Master Bash Conditional Logic: Essential Operators and Best Practices

1. Logical Operators

Shell provides a variety of test flags that can be used to evaluate files, directories, and other attributes, enabling concise and powerful conditional statements. -f – checks if a file exists (e.g., if [ -f filename ]) -d – checks if a directory exists -b – checks for a block device file -c – checks for a character device file -S – checks for a socket file -L – checks for a symbolic link -e – checks if a path exists -G – true if the file is owned by the effective group ID -O – true if the file is owned by the effective user ID -p – checks for a named pipe (FIFO) -r – checks if the file is readable -w – checks if the file is writable -x – checks if the file is executable -s – checks if the file size is greater than zero (non‑empty) -u – checks for the SUID bit -g – checks for the SGID bit -k – checks for the sticky bit

File‑to‑file comparisons include: -nt – first file is newer than the second -ot – first file is older than the second -ef – both names refer to the same inode (hard link)

Logical conjunction and disjunction can be expressed with && (AND) and || (OR).

2. Operator Symbols

When using test or [ ], the following symbols are common: = – string equality != – string inequality < – string less‑than (requires escaping in [ ]) > – string greater‑than (requires escaping in [ ]) -eq – numeric equality -ne – numeric inequality -lt – numeric less‑than -gt – numeric greater‑than -le – numeric less‑or‑equal -ge – numeric greater‑or‑equal -a – logical AND (alternative syntax) -o – logical OR (alternative syntax) -z – tests for an empty string -n – tests for a non‑empty string

3. Logical Expressions

test command

Usage: test EXPRESSION Examples:

[root@localhost ~]# test 1 = 1 && echo 'ok'
ok

[root@localhost ~]# test -d /etc/ && echo 'ok'
ok

[root@localhost ~]# test 1 -eq 1 && echo 'ok'
ok

[root@localhost ~]# if test 1 = 1 ; then echo 'ok'; fi
ok

Note: All operators and operands must be separated by spaces.

Simplified expressions [] expression:

[root@localhost ~]# [ 1 -eq 1 ] && echo 'ok'
ok

[root@localhost ~]# [ 2 < 1 ] && echo 'ok'
-bash: 2: No such file or directory

[root@localhost ~]# [ 2 -gt 1 -a 3 -lt 4 ] && echo 'ok'
ok

When using [], the characters < and > must be escaped because they are interpreted as redirection operators.

[[]] expression

[root@localhost ~]$ [[ 2 < 3 ]] && echo 'ok'
ok

[root@localhost ~]$ [[ 2 < 3 && 4 > 5 ]] && echo 'ok'
ok

The [[ ]] construct extends [] by allowing unescaped < and > for string comparison and supports logical operators || and && directly.

Performance comparison

Three common conditional forms were timed over 100,000 iterations:

time (for m in {1..100000}; do test -d .; done;)
real 0m0.658s
user 0m0.558s
sys 0m0.100s

time (for m in {1..100000}; do [ -d . ]; done;)
real 0m0.609s
user 0m0.524s
sys 0m0.085s

time (for m in {1..100000}; do [[ -d . ]]; done;)
real 0m0.311s
user 0m0.275s
sys 0m0.036s

When compatibility with older Bash versions or POSIX sh is not required, [[ ]] offers the best performance and flexibility for conditional checks.