
"${PSQL[@]}" "${QUERY}" # > ${FILENAME}

Explanation. The first line defines a new array named PSQL. You can think of it as:

PSQL[0] = psql
PSQL[1] = -a
PSQL[2] = -e
PSQL[3] = -d
PSQL[4] = users
PSQL[5] = -U
PSQL[6] = postgres
PSQL[7] = --no-align
PSQL[8] = -F<tab>
PSQL[9] = -c

In the last line, the (double-quoted) term "${PSQL[@]}" will expand to the 10 “words” that constitute the array PSQL and the (double-quoted) term "${QUERY}" will expand to the string select * from users limit 1; considered a single word. To make things clearer, I’m going to show you how the expansion works using { and } to group each argument bash sees when expanding the line "${PSQL[@]}" "${QUERY}":

{ psql } { -a } { -e } { -d } { users } { -U } { postgres } { --no-align } { -F<tab> } { -c } { select * from users limit 1; }

Remark. Using all upper-case variable names in bash is considered bad practice.