Q. I see [ command in /usr/bin directory. What is the /usr/bin/[ command used for?
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A.This is test command and it is used to check file types and compare values.
General syntax is as follows:
[ EXPRESSION ]
For example, find out if /etc/passwd exists or not:
[ -f /etc/passwd ] && echo "Yes" || echo "No"
Regularly you write it as follows:
if [ -f /etc/passwd ]
then
echo "Yes"
else
echo "No"
fi |
if [ -f /etc/passwd ]
then
echo "Yes"
else
echo "No"
fi
OR
if test -f /etc/passwd
then
echo "Yes"
else
echo "No"
fi |
if test -f /etc/passwd
then
echo "Yes"
else
echo "No"
fi
Read test or [ command man page for all other expressions.
man [
man test
Posted by: Vivek Gite
The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.
1 comment
Doesn’t work as written in all shells (e.g. BASH – not tested in others)
root@localhost:~/root# ls -l
total 0
-rw——-. 1 root root 0 Oct 25 14:19 test
root@localhost:~/root# [ -a test] echo “Y” || echo “N” // <– Fails
bash: [: missing `]'
N
root@localhost:~/root# [ -a test ]; echo "Y" || echo "N" // <– Requires ';' after ']'
Y
In Bash, the semi-colon ';' separates commands on the same line allowing multiple commands on a single line
–PB–