bash unary operator expected error - bash

Question

This is the script I came up with
#!/bin/bash
expression=$1
field=$2
if [ -z "$expression" ]; then
echo "expression is missing"
exit 1
fi
if [ -f /home/miked/table ]; then
if [ -f table ] && [ grep "$expression table" ]; then
grep "$expression" table | cut -d: -f${2:-3} | clipit
else
echo "no match found"
fi
else
echo "there is no table file"
fi
As a matter of fact I know how to fix it, but I don't know why
it's fixed.
If I remove the space between grep and ", all is working fine, I just can't seem to understand why.
If I do grep something file directly to the command line, it's working good. Why do I to stick the grep to the " in the script?

Answer 1

You do not need to wrap grep call inside square brackets. [ is alias to test command (in general, however most shells replicate that using builtin). You can see syntax of that command using man test. What you want do is to check if $expression table exist in some file, so instead you need to write it as:
#!/bin/bash
expression="$1"
field="$2"
if [ -z "$expression" ]; then
echo "expression is missing"
exit 1
fi
if [ -f /home/miked/table ]; then
if [ -f table ] && grep "$expression table"; then
grep "$expression" table | cut -d: -f${2:-3} | clipit
else
echo "no match found"
fi
else
echo "there is no table file"
fi
However there are more problems with your script.
You print errors to stdout instead of stderr which makes them invisible when piping output of your script to other tools, instead you should use echo "error" >&2, ideally use separate function for that.
You pass only 1 argument to grep and I believe that there should be 2: grep "$expression" table.
Your first grep call will also print to stdout and I believe you want to surpass that, so instead use -q flag.
It is good idea to enable "exit on error" using set -e and "exit on pipe error" using set -o pipefail
You do not use outer file, so you can just remove check for that.
You do not use your $field variable.
Use guard clausules instead of ifs for fatal error checking as it will make easier to refactor your script.
So whole file could be written as:
#!/bin/bash
set -eo pipefail
perror() {
echo "$1" >&2 && exit 1
}
expression=$1
field=${2:-3}
file=${3:table}
[ -z "$expression" ] || perror "expression is missing"
[ -f "$file" ] || perror "there is no '$file' file"
grep "$expression" "$file" | cut -d: -f"${field}" | clipit || perror "no match found"

Answer 2

You don't need [ ] at all here. You're interested in the exit status of grep and it gives you one already; to suppress its output, you can use the -q option:
if [ -f table ] && grep -q "$expression" table; then
The filename should not be within the quotes, either.
If you use [ ] without any test, it defaults to -n: "true if string is not empty". This test expects a single argument, which is why it seems to work for you if you remove the space: it just checks if the string grep$expression table expands to is non-zero, and it always is.

Answer 3

There are quite a few things that could be done to fix this, your main problem is the following line:
if [ -f table ] && [ grep "$expression table" ]; then
You've already tested if "table" exists, so you're doing it again and once it succeeds, the expression [ grep "$expression table" ] is being evaluated, which is broken down to '[' grep 'expression table' ']' which means essentially nothing.
You should instead use $() and evaluate the number of occurrences, or like Benjamin mentions, skip it altogether if that's what you want.
I would suggest something like this
#!/bin/bash
expression=$1
field=$2
table_file=/home/miked/table
if [ -z "$expression" ]; then
echo "expression is missing"
exit 1
fi
if [ -f $table_file ]; then
if [ $(grep -q "$expression $table_file") -gt 0 ]; then
grep "$expression" $table_file | cut -d: -f${2:-3} | clipit
else
echo "no match found"
fi
else
echo "there is no table file"
fi
Notice how we're still using a test, this could be condensed to:
#!/bin/bash
expression=$1
field=$2
table_file=/home/miked/table
if [ -z "$expression" ]; then
echo "expression is missing"
exit 1
fi
if [ -f $table_file ]; then
grep -q $expression $table_file && grep "$expression" $table_file | cut -d: -f${2:-3} | clipit || echo "no match found"
else
echo "there is no table file"
fi