Master Shell Logic: Essential Operators and Test Expressions Explained

1. Logical Operators

-f checks if a file exists, e.g., if [ -f filename ]. -d checks for a directory. -b tests for a block device. -c tests for a character device. -S tests for a socket. -L tests for a symbolic link. -e tests if a path exists.

Program ownership tests: -G checks if a file is owned by a group ID. -O checks if a file is owned by a user ID. -p checks for a named pipe or FIFO.

File attribute tests: -r readable, -w writable, -x executable. -s non‑empty. -u set‑uid, -g set‑gid, -k sticky bit.

File comparison operators: -nt newer than, -ot older than, -ef same file (hard link).

Logical AND/OR symbols: && (AND), || (OR).

2. Operator Symbol Meanings

String comparison inside []: = (equal), != (not equal), < (less than), > (greater than). Integer comparison uses -eq, -ne, -lt, -gt, -le, -ge.

Combined tests: -a (AND), -o (OR), -z (empty string), -n (non‑empty string).

3. Logical Expressions

1. test command

Usage: test EXPRESSION. Example:

test 1 = 1 && echo 'ok'
# prints ok

Another example: test -d /etc/ && echo 'ok' Note: operators and operands must be separated by spaces.

2. Simplified [] expression

Example: [ 1 -eq 1 ] && echo 'ok' When using < or > inside [], they must be escaped because they are interpreted as redirection.

Logical AND/OR inside [] are expressed with -a and -o respectively.

3. [[]] expression

Example:

[[ 2 < 3 ]] && echo 'ok'
[[ 2 < 3 && 4 > 5 ]] && echo 'ok'

[[ ]]

supports < and > without escaping and allows logical operators || and &&.

Performance comparison

Benchmarks (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

Conclusion: [[ ]] is the fastest and, when compatibility with modern Bash is acceptable, is the preferred choice for conditional tests.